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Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	6	<title>LLVM Programmer's Manual</title>
				7	<link rel="stylesheet" href="llvm.css" type="text/css">
				8	</head>
				9	<body>
				10
				11	<div class="doc_title">
				12	LLVM Programmer's Manual
				13	</div>
				14
				15	<ol>
				16	<li><a href="#introduction">Introduction</a></li>
				17	<li><a href="#general">General Information</a>
				18	<ul>
				19	<li><a href="#stl">The C++ Standard Template Library</a></li>
				20	<!--
				21	<li>The <tt>-time-passes</tt> option</li>
				22	<li>How to use the LLVM Makefile system</li>
				23	<li>How to write a regression test</li>
				24
				25	-->
				26	</ul>
				27	</li>
				28	<li><a href="#apis">Important and useful LLVM APIs</a>
				29	<ul>
				30	<li><a href="#isa">The <tt>isa<></tt>, <tt>cast<></tt>
				31	and <tt>dyn_cast<></tt> templates</a> </li>
Daniel Dunbar	e3572ba	2009-07-25 04:41:11 +0000	[diff] [blame]	32	<li><a href="#string_apis">Passing strings (the <tt>StringRef</tt>
Benjamin Kramer	7b2136d	2009-08-05 15:42:44 +0000	[diff] [blame]	33	and <tt>Twine</tt> classes)</a>
Daniel Dunbar	e3572ba	2009-07-25 04:41:11 +0000	[diff] [blame]	34	<ul>
				35	<li><a href="#StringRef">The <tt>StringRef</tt> class</a> </li>
				36	<li><a href="#Twine">The <tt>Twine</tt> class</a> </li>
				37	</ul>
Benjamin Kramer	7b2136d	2009-08-05 15:42:44 +0000	[diff] [blame]	38	</li>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	39	<li><a href="#DEBUG">The <tt>DEBUG()</tt> macro and <tt>-debug</tt>
				40	option</a>
				41	<ul>
				42	<li><a href="#DEBUG_TYPE">Fine grained debug info with <tt>DEBUG_TYPE</tt>
				43	and the <tt>-debug-only</tt> option</a> </li>
				44	</ul>
				45	</li>
				46	<li><a href="#Statistic">The <tt>Statistic</tt> class & <tt>-stats</tt>
				47	option</a></li>
				48	<!--
				49	<li>The <tt>InstVisitor</tt> template
				50	<li>The general graph API
				51	-->
				52	<li><a href="#ViewGraph">Viewing graphs while debugging code</a></li>
				53	</ul>
				54	</li>
				55	<li><a href="#datastructure">Picking the Right Data Structure for a Task</a>
				56	<ul>
				57	<li><a href="#ds_sequential">Sequential Containers (std::vector, std::list, etc)</a>
				58	<ul>
				59	<li><a href="#dss_fixedarrays">Fixed Size Arrays</a></li>
				60	<li><a href="#dss_heaparrays">Heap Allocated Arrays</a></li>
				61	<li><a href="#dss_smallvector">"llvm/ADT/SmallVector.h"</a></li>
				62	<li><a href="#dss_vector"><vector></a></li>
				63	<li><a href="#dss_deque"><deque></a></li>
				64	<li><a href="#dss_list"><list></a></li>
Gabor Greif	bb17f65	2009-02-27 11:37:41 +0000	[diff] [blame]	65	<li><a href="#dss_ilist">llvm/ADT/ilist.h</a></li>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	66	<li><a href="#dss_other">Other Sequential Container Options</a></li>
				67	</ul></li>
				68	<li><a href="#ds_set">Set-Like Containers (std::set, SmallSet, SetVector, etc)</a>
				69	<ul>
				70	<li><a href="#dss_sortedvectorset">A sorted 'vector'</a></li>
				71	<li><a href="#dss_smallset">"llvm/ADT/SmallSet.h"</a></li>
				72	<li><a href="#dss_smallptrset">"llvm/ADT/SmallPtrSet.h"</a></li>
Chris Lattner	77ab46d	2007-09-30 00:58:59 +0000	[diff] [blame]	73	<li><a href="#dss_denseset">"llvm/ADT/DenseSet.h"</a></li>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	74	<li><a href="#dss_FoldingSet">"llvm/ADT/FoldingSet.h"</a></li>
				75	<li><a href="#dss_set"><set></a></li>
				76	<li><a href="#dss_setvector">"llvm/ADT/SetVector.h"</a></li>
				77	<li><a href="#dss_uniquevector">"llvm/ADT/UniqueVector.h"</a></li>
				78	<li><a href="#dss_otherset">Other Set-Like ContainerOptions</a></li>
				79	</ul></li>
				80	<li><a href="#ds_map">Map-Like Containers (std::map, DenseMap, etc)</a>
				81	<ul>
				82	<li><a href="#dss_sortedvectormap">A sorted 'vector'</a></li>
				83	<li><a href="#dss_stringmap">"llvm/ADT/StringMap.h"</a></li>
				84	<li><a href="#dss_indexedmap">"llvm/ADT/IndexedMap.h"</a></li>
				85	<li><a href="#dss_densemap">"llvm/ADT/DenseMap.h"</a></li>
Jeffrey Yasskin	50e846d	2009-10-22 22:11:22 +0000	[diff] [blame]	86	<li><a href="#dss_valuemap">"llvm/ADT/ValueMap.h"</a></li>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	87	<li><a href="#dss_map"><map></a></li>
				88	<li><a href="#dss_othermap">Other Map-Like Container Options</a></li>
				89	</ul></li>
Chris Lattner	d8b95b7	2009-07-25 07:22:20 +0000	[diff] [blame]	90	<li><a href="#ds_string">String-like containers</a>
Benjamin Kramer	7b2136d	2009-08-05 15:42:44 +0000	[diff] [blame]	91	<!--<ul>
				92	todo
				93	</ul>--></li>
Daniel Berlin	7ea44dc	2007-09-24 17:52:25 +0000	[diff] [blame]	94	<li><a href="#ds_bit">BitVector-like containers</a>
				95	<ul>
				96	<li><a href="#dss_bitvector">A dense bitvector</a></li>
Dan Gohman	c83a2b4	2010-01-05 18:24:00 +0000	[diff] [blame]	97	<li><a href="#dss_smallbitvector">A "small" dense bitvector</a></li>
Daniel Berlin	7ea44dc	2007-09-24 17:52:25 +0000	[diff] [blame]	98	<li><a href="#dss_sparsebitvector">A sparse bitvector</a></li>
				99	</ul></li>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	100	</ul>
				101	</li>
				102	<li><a href="#common">Helpful Hints for Common Operations</a>
				103	<ul>
				104	<li><a href="#inspection">Basic Inspection and Traversal Routines</a>
				105	<ul>
				106	<li><a href="#iterate_function">Iterating over the <tt>BasicBlock</tt>s
				107	in a <tt>Function</tt></a> </li>
				108	<li><a href="#iterate_basicblock">Iterating over the <tt>Instruction</tt>s
				109	in a <tt>BasicBlock</tt></a> </li>
				110	<li><a href="#iterate_institer">Iterating over the <tt>Instruction</tt>s
				111	in a <tt>Function</tt></a> </li>
				112	<li><a href="#iterate_convert">Turning an iterator into a
				113	class pointer</a> </li>
				114	<li><a href="#iterate_complex">Finding call sites: a more
				115	complex example</a> </li>
				116	<li><a href="#calls_and_invokes">Treating calls and invokes
				117	the same way</a> </li>
				118	<li><a href="#iterate_chains">Iterating over def-use &
				119	use-def chains</a> </li>
Chris Lattner	0665e1f	2008-01-03 16:56:04 +0000	[diff] [blame]	120	<li><a href="#iterate_preds">Iterating over predecessors &
				121	successors of blocks</a></li>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	122	</ul>
				123	</li>
				124	<li><a href="#simplechanges">Making simple changes</a>
				125	<ul>
				126	<li><a href="#schanges_creating">Creating and inserting new
				127	<tt>Instruction</tt>s</a> </li>
				128	<li><a href="#schanges_deleting">Deleting <tt>Instruction</tt>s</a> </li>
				129	<li><a href="#schanges_replacing">Replacing an <tt>Instruction</tt>
				130	with another <tt>Value</tt></a> </li>
				131	<li><a href="#schanges_deletingGV">Deleting <tt>GlobalVariable</tt>s</a> </li>
				132	</ul>
				133	</li>
Jeffrey Yasskin	7ebb6ac	2009-04-30 22:33:41 +0000	[diff] [blame]	134	<li><a href="#create_types">How to Create Types</a></li>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	135	<!--
				136	<li>Working with the Control Flow Graph
				137	<ul>
				138	<li>Accessing predecessors and successors of a <tt>BasicBlock</tt>
				139	<li>
				140	<li>
				141	</ul>
				142	-->
				143	</ul>
				144	</li>
				145
Owen Anderson	e8370c0	2009-06-16 01:17:16 +0000	[diff] [blame]	146	<li><a href="#threading">Threads and LLVM</a>
				147	<ul>
Owen Anderson	b418690	2009-06-16 18:04:19 +0000	[diff] [blame]	148	<li><a href="#startmultithreaded">Entering and Exiting Multithreaded Mode
				149	</a></li>
Owen Anderson	e8370c0	2009-06-16 01:17:16 +0000	[diff] [blame]	150	<li><a href="#shutdown">Ending execution with <tt>llvm_shutdown()</tt></a></li>
				151	<li><a href="#managedstatic">Lazy initialization with <tt>ManagedStatic</tt></a></li>
Owen Anderson	3f2da0e	2009-08-19 17:58:52 +0000	[diff] [blame]	152	<li><a href="#llvmcontext">Achieving Isolation with <tt>LLVMContext</tt></a></li>
Jeffrey Yasskin	4071514	2010-01-29 19:10:38 +0000	[diff] [blame]	153	<li><a href="#jitthreading">Threads and the JIT</a></li>
Owen Anderson	e8370c0	2009-06-16 01:17:16 +0000	[diff] [blame]	154	</ul>
				155	</li>
				156
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	157	<li><a href="#advanced">Advanced Topics</a>
				158	<ul>
				159	<li><a href="#TypeResolve">LLVM Type Resolution</a>
				160	<ul>
				161	<li><a href="#BuildRecType">Basic Recursive Type Construction</a></li>
				162	<li><a href="#refineAbstractTypeTo">The <tt>refineAbstractTypeTo</tt> method</a></li>
				163	<li><a href="#PATypeHolder">The PATypeHolder Class</a></li>
				164	<li><a href="#AbstractTypeUser">The AbstractTypeUser Class</a></li>
				165	</ul></li>
				166
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	167	<li><a href="#SymbolTable">The <tt>ValueSymbolTable</tt> and <tt>TypeSymbolTable</tt> classes</a></li>
				168	<li><a href="#UserLayout">The <tt>User</tt> and owned <tt>Use</tt> classes' memory layout</a></li>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	169	</ul></li>
				170
				171	<li><a href="#coreclasses">The Core LLVM Class Hierarchy Reference</a>
				172	<ul>
				173	<li><a href="#Type">The <tt>Type</tt> class</a> </li>
				174	<li><a href="#Module">The <tt>Module</tt> class</a></li>
				175	<li><a href="#Value">The <tt>Value</tt> class</a>
				176	<ul>
				177	<li><a href="#User">The <tt>User</tt> class</a>
				178	<ul>
				179	<li><a href="#Instruction">The <tt>Instruction</tt> class</a></li>
				180	<li><a href="#Constant">The <tt>Constant</tt> class</a>
				181	<ul>
				182	<li><a href="#GlobalValue">The <tt>GlobalValue</tt> class</a>
				183	<ul>
				184	<li><a href="#Function">The <tt>Function</tt> class</a></li>
				185	<li><a href="#GlobalVariable">The <tt>GlobalVariable</tt> class</a></li>
				186	</ul>
				187	</li>
				188	</ul>
				189	</li>
				190	</ul>
				191	</li>
				192	<li><a href="#BasicBlock">The <tt>BasicBlock</tt> class</a></li>
				193	<li><a href="#Argument">The <tt>Argument</tt> class</a></li>
				194	</ul>
				195	</li>
				196	</ul>
				197	</li>
				198	</ol>
				199
				200	<div class="doc_author">
				201	<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>,
				202	<a href="mailto:dhurjati@cs.uiuc.edu">Dinakar Dhurjati</a>,
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	203	<a href="mailto:ggreif@gmail.com">Gabor Greif</a>,
Owen Anderson	e8370c0	2009-06-16 01:17:16 +0000	[diff] [blame]	204	<a href="mailto:jstanley@cs.uiuc.edu">Joel Stanley</a>,
				205	<a href="mailto:rspencer@x10sys.com">Reid Spencer</a> and
				206	<a href="mailto:owen@apple.com">Owen Anderson</a></p>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	207	</div>
				208
				209	<!-- *********************************************************************** -->
				210	<div class="doc_section">
				211	<a name="introduction">Introduction </a>
				212	</div>
				213	<!-- *********************************************************************** -->
				214
				215	<div class="doc_text">
				216
				217	<p>This document is meant to highlight some of the important classes and
				218	interfaces available in the LLVM source-base. This manual is not
				219	intended to explain what LLVM is, how it works, and what LLVM code looks
				220	like. It assumes that you know the basics of LLVM and are interested
				221	in writing transformations or otherwise analyzing or manipulating the
				222	code.</p>
				223
				224	<p>This document should get you oriented so that you can find your
				225	way in the continuously growing source code that makes up the LLVM
				226	infrastructure. Note that this manual is not intended to serve as a
				227	replacement for reading the source code, so if you think there should be
				228	a method in one of these classes to do something, but it's not listed,
				229	check the source. Links to the <a href="/doxygen/">doxygen</a> sources
				230	are provided to make this as easy as possible.</p>
				231
				232	<p>The first section of this document describes general information that is
				233	useful to know when working in the LLVM infrastructure, and the second describes
				234	the Core LLVM classes. In the future this manual will be extended with
				235	information describing how to use extension libraries, such as dominator
				236	information, CFG traversal routines, and useful utilities like the <tt><a
				237	href="/doxygen/InstVisitor_8h-source.html">InstVisitor</a></tt> template.</p>
				238
				239	</div>
				240
				241	<!-- *********************************************************************** -->
				242	<div class="doc_section">
				243	<a name="general">General Information</a>
				244	</div>
				245	<!-- *********************************************************************** -->
				246
				247	<div class="doc_text">
				248
				249	<p>This section contains general information that is useful if you are working
				250	in the LLVM source-base, but that isn't specific to any particular API.</p>
				251
				252	</div>
				253
				254	<!-- ======================================================================= -->
				255	<div class="doc_subsection">
				256	<a name="stl">The C++ Standard Template Library</a>
				257	</div>
				258
				259	<div class="doc_text">
				260
				261	<p>LLVM makes heavy use of the C++ Standard Template Library (STL),
				262	perhaps much more than you are used to, or have seen before. Because of
				263	this, you might want to do a little background reading in the
				264	techniques used and capabilities of the library. There are many good
				265	pages that discuss the STL, and several books on the subject that you
				266	can get, so it will not be discussed in this document.</p>
				267
				268	<p>Here are some useful links:</p>
				269
				270	<ol>
				271
				272	<li><a href="http://www.dinkumware.com/refxcpp.html">Dinkumware C++ Library
				273	reference</a> - an excellent reference for the STL and other parts of the
				274	standard C++ library.</li>
				275
				276	<li><a href="http://www.tempest-sw.com/cpp/">C++ In a Nutshell</a> - This is an
Gabor Greif	e39f0e7	2009-03-12 09:47:03 +0000	[diff] [blame]	277	O'Reilly book in the making. It has a decent Standard Library
				278	Reference that rivals Dinkumware's, and is unfortunately no longer free since the
				279	book has been published.</li>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	280
				281	<li><a href="http://www.parashift.com/c++-faq-lite/">C++ Frequently Asked
				282	Questions</a></li>
				283
				284	<li><a href="http://www.sgi.com/tech/stl/">SGI's STL Programmer's Guide</a> -
				285	Contains a useful <a
				286	href="http://www.sgi.com/tech/stl/stl_introduction.html">Introduction to the
				287	STL</a>.</li>
				288
				289	<li><a href="http://www.research.att.com/%7Ebs/C++.html">Bjarne Stroustrup's C++
				290	Page</a></li>
				291
				292	<li><a href="http://64.78.49.204/">
				293	Bruce Eckel's Thinking in C++, 2nd ed. Volume 2 Revision 4.0 (even better, get
				294	the book).</a></li>
				295
				296	</ol>
				297
				298	<p>You are also encouraged to take a look at the <a
				299	href="CodingStandards.html">LLVM Coding Standards</a> guide which focuses on how
				300	to write maintainable code more than where to put your curly braces.</p>
				301
				302	</div>
				303
				304	<!-- ======================================================================= -->
				305	<div class="doc_subsection">
				306	<a name="stl">Other useful references</a>
				307	</div>
				308
				309	<div class="doc_text">
				310
				311	<ol>
				312	<li><a href="http://www.psc.edu/%7Esemke/cvs_branches.html">CVS
				313	Branch and Tag Primer</a></li>
				314	<li><a href="http://www.fortran-2000.com/ArnaudRecipes/sharedlib.html">Using
				315	static and shared libraries across platforms</a></li>
				316	</ol>
				317
				318	</div>
				319
				320	<!-- *********************************************************************** -->
				321	<div class="doc_section">
				322	<a name="apis">Important and useful LLVM APIs</a>
				323	</div>
				324	<!-- *********************************************************************** -->
				325
				326	<div class="doc_text">
				327
				328	<p>Here we highlight some LLVM APIs that are generally useful and good to
				329	know about when writing transformations.</p>
				330
				331	</div>
				332
				333	<!-- ======================================================================= -->
				334	<div class="doc_subsection">
				335	<a name="isa">The <tt>isa<></tt>, <tt>cast<></tt> and
				336	<tt>dyn_cast<></tt> templates</a>
				337	</div>
				338
				339	<div class="doc_text">
				340
				341	<p>The LLVM source-base makes extensive use of a custom form of RTTI.
				342	These templates have many similarities to the C++ <tt>dynamic_cast<></tt>
				343	operator, but they don't have some drawbacks (primarily stemming from
				344	the fact that <tt>dynamic_cast<></tt> only works on classes that
				345	have a v-table). Because they are used so often, you must know what they
				346	do and how they work. All of these templates are defined in the <a
				347	href="/doxygen/Casting_8h-source.html"><tt>llvm/Support/Casting.h</tt></a>
				348	file (note that you very rarely have to include this file directly).</p>
				349
				350	<dl>
				351	<dt><tt>isa<></tt>: </dt>
				352
				353	<dd><p>The <tt>isa<></tt> operator works exactly like the Java
				354	"<tt>instanceof</tt>" operator. It returns true or false depending on whether
				355	a reference or pointer points to an instance of the specified class. This can
				356	be very useful for constraint checking of various sorts (example below).</p>
				357	</dd>
				358
				359	<dt><tt>cast<></tt>: </dt>
				360
				361	<dd><p>The <tt>cast<></tt> operator is a "checked cast" operation. It
Chris Lattner	1d5610a	2008-06-20 05:03:17 +0000	[diff] [blame]	362	converts a pointer or reference from a base class to a derived class, causing
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	363	an assertion failure if it is not really an instance of the right type. This
				364	should be used in cases where you have some information that makes you believe
				365	that something is of the right type. An example of the <tt>isa<></tt>
				366	and <tt>cast<></tt> template is:</p>
				367
				368	<div class="doc_code">
				369	<pre>
				370	static bool isLoopInvariant(const <a href="#Value">Value</a> V, const Loop L) {
				371	if (isa<<a href="#Constant">Constant</a>>(V) \|\| isa<<a href="#Argument">Argument</a>>(V) \|\| isa<<a href="#GlobalValue">GlobalValue</a>>(V))
				372	return true;
				373
				374	// <i>Otherwise, it must be an instruction...</i>
				375	return !L->contains(cast<<a href="#Instruction">Instruction</a>>(V)->getParent());
				376	}
				377	</pre>
				378	</div>
				379
				380	<p>Note that you should <b>not</b> use an <tt>isa<></tt> test followed
				381	by a <tt>cast<></tt>, for that use the <tt>dyn_cast<></tt>
				382	operator.</p>
				383
				384	</dd>
				385
				386	<dt><tt>dyn_cast<></tt>:</dt>
				387
				388	<dd><p>The <tt>dyn_cast<></tt> operator is a "checking cast" operation.
				389	It checks to see if the operand is of the specified type, and if so, returns a
				390	pointer to it (this operator does not work with references). If the operand is
				391	not of the correct type, a null pointer is returned. Thus, this works very
				392	much like the <tt>dynamic_cast<></tt> operator in C++, and should be
				393	used in the same circumstances. Typically, the <tt>dyn_cast<></tt>
				394	operator is used in an <tt>if</tt> statement or some other flow control
				395	statement like this:</p>
				396
				397	<div class="doc_code">
				398	<pre>
				399	if (<a href="#AllocationInst">AllocationInst</a> *AI = dyn_cast<<a href="#AllocationInst">AllocationInst</a>>(Val)) {
				400	// <i>...</i>
				401	}
				402	</pre>
				403	</div>
				404
				405	<p>This form of the <tt>if</tt> statement effectively combines together a call
				406	to <tt>isa<></tt> and a call to <tt>cast<></tt> into one
				407	statement, which is very convenient.</p>
				408
				409	<p>Note that the <tt>dyn_cast<></tt> operator, like C++'s
				410	<tt>dynamic_cast<></tt> or Java's <tt>instanceof</tt> operator, can be
				411	abused. In particular, you should not use big chained <tt>if/then/else</tt>
				412	blocks to check for lots of different variants of classes. If you find
				413	yourself wanting to do this, it is much cleaner and more efficient to use the
				414	<tt>InstVisitor</tt> class to dispatch over the instruction type directly.</p>
				415
				416	</dd>
				417
				418	<dt><tt>cast_or_null<></tt>: </dt>
				419
				420	<dd><p>The <tt>cast_or_null<></tt> operator works just like the
				421	<tt>cast<></tt> operator, except that it allows for a null pointer as an
				422	argument (which it then propagates). This can sometimes be useful, allowing
				423	you to combine several null checks into one.</p></dd>
				424
				425	<dt><tt>dyn_cast_or_null<></tt>: </dt>
				426
				427	<dd><p>The <tt>dyn_cast_or_null<></tt> operator works just like the
				428	<tt>dyn_cast<></tt> operator, except that it allows for a null pointer
				429	as an argument (which it then propagates). This can sometimes be useful,
				430	allowing you to combine several null checks into one.</p></dd>
				431
				432	</dl>
				433
				434	<p>These five templates can be used with any classes, whether they have a
				435	v-table or not. To add support for these templates, you simply need to add
				436	<tt>classof</tt> static methods to the class you are interested casting
				437	to. Describing this is currently outside the scope of this document, but there
				438	are lots of examples in the LLVM source base.</p>
				439
				440	</div>
				441
Daniel Dunbar	e3572ba	2009-07-25 04:41:11 +0000	[diff] [blame]	442
				443	<!-- ======================================================================= -->
				444	<div class="doc_subsection">
				445	<a name="string_apis">Passing strings (the <tt>StringRef</tt>
				446	and <tt>Twine</tt> classes)</a>
				447	</div>
				448
				449	<div class="doc_text">
				450
				451	<p>Although LLVM generally does not do much string manipulation, we do have
Chris Lattner	c00114f	2009-07-25 07:16:59 +0000	[diff] [blame]	452	several important APIs which take strings. Two important examples are the
Daniel Dunbar	e3572ba	2009-07-25 04:41:11 +0000	[diff] [blame]	453	Value class -- which has names for instructions, functions, etc. -- and the
				454	StringMap class which is used extensively in LLVM and Clang.</p>
				455
				456	<p>These are generic classes, and they need to be able to accept strings which
				457	may have embedded null characters. Therefore, they cannot simply take
Chris Lattner	c00114f	2009-07-25 07:16:59 +0000	[diff] [blame]	458	a <tt>const char *</tt>, and taking a <tt>const std::string&</tt> requires
Daniel Dunbar	e3572ba	2009-07-25 04:41:11 +0000	[diff] [blame]	459	clients to perform a heap allocation which is usually unnecessary. Instead,
Chris Lattner	c00114f	2009-07-25 07:16:59 +0000	[diff] [blame]	460	many LLVM APIs use a <tt>const StringRef&</tt> or a <tt>const
				461	Twine&</tt> for passing strings efficiently.</p>
Daniel Dunbar	e3572ba	2009-07-25 04:41:11 +0000	[diff] [blame]	462
				463	</div>
				464
				465	<!-- _______________________________________________________________________ -->
				466	<div class="doc_subsubsection">
				467	<a name="StringRef">The <tt>StringRef</tt> class</a>
				468	</div>
				469
				470	<div class="doc_text">
				471
				472	<p>The <tt>StringRef</tt> data type represents a reference to a constant string
				473	(a character array and a length) and supports the common operations available
				474	on <tt>std:string</tt>, but does not require heap allocation.</p>
				475
Chris Lattner	c00114f	2009-07-25 07:16:59 +0000	[diff] [blame]	476	<p>It can be implicitly constructed using a C style null-terminated string,
				477	an <tt>std::string</tt>, or explicitly with a character pointer and length.
Daniel Dunbar	e3572ba	2009-07-25 04:41:11 +0000	[diff] [blame]	478	For example, the <tt>StringRef</tt> find function is declared as:</p>
Chris Lattner	c00114f	2009-07-25 07:16:59 +0000	[diff] [blame]	479
Daniel Dunbar	e3572ba	2009-07-25 04:41:11 +0000	[diff] [blame]	480	<div class="doc_code">
Chris Lattner	c00114f	2009-07-25 07:16:59 +0000	[diff] [blame]	481	iterator find(const StringRef &Key);
Daniel Dunbar	e3572ba	2009-07-25 04:41:11 +0000	[diff] [blame]	482	</div>
				483
				484	<p>and clients can call it using any one of:</p>
				485
				486	<div class="doc_code">
				487	<pre>
				488	Map.find("foo"); <i>// Lookup "foo"</i>
				489	Map.find(std::string("bar")); <i>// Lookup "bar"</i>
				490	Map.find(StringRef("\0baz", 4)); <i>// Lookup "\0baz"</i>
				491	</pre>
				492	</div>
				493
				494	<p>Similarly, APIs which need to return a string may return a <tt>StringRef</tt>
				495	instance, which can be used directly or converted to an <tt>std::string</tt>
				496	using the <tt>str</tt> member function. See
				497	"<tt><a href="/doxygen/classllvm_1_1StringRef_8h-source.html">llvm/ADT/StringRef.h</a></tt>"
				498	for more information.</p>
				499
				500	<p>You should rarely use the <tt>StringRef</tt> class directly, because it contains
				501	pointers to external memory it is not generally safe to store an instance of the
Chris Lattner	c00114f	2009-07-25 07:16:59 +0000	[diff] [blame]	502	class (unless you know that the external storage will not be freed).</p>
Daniel Dunbar	e3572ba	2009-07-25 04:41:11 +0000	[diff] [blame]	503
				504	</div>
				505
				506	<!-- _______________________________________________________________________ -->
				507	<div class="doc_subsubsection">
				508	<a name="Twine">The <tt>Twine</tt> class</a>
				509	</div>
				510
				511	<div class="doc_text">
				512
				513	<p>The <tt>Twine</tt> class is an efficient way for APIs to accept concatenated
				514	strings. For example, a common LLVM paradigm is to name one instruction based on
				515	the name of another instruction with a suffix, for example:</p>
				516
				517	<div class="doc_code">
				518	<pre>
				519	New = CmpInst::Create(<i>...</i>, SO->getName() + ".cmp");
				520	</pre>
				521	</div>
				522
				523	<p>The <tt>Twine</tt> class is effectively a
				524	lightweight <a href="http://en.wikipedia.org/wiki/Rope_(computer_science)">rope</a>
				525	which points to temporary (stack allocated) objects. Twines can be implicitly
				526	constructed as the result of the plus operator applied to strings (i.e., a C
				527	strings, an <tt>std::string</tt>, or a <tt>StringRef</tt>). The twine delays the
Dan Gohman	b1d8d90	2010-02-25 23:51:27 +0000	[diff] [blame]	528	actual concatenation of strings until it is actually required, at which point
Daniel Dunbar	e3572ba	2009-07-25 04:41:11 +0000	[diff] [blame]	529	it can be efficiently rendered directly into a character array. This avoids
				530	unnecessary heap allocation involved in constructing the temporary results of
				531	string concatenation. See
				532	"<tt><a href="/doxygen/classllvm_1_1Twine_8h-source.html">llvm/ADT/Twine.h</a></tt>"
Benjamin Kramer	7b2136d	2009-08-05 15:42:44 +0000	[diff] [blame]	533	for more information.</p>
Daniel Dunbar	e3572ba	2009-07-25 04:41:11 +0000	[diff] [blame]	534
				535	<p>As with a <tt>StringRef</tt>, <tt>Twine</tt> objects point to external memory
				536	and should almost never be stored or mentioned directly. They are intended
				537	solely for use when defining a function which should be able to efficiently
				538	accept concatenated strings.</p>
				539
				540	</div>
				541
				542
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	543	<!-- ======================================================================= -->
				544	<div class="doc_subsection">
				545	<a name="DEBUG">The <tt>DEBUG()</tt> macro and <tt>-debug</tt> option</a>
				546	</div>
				547
				548	<div class="doc_text">
				549
				550	<p>Often when working on your pass you will put a bunch of debugging printouts
				551	and other code into your pass. After you get it working, you want to remove
				552	it, but you may need it again in the future (to work out new bugs that you run
				553	across).</p>
				554
				555	<p> Naturally, because of this, you don't want to delete the debug printouts,
				556	but you don't want them to always be noisy. A standard compromise is to comment
				557	them out, allowing you to enable them if you need them in the future.</p>
				558
				559	<p>The "<tt><a href="/doxygen/Debug_8h-source.html">llvm/Support/Debug.h</a></tt>"
				560	file provides a macro named <tt>DEBUG()</tt> that is a much nicer solution to
				561	this problem. Basically, you can put arbitrary code into the argument of the
				562	<tt>DEBUG</tt> macro, and it is only executed if '<tt>opt</tt>' (or any other
				563	tool) is run with the '<tt>-debug</tt>' command line argument:</p>
				564
				565	<div class="doc_code">
				566	<pre>
Daniel Dunbar	e32f9b2	2009-07-25 01:55:32 +0000	[diff] [blame]	567	DEBUG(errs() << "I am here!\n");
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	568	</pre>
				569	</div>
				570
				571	<p>Then you can run your pass like this:</p>
				572
				573	<div class="doc_code">
				574	<pre>
				575	$ opt < a.bc > /dev/null -mypass
				576	<i><no output></i>
				577	$ opt < a.bc > /dev/null -mypass -debug
				578	I am here!
				579	</pre>
				580	</div>
				581
				582	<p>Using the <tt>DEBUG()</tt> macro instead of a home-brewed solution allows you
				583	to not have to create "yet another" command line option for the debug output for
				584	your pass. Note that <tt>DEBUG()</tt> macros are disabled for optimized builds,
				585	so they do not cause a performance impact at all (for the same reason, they
				586	should also not contain side-effects!).</p>
				587
				588	<p>One additional nice thing about the <tt>DEBUG()</tt> macro is that you can
				589	enable or disable it directly in gdb. Just use "<tt>set DebugFlag=0</tt>" or
				590	"<tt>set DebugFlag=1</tt>" from the gdb if the program is running. If the
				591	program hasn't been started yet, you can always just run it with
				592	<tt>-debug</tt>.</p>
				593
				594	</div>
				595
				596	<!-- _______________________________________________________________________ -->
				597	<div class="doc_subsubsection">
				598	<a name="DEBUG_TYPE">Fine grained debug info with <tt>DEBUG_TYPE</tt> and
				599	the <tt>-debug-only</tt> option</a>
				600	</div>
				601
				602	<div class="doc_text">
				603
				604	<p>Sometimes you may find yourself in a situation where enabling <tt>-debug</tt>
				605	just turns on <b>too much</b> information (such as when working on the code
				606	generator). If you want to enable debug information with more fine-grained
				607	control, you define the <tt>DEBUG_TYPE</tt> macro and the <tt>-debug</tt> only
				608	option as follows:</p>
				609
				610	<div class="doc_code">
				611	<pre>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	612	#undef DEBUG_TYPE
Daniel Dunbar	e32f9b2	2009-07-25 01:55:32 +0000	[diff] [blame]	613	DEBUG(errs() << "No debug type\n");
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	614	#define DEBUG_TYPE "foo"
Daniel Dunbar	e32f9b2	2009-07-25 01:55:32 +0000	[diff] [blame]	615	DEBUG(errs() << "'foo' debug type\n");
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	616	#undef DEBUG_TYPE
				617	#define DEBUG_TYPE "bar"
Daniel Dunbar	e32f9b2	2009-07-25 01:55:32 +0000	[diff] [blame]	618	DEBUG(errs() << "'bar' debug type\n"));
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	619	#undef DEBUG_TYPE
				620	#define DEBUG_TYPE ""
Daniel Dunbar	e32f9b2	2009-07-25 01:55:32 +0000	[diff] [blame]	621	DEBUG(errs() << "No debug type (2)\n");
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	622	</pre>
				623	</div>
				624
				625	<p>Then you can run your pass like this:</p>
				626
				627	<div class="doc_code">
				628	<pre>
				629	$ opt < a.bc > /dev/null -mypass
				630	<i><no output></i>
				631	$ opt < a.bc > /dev/null -mypass -debug
				632	No debug type
				633	'foo' debug type
				634	'bar' debug type
				635	No debug type (2)
				636	$ opt < a.bc > /dev/null -mypass -debug-only=foo
				637	'foo' debug type
				638	$ opt < a.bc > /dev/null -mypass -debug-only=bar
				639	'bar' debug type
				640	</pre>
				641	</div>
				642
				643	<p>Of course, in practice, you should only set <tt>DEBUG_TYPE</tt> at the top of
				644	a file, to specify the debug type for the entire module (if you do this before
				645	you <tt>#include "llvm/Support/Debug.h"</tt>, you don't have to insert the ugly
				646	<tt>#undef</tt>'s). Also, you should use names more meaningful than "foo" and
				647	"bar", because there is no system in place to ensure that names do not
				648	conflict. If two different modules use the same string, they will all be turned
				649	on when the name is specified. This allows, for example, all debug information
				650	for instruction scheduling to be enabled with <tt>-debug-type=InstrSched</tt>,
				651	even if the source lives in multiple files.</p>
				652
Daniel Dunbar	8350419	2009-08-07 23:48:59 +0000	[diff] [blame]	653	<p>The <tt>DEBUG_WITH_TYPE</tt> macro is also available for situations where you
				654	would like to set <tt>DEBUG_TYPE</tt>, but only for one specific <tt>DEBUG</tt>
				655	statement. It takes an additional first parameter, which is the type to use. For
Benjamin Kramer	5fb9d7e	2009-10-12 14:46:08 +0000	[diff] [blame]	656	example, the preceding example could be written as:</p>
Daniel Dunbar	8350419	2009-08-07 23:48:59 +0000	[diff] [blame]	657
				658
				659	<div class="doc_code">
				660	<pre>
				661	DEBUG_WITH_TYPE("", errs() << "No debug type\n");
				662	DEBUG_WITH_TYPE("foo", errs() << "'foo' debug type\n");
				663	DEBUG_WITH_TYPE("bar", errs() << "'bar' debug type\n"));
				664	DEBUG_WITH_TYPE("", errs() << "No debug type (2)\n");
				665	</pre>
				666	</div>
				667
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	668	</div>
				669
				670	<!-- ======================================================================= -->
				671	<div class="doc_subsection">
				672	<a name="Statistic">The <tt>Statistic</tt> class & <tt>-stats</tt>
				673	option</a>
				674	</div>
				675
				676	<div class="doc_text">
				677
				678	<p>The "<tt><a
				679	href="/doxygen/Statistic_8h-source.html">llvm/ADT/Statistic.h</a></tt>" file
				680	provides a class named <tt>Statistic</tt> that is used as a unified way to
				681	keep track of what the LLVM compiler is doing and how effective various
				682	optimizations are. It is useful to see what optimizations are contributing to
				683	making a particular program run faster.</p>
				684
				685	<p>Often you may run your pass on some big program, and you're interested to see
				686	how many times it makes a certain transformation. Although you can do this with
				687	hand inspection, or some ad-hoc method, this is a real pain and not very useful
				688	for big programs. Using the <tt>Statistic</tt> class makes it very easy to
				689	keep track of this information, and the calculated information is presented in a
				690	uniform manner with the rest of the passes being executed.</p>
				691
				692	<p>There are many examples of <tt>Statistic</tt> uses, but the basics of using
				693	it are as follows:</p>
				694
				695	<ol>
				696	<li><p>Define your statistic like this:</p>
				697
				698	<div class="doc_code">
				699	<pre>
				700	#define <a href="#DEBUG_TYPE">DEBUG_TYPE</a> "mypassname" <i>// This goes before any #includes.</i>
				701	STATISTIC(NumXForms, "The # of times I did stuff");
				702	</pre>
				703	</div>
				704
				705	<p>The <tt>STATISTIC</tt> macro defines a static variable, whose name is
				706	specified by the first argument. The pass name is taken from the DEBUG_TYPE
				707	macro, and the description is taken from the second argument. The variable
				708	defined ("NumXForms" in this case) acts like an unsigned integer.</p></li>
				709
				710	<li><p>Whenever you make a transformation, bump the counter:</p>
				711
				712	<div class="doc_code">
				713	<pre>
				714	++NumXForms; // <i>I did stuff!</i>
				715	</pre>
				716	</div>
				717
				718	</li>
				719	</ol>
				720
				721	<p>That's all you have to do. To get '<tt>opt</tt>' to print out the
				722	statistics gathered, use the '<tt>-stats</tt>' option:</p>
				723
				724	<div class="doc_code">
				725	<pre>
				726	$ opt -stats -mypassname < program.bc > /dev/null
				727	<i>... statistics output ...</i>
				728	</pre>
				729	</div>
				730
				731	<p> When running <tt>opt</tt> on a C file from the SPEC benchmark
				732	suite, it gives a report that looks like this:</p>
				733
				734	<div class="doc_code">
				735	<pre>
				736	7646 bitcodewriter - Number of normal instructions
				737	725 bitcodewriter - Number of oversized instructions
				738	129996 bitcodewriter - Number of bitcode bytes written
				739	2817 raise - Number of insts DCEd or constprop'd
				740	3213 raise - Number of cast-of-self removed
				741	5046 raise - Number of expression trees converted
				742	75 raise - Number of other getelementptr's formed
				743	138 raise - Number of load/store peepholes
				744	42 deadtypeelim - Number of unused typenames removed from symtab
				745	392 funcresolve - Number of varargs functions resolved
				746	27 globaldce - Number of global variables removed
				747	2 adce - Number of basic blocks removed
				748	134 cee - Number of branches revectored
				749	49 cee - Number of setcc instruction eliminated
				750	532 gcse - Number of loads removed
				751	2919 gcse - Number of instructions removed
				752	86 indvars - Number of canonical indvars added
				753	87 indvars - Number of aux indvars removed
				754	25 instcombine - Number of dead inst eliminate
				755	434 instcombine - Number of insts combined
				756	248 licm - Number of load insts hoisted
				757	1298 licm - Number of insts hoisted to a loop pre-header
				758	3 licm - Number of insts hoisted to multiple loop preds (bad, no loop pre-header)
				759	75 mem2reg - Number of alloca's promoted
				760	1444 cfgsimplify - Number of blocks simplified
				761	</pre>
				762	</div>
				763
				764	<p>Obviously, with so many optimizations, having a unified framework for this
				765	stuff is very nice. Making your pass fit well into the framework makes it more
				766	maintainable and useful.</p>
				767
				768	</div>
				769
				770	<!-- ======================================================================= -->
				771	<div class="doc_subsection">
				772	<a name="ViewGraph">Viewing graphs while debugging code</a>
				773	</div>
				774
				775	<div class="doc_text">
				776
				777	<p>Several of the important data structures in LLVM are graphs: for example
				778	CFGs made out of LLVM <a href="#BasicBlock">BasicBlock</a>s, CFGs made out of
				779	LLVM <a href="CodeGenerator.html#machinebasicblock">MachineBasicBlock</a>s, and
				780	<a href="CodeGenerator.html#selectiondag_intro">Instruction Selection
				781	DAGs</a>. In many cases, while debugging various parts of the compiler, it is
				782	nice to instantly visualize these graphs.</p>
				783
				784	<p>LLVM provides several callbacks that are available in a debug build to do
				785	exactly that. If you call the <tt>Function::viewCFG()</tt> method, for example,
				786	the current LLVM tool will pop up a window containing the CFG for the function
				787	where each basic block is a node in the graph, and each node contains the
				788	instructions in the block. Similarly, there also exists
				789	<tt>Function::viewCFGOnly()</tt> (does not include the instructions), the
				790	<tt>MachineFunction::viewCFG()</tt> and <tt>MachineFunction::viewCFGOnly()</tt>,
				791	and the <tt>SelectionDAG::viewGraph()</tt> methods. Within GDB, for example,
				792	you can usually use something like <tt>call DAG.viewGraph()</tt> to pop
				793	up a window. Alternatively, you can sprinkle calls to these functions in your
				794	code in places you want to debug.</p>
				795
				796	<p>Getting this to work requires a small amount of configuration. On Unix
				797	systems with X11, install the <a href="http://www.graphviz.org">graphviz</a>
				798	toolkit, and make sure 'dot' and 'gv' are in your path. If you are running on
				799	Mac OS/X, download and install the Mac OS/X <a
				800	href="http://www.pixelglow.com/graphviz/">Graphviz program</a>, and add
				801	<tt>/Applications/Graphviz.app/Contents/MacOS/</tt> (or wherever you install
				802	it) to your path. Once in your system and path are set up, rerun the LLVM
				803	configure script and rebuild LLVM to enable this functionality.</p>
				804
				805	<p><tt>SelectionDAG</tt> has been extended to make it easier to locate
				806	<i>interesting</i> nodes in large complex graphs. From gdb, if you
				807	<tt>call DAG.setGraphColor(<i>node</i>, "<i>color</i>")</tt>, then the
				808	next <tt>call DAG.viewGraph()</tt> would highlight the node in the
				809	specified color (choices of colors can be found at <a
				810	href="http://www.graphviz.org/doc/info/colors.html">colors</a>.) More
				811	complex node attributes can be provided with <tt>call
				812	DAG.setGraphAttrs(<i>node</i>, "<i>attributes</i>")</tt> (choices can be
				813	found at <a href="http://www.graphviz.org/doc/info/attrs.html">Graph
				814	Attributes</a>.) If you want to restart and clear all the current graph
				815	attributes, then you can <tt>call DAG.clearGraphAttrs()</tt>. </p>
				816
				817	</div>
				818
				819	<!-- *********************************************************************** -->
				820	<div class="doc_section">
				821	<a name="datastructure">Picking the Right Data Structure for a Task</a>
				822	</div>
				823	<!-- *********************************************************************** -->
				824
				825	<div class="doc_text">
				826
				827	<p>LLVM has a plethora of data structures in the <tt>llvm/ADT/</tt> directory,
				828	and we commonly use STL data structures. This section describes the trade-offs
				829	you should consider when you pick one.</p>
				830
				831	<p>
				832	The first step is a choose your own adventure: do you want a sequential
				833	container, a set-like container, or a map-like container? The most important
				834	thing when choosing a container is the algorithmic properties of how you plan to
				835	access the container. Based on that, you should use:</p>
				836
				837	<ul>
				838	<li>a <a href="#ds_map">map-like</a> container if you need efficient look-up
				839	of an value based on another value. Map-like containers also support
				840	efficient queries for containment (whether a key is in the map). Map-like
				841	containers generally do not support efficient reverse mapping (values to
				842	keys). If you need that, use two maps. Some map-like containers also
				843	support efficient iteration through the keys in sorted order. Map-like
				844	containers are the most expensive sort, only use them if you need one of
				845	these capabilities.</li>
				846
				847	<li>a <a href="#ds_set">set-like</a> container if you need to put a bunch of
				848	stuff into a container that automatically eliminates duplicates. Some
				849	set-like containers support efficient iteration through the elements in
				850	sorted order. Set-like containers are more expensive than sequential
				851	containers.
				852	</li>
				853
				854	<li>a <a href="#ds_sequential">sequential</a> container provides
				855	the most efficient way to add elements and keeps track of the order they are
				856	added to the collection. They permit duplicates and support efficient
				857	iteration, but do not support efficient look-up based on a key.
				858	</li>
				859
Chris Lattner	d8b95b7	2009-07-25 07:22:20 +0000	[diff] [blame]	860	<li>a <a href="#ds_string">string</a> container is a specialized sequential
				861	container or reference structure that is used for character or byte
				862	arrays.</li>
				863
Daniel Berlin	7ea44dc	2007-09-24 17:52:25 +0000	[diff] [blame]	864	<li>a <a href="#ds_bit">bit</a> container provides an efficient way to store and
				865	perform set operations on sets of numeric id's, while automatically
				866	eliminating duplicates. Bit containers require a maximum of 1 bit for each
				867	identifier you want to store.
				868	</li>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	869	</ul>
				870
				871	<p>
				872	Once the proper category of container is determined, you can fine tune the
				873	memory use, constant factors, and cache behaviors of access by intelligently
				874	picking a member of the category. Note that constant factors and cache behavior
				875	can be a big deal. If you have a vector that usually only contains a few
				876	elements (but could contain many), for example, it's much better to use
				877	<a href="#dss_smallvector">SmallVector</a> than <a href="#dss_vector">vector</a>
				878	. Doing so avoids (relatively) expensive malloc/free calls, which dwarf the
				879	cost of adding the elements to the container. </p>
				880
				881	</div>
				882
				883	<!-- ======================================================================= -->
				884	<div class="doc_subsection">
				885	<a name="ds_sequential">Sequential Containers (std::vector, std::list, etc)</a>
				886	</div>
				887
				888	<div class="doc_text">
				889	There are a variety of sequential containers available for you, based on your
				890	needs. Pick the first in this section that will do what you want.
				891	</div>
				892
				893	<!-- _______________________________________________________________________ -->
				894	<div class="doc_subsubsection">
				895	<a name="dss_fixedarrays">Fixed Size Arrays</a>
				896	</div>
				897
				898	<div class="doc_text">
				899	<p>Fixed size arrays are very simple and very fast. They are good if you know
				900	exactly how many elements you have, or you have a (low) upper bound on how many
				901	you have.</p>
				902	</div>
				903
				904	<!-- _______________________________________________________________________ -->
				905	<div class="doc_subsubsection">
				906	<a name="dss_heaparrays">Heap Allocated Arrays</a>
				907	</div>
				908
				909	<div class="doc_text">
				910	<p>Heap allocated arrays (new[] + delete[]) are also simple. They are good if
				911	the number of elements is variable, if you know how many elements you will need
				912	before the array is allocated, and if the array is usually large (if not,
				913	consider a <a href="#dss_smallvector">SmallVector</a>). The cost of a heap
				914	allocated array is the cost of the new/delete (aka malloc/free). Also note that
				915	if you are allocating an array of a type with a constructor, the constructor and
				916	destructors will be run for every element in the array (re-sizable vectors only
				917	construct those elements actually used).</p>
				918	</div>
				919
				920	<!-- _______________________________________________________________________ -->
				921	<div class="doc_subsubsection">
				922	<a name="dss_smallvector">"llvm/ADT/SmallVector.h"</a>
				923	</div>
				924
				925	<div class="doc_text">
				926	<p><tt>SmallVector<Type, N></tt> is a simple class that looks and smells
				927	just like <tt>vector<Type></tt>:
				928	it supports efficient iteration, lays out elements in memory order (so you can
				929	do pointer arithmetic between elements), supports efficient push_back/pop_back
				930	operations, supports efficient random access to its elements, etc.</p>
				931
				932	<p>The advantage of SmallVector is that it allocates space for
				933	some number of elements (N) <b>in the object itself</b>. Because of this, if
				934	the SmallVector is dynamically smaller than N, no malloc is performed. This can
				935	be a big win in cases where the malloc/free call is far more expensive than the
				936	code that fiddles around with the elements.</p>
				937
				938	<p>This is good for vectors that are "usually small" (e.g. the number of
				939	predecessors/successors of a block is usually less than 8). On the other hand,
				940	this makes the size of the SmallVector itself large, so you don't want to
				941	allocate lots of them (doing so will waste a lot of space). As such,
				942	SmallVectors are most useful when on the stack.</p>
				943
				944	<p>SmallVector also provides a nice portable and efficient replacement for
				945	<tt>alloca</tt>.</p>
				946
				947	</div>
				948
				949	<!-- _______________________________________________________________________ -->
				950	<div class="doc_subsubsection">
				951	<a name="dss_vector"><vector></a>
				952	</div>
				953
				954	<div class="doc_text">
				955	<p>
				956	std::vector is well loved and respected. It is useful when SmallVector isn't:
				957	when the size of the vector is often large (thus the small optimization will
				958	rarely be a benefit) or if you will be allocating many instances of the vector
				959	itself (which would waste space for elements that aren't in the container).
				960	vector is also useful when interfacing with code that expects vectors :).
				961	</p>
				962
				963	<p>One worthwhile note about std::vector: avoid code like this:</p>
				964
				965	<div class="doc_code">
				966	<pre>
				967	for ( ... ) {
				968	std::vector<foo> V;
				969	use V;
				970	}
				971	</pre>
				972	</div>
				973
				974	<p>Instead, write this as:</p>
				975
				976	<div class="doc_code">
				977	<pre>
				978	std::vector<foo> V;
				979	for ( ... ) {
				980	use V;
				981	V.clear();
				982	}
				983	</pre>
				984	</div>
				985
				986	<p>Doing so will save (at least) one heap allocation and free per iteration of
				987	the loop.</p>
				988
				989	</div>
				990
				991	<!-- _______________________________________________________________________ -->
				992	<div class="doc_subsubsection">
				993	<a name="dss_deque"><deque></a>
				994	</div>
				995
				996	<div class="doc_text">
				997	<p>std::deque is, in some senses, a generalized version of std::vector. Like
				998	std::vector, it provides constant time random access and other similar
				999	properties, but it also provides efficient access to the front of the list. It
				1000	does not guarantee continuity of elements within memory.</p>
				1001
				1002	<p>In exchange for this extra flexibility, std::deque has significantly higher
				1003	constant factor costs than std::vector. If possible, use std::vector or
				1004	something cheaper.</p>
				1005	</div>
				1006
				1007	<!-- _______________________________________________________________________ -->
				1008	<div class="doc_subsubsection">
				1009	<a name="dss_list"><list></a>
				1010	</div>
				1011
				1012	<div class="doc_text">
				1013	<p>std::list is an extremely inefficient class that is rarely useful.
				1014	It performs a heap allocation for every element inserted into it, thus having an
				1015	extremely high constant factor, particularly for small data types. std::list
				1016	also only supports bidirectional iteration, not random access iteration.</p>
				1017
				1018	<p>In exchange for this high cost, std::list supports efficient access to both
				1019	ends of the list (like std::deque, but unlike std::vector or SmallVector). In
				1020	addition, the iterator invalidation characteristics of std::list are stronger
				1021	than that of a vector class: inserting or removing an element into the list does
				1022	not invalidate iterator or pointers to other elements in the list.</p>
				1023	</div>
				1024
				1025	<!-- _______________________________________________________________________ -->
				1026	<div class="doc_subsubsection">
Gabor Greif	bb17f65	2009-02-27 11:37:41 +0000	[diff] [blame]	1027	<a name="dss_ilist">llvm/ADT/ilist.h</a>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1028	</div>
				1029
				1030	<div class="doc_text">
				1031	<p><tt>ilist<T></tt> implements an 'intrusive' doubly-linked list. It is
				1032	intrusive, because it requires the element to store and provide access to the
				1033	prev/next pointers for the list.</p>
				1034
Gabor Greif	b6b21ec	2009-02-27 12:02:19 +0000	[diff] [blame]	1035	<p><tt>ilist</tt> has the same drawbacks as <tt>std::list</tt>, and additionally
				1036	requires an <tt>ilist_traits</tt> implementation for the element type, but it
				1037	provides some novel characteristics. In particular, it can efficiently store
				1038	polymorphic objects, the traits class is informed when an element is inserted or
Gabor Greif	e39f0e7	2009-03-12 09:47:03 +0000	[diff] [blame]	1039	removed from the list, and <tt>ilist</tt>s are guaranteed to support a
				1040	constant-time splice operation.</p>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1041
Gabor Greif	e39f0e7	2009-03-12 09:47:03 +0000	[diff] [blame]	1042	<p>These properties are exactly what we want for things like
				1043	<tt>Instruction</tt>s and basic blocks, which is why these are implemented with
				1044	<tt>ilist</tt>s.</p>
Gabor Greif	bb17f65	2009-02-27 11:37:41 +0000	[diff] [blame]	1045
				1046	Related classes of interest are explained in the following subsections:
				1047	<ul>
Gabor Greif	a17abe1	2009-02-27 13:28:07 +0000	[diff] [blame]	1048	<li><a href="#dss_ilist_traits">ilist_traits</a></li>
Gabor Greif	b6b21ec	2009-02-27 12:02:19 +0000	[diff] [blame]	1049	<li><a href="#dss_iplist">iplist</a></li>
Gabor Greif	bb17f65	2009-02-27 11:37:41 +0000	[diff] [blame]	1050	<li><a href="#dss_ilist_node">llvm/ADT/ilist_node.h</a></li>
Gabor Greif	d970d69	2009-03-12 10:30:31 +0000	[diff] [blame]	1051	<li><a href="#dss_ilist_sentinel">Sentinels</a></li>
Gabor Greif	bb17f65	2009-02-27 11:37:41 +0000	[diff] [blame]	1052	</ul>
				1053	</div>
				1054
				1055	<!-- _______________________________________________________________________ -->
				1056	<div class="doc_subsubsection">
Gabor Greif	a17abe1	2009-02-27 13:28:07 +0000	[diff] [blame]	1057	<a name="dss_ilist_traits">ilist_traits</a>
				1058	</div>
				1059
				1060	<div class="doc_text">
				1061	<p><tt>ilist_traits<T></tt> is <tt>ilist<T></tt>'s customization
				1062	mechanism. <tt>iplist<T></tt> (and consequently <tt>ilist<T></tt>)
				1063	publicly derive from this traits class.</p>
				1064	</div>
				1065
				1066	<!-- _______________________________________________________________________ -->
				1067	<div class="doc_subsubsection">
Gabor Greif	b6b21ec	2009-02-27 12:02:19 +0000	[diff] [blame]	1068	<a name="dss_iplist">iplist</a>
				1069	</div>
				1070
				1071	<div class="doc_text">
				1072	<p><tt>iplist<T></tt> is <tt>ilist<T></tt>'s base and as such
Gabor Greif	e39f0e7	2009-03-12 09:47:03 +0000	[diff] [blame]	1073	supports a slightly narrower interface. Notably, inserters from
				1074	<tt>T&</tt> are absent.</p>
Gabor Greif	a17abe1	2009-02-27 13:28:07 +0000	[diff] [blame]	1075
				1076	<p><tt>ilist_traits<T></tt> is a public base of this class and can be
				1077	used for a wide variety of customizations.</p>
Gabor Greif	b6b21ec	2009-02-27 12:02:19 +0000	[diff] [blame]	1078	</div>
				1079
				1080	<!-- _______________________________________________________________________ -->
				1081	<div class="doc_subsubsection">
Gabor Greif	bb17f65	2009-02-27 11:37:41 +0000	[diff] [blame]	1082	<a name="dss_ilist_node">llvm/ADT/ilist_node.h</a>
				1083	</div>
				1084
				1085	<div class="doc_text">
				1086	<p><tt>ilist_node<T></tt> implements a the forward and backward links
				1087	that are expected by the <tt>ilist<T></tt> (and analogous containers)
				1088	in the default manner.</p>
				1089
				1090	<p><tt>ilist_node<T></tt>s are meant to be embedded in the node type
Gabor Greif	e39f0e7	2009-03-12 09:47:03 +0000	[diff] [blame]	1091	<tt>T</tt>, usually <tt>T</tt> publicly derives from
				1092	<tt>ilist_node<T></tt>.</p>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1093	</div>
				1094
				1095	<!-- _______________________________________________________________________ -->
				1096	<div class="doc_subsubsection">
Gabor Greif	d970d69	2009-03-12 10:30:31 +0000	[diff] [blame]	1097	<a name="dss_ilist_sentinel">Sentinels</a>
				1098	</div>
				1099
				1100	<div class="doc_text">
Dan Gohman	b1d8d90	2010-02-25 23:51:27 +0000	[diff] [blame]	1101	<p><tt>ilist</tt>s have another specialty that must be considered. To be a good
Gabor Greif	d970d69	2009-03-12 10:30:31 +0000	[diff] [blame]	1102	citizen in the C++ ecosystem, it needs to support the standard container
				1103	operations, such as <tt>begin</tt> and <tt>end</tt> iterators, etc. Also, the
				1104	<tt>operator--</tt> must work correctly on the <tt>end</tt> iterator in the
				1105	case of non-empty <tt>ilist</tt>s.</p>
				1106
				1107	<p>The only sensible solution to this problem is to allocate a so-called
				1108	<i>sentinel</i> along with the intrusive list, which serves as the <tt>end</tt>
				1109	iterator, providing the back-link to the last element. However conforming to the
				1110	C++ convention it is illegal to <tt>operator++</tt> beyond the sentinel and it
				1111	also must not be dereferenced.</p>
				1112
				1113	<p>These constraints allow for some implementation freedom to the <tt>ilist</tt>
				1114	how to allocate and store the sentinel. The corresponding policy is dictated
				1115	by <tt>ilist_traits<T></tt>. By default a <tt>T</tt> gets heap-allocated
				1116	whenever the need for a sentinel arises.</p>
				1117
				1118	<p>While the default policy is sufficient in most cases, it may break down when
				1119	<tt>T</tt> does not provide a default constructor. Also, in the case of many
				1120	instances of <tt>ilist</tt>s, the memory overhead of the associated sentinels
				1121	is wasted. To alleviate the situation with numerous and voluminous
				1122	<tt>T</tt>-sentinels, sometimes a trick is employed, leading to <i>ghostly
				1123	sentinels</i>.</p>
				1124
				1125	<p>Ghostly sentinels are obtained by specially-crafted <tt>ilist_traits<T></tt>
				1126	which superpose the sentinel with the <tt>ilist</tt> instance in memory. Pointer
				1127	arithmetic is used to obtain the sentinel, which is relative to the
				1128	<tt>ilist</tt>'s <tt>this</tt> pointer. The <tt>ilist</tt> is augmented by an
				1129	extra pointer, which serves as the back-link of the sentinel. This is the only
				1130	field in the ghostly sentinel which can be legally accessed.</p>
				1131	</div>
				1132
				1133	<!-- _______________________________________________________________________ -->
				1134	<div class="doc_subsubsection">
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1135	<a name="dss_other">Other Sequential Container options</a>
				1136	</div>
				1137
				1138	<div class="doc_text">
				1139	<p>Other STL containers are available, such as std::string.</p>
				1140
				1141	<p>There are also various STL adapter classes such as std::queue,
				1142	std::priority_queue, std::stack, etc. These provide simplified access to an
				1143	underlying container but don't affect the cost of the container itself.</p>
				1144
				1145	</div>
				1146
				1147
				1148	<!-- ======================================================================= -->
				1149	<div class="doc_subsection">
				1150	<a name="ds_set">Set-Like Containers (std::set, SmallSet, SetVector, etc)</a>
				1151	</div>
				1152
				1153	<div class="doc_text">
				1154
				1155	<p>Set-like containers are useful when you need to canonicalize multiple values
				1156	into a single representation. There are several different choices for how to do
				1157	this, providing various trade-offs.</p>
				1158
				1159	</div>
				1160
				1161
				1162	<!-- _______________________________________________________________________ -->
				1163	<div class="doc_subsubsection">
				1164	<a name="dss_sortedvectorset">A sorted 'vector'</a>
				1165	</div>
				1166
				1167	<div class="doc_text">
				1168
				1169	<p>If you intend to insert a lot of elements, then do a lot of queries, a
				1170	great approach is to use a vector (or other sequential container) with
				1171	std::sort+std::unique to remove duplicates. This approach works really well if
				1172	your usage pattern has these two distinct phases (insert then query), and can be
				1173	coupled with a good choice of <a href="#ds_sequential">sequential container</a>.
				1174	</p>
				1175
				1176	<p>
				1177	This combination provides the several nice properties: the result data is
				1178	contiguous in memory (good for cache locality), has few allocations, is easy to
				1179	address (iterators in the final vector are just indices or pointers), and can be
				1180	efficiently queried with a standard binary or radix search.</p>
				1181
				1182	</div>
				1183
				1184	<!-- _______________________________________________________________________ -->
				1185	<div class="doc_subsubsection">
				1186	<a name="dss_smallset">"llvm/ADT/SmallSet.h"</a>
				1187	</div>
				1188
				1189	<div class="doc_text">
				1190
				1191	<p>If you have a set-like data structure that is usually small and whose elements
				1192	are reasonably small, a <tt>SmallSet<Type, N></tt> is a good choice. This set
				1193	has space for N elements in place (thus, if the set is dynamically smaller than
				1194	N, no malloc traffic is required) and accesses them with a simple linear search.
				1195	When the set grows beyond 'N' elements, it allocates a more expensive representation that
				1196	guarantees efficient access (for most types, it falls back to std::set, but for
				1197	pointers it uses something far better, <a
				1198	href="#dss_smallptrset">SmallPtrSet</a>).</p>
				1199
				1200	<p>The magic of this class is that it handles small sets extremely efficiently,
				1201	but gracefully handles extremely large sets without loss of efficiency. The
				1202	drawback is that the interface is quite small: it supports insertion, queries
				1203	and erasing, but does not support iteration.</p>
				1204
				1205	</div>
				1206
				1207	<!-- _______________________________________________________________________ -->
				1208	<div class="doc_subsubsection">
				1209	<a name="dss_smallptrset">"llvm/ADT/SmallPtrSet.h"</a>
				1210	</div>
				1211
				1212	<div class="doc_text">
				1213
Gabor Greif	0029985	2010-03-26 19:30:47 +0000	[diff] [blame]	1214	<p>SmallPtrSet has all the advantages of <tt>SmallSet</tt> (and a <tt>SmallSet</tt> of pointers is
				1215	transparently implemented with a <tt>SmallPtrSet</tt>), but also supports iterators. If
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1216	more than 'N' insertions are performed, a single quadratically
				1217	probed hash table is allocated and grows as needed, providing extremely
				1218	efficient access (constant time insertion/deleting/queries with low constant
				1219	factors) and is very stingy with malloc traffic.</p>
				1220
Gabor Greif	0029985	2010-03-26 19:30:47 +0000	[diff] [blame]	1221	<p>Note that, unlike <tt>std::set</tt>, the iterators of <tt>SmallPtrSet</tt> are invalidated
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1222	whenever an insertion occurs. Also, the values visited by the iterators are not
				1223	visited in sorted order.</p>
				1224
				1225	</div>
				1226
				1227	<!-- _______________________________________________________________________ -->
				1228	<div class="doc_subsubsection">
Chris Lattner	77ab46d	2007-09-30 00:58:59 +0000	[diff] [blame]	1229	<a name="dss_denseset">"llvm/ADT/DenseSet.h"</a>
				1230	</div>
				1231
				1232	<div class="doc_text">
				1233
				1234	<p>
				1235	DenseSet is a simple quadratically probed hash table. It excels at supporting
				1236	small values: it uses a single allocation to hold all of the pairs that
				1237	are currently inserted in the set. DenseSet is a great way to unique small
				1238	values that are not simple pointers (use <a
				1239	href="#dss_smallptrset">SmallPtrSet</a> for pointers). Note that DenseSet has
				1240	the same requirements for the value type that <a
				1241	href="#dss_densemap">DenseMap</a> has.
				1242	</p>
				1243
				1244	</div>
				1245
				1246	<!-- _______________________________________________________________________ -->
				1247	<div class="doc_subsubsection">
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1248	<a name="dss_FoldingSet">"llvm/ADT/FoldingSet.h"</a>
				1249	</div>
				1250
				1251	<div class="doc_text">
				1252
				1253	<p>
				1254	FoldingSet is an aggregate class that is really good at uniquing
				1255	expensive-to-create or polymorphic objects. It is a combination of a chained
				1256	hash table with intrusive links (uniqued objects are required to inherit from
				1257	FoldingSetNode) that uses <a href="#dss_smallvector">SmallVector</a> as part of
				1258	its ID process.</p>
				1259
				1260	<p>Consider a case where you want to implement a "getOrCreateFoo" method for
				1261	a complex object (for example, a node in the code generator). The client has a
				1262	description of what it wants to generate (it knows the opcode and all the
				1263	operands), but we don't want to 'new' a node, then try inserting it into a set
				1264	only to find out it already exists, at which point we would have to delete it
				1265	and return the node that already exists.
				1266	</p>
				1267
				1268	<p>To support this style of client, FoldingSet perform a query with a
				1269	FoldingSetNodeID (which wraps SmallVector) that can be used to describe the
				1270	element that we want to query for. The query either returns the element
				1271	matching the ID or it returns an opaque ID that indicates where insertion should
				1272	take place. Construction of the ID usually does not require heap traffic.</p>
				1273
				1274	<p>Because FoldingSet uses intrusive links, it can support polymorphic objects
				1275	in the set (for example, you can have SDNode instances mixed with LoadSDNodes).
				1276	Because the elements are individually allocated, pointers to the elements are
				1277	stable: inserting or removing elements does not invalidate any pointers to other
				1278	elements.
				1279	</p>
				1280
				1281	</div>
				1282
				1283	<!-- _______________________________________________________________________ -->
				1284	<div class="doc_subsubsection">
				1285	<a name="dss_set"><set></a>
				1286	</div>
				1287
				1288	<div class="doc_text">
				1289
				1290	<p><tt>std::set</tt> is a reasonable all-around set class, which is decent at
				1291	many things but great at nothing. std::set allocates memory for each element
				1292	inserted (thus it is very malloc intensive) and typically stores three pointers
				1293	per element in the set (thus adding a large amount of per-element space
				1294	overhead). It offers guaranteed log(n) performance, which is not particularly
				1295	fast from a complexity standpoint (particularly if the elements of the set are
				1296	expensive to compare, like strings), and has extremely high constant factors for
				1297	lookup, insertion and removal.</p>
				1298
				1299	<p>The advantages of std::set are that its iterators are stable (deleting or
				1300	inserting an element from the set does not affect iterators or pointers to other
				1301	elements) and that iteration over the set is guaranteed to be in sorted order.
				1302	If the elements in the set are large, then the relative overhead of the pointers
				1303	and malloc traffic is not a big deal, but if the elements of the set are small,
				1304	std::set is almost never a good choice.</p>
				1305
				1306	</div>
				1307
				1308	<!-- _______________________________________________________________________ -->
				1309	<div class="doc_subsubsection">
				1310	<a name="dss_setvector">"llvm/ADT/SetVector.h"</a>
				1311	</div>
				1312
				1313	<div class="doc_text">
				1314	<p>LLVM's SetVector<Type> is an adapter class that combines your choice of
				1315	a set-like container along with a <a href="#ds_sequential">Sequential
				1316	Container</a>. The important property
				1317	that this provides is efficient insertion with uniquing (duplicate elements are
				1318	ignored) with iteration support. It implements this by inserting elements into
				1319	both a set-like container and the sequential container, using the set-like
				1320	container for uniquing and the sequential container for iteration.
				1321	</p>
				1322
				1323	<p>The difference between SetVector and other sets is that the order of
				1324	iteration is guaranteed to match the order of insertion into the SetVector.
				1325	This property is really important for things like sets of pointers. Because
				1326	pointer values are non-deterministic (e.g. vary across runs of the program on
				1327	different machines), iterating over the pointers in the set will
				1328	not be in a well-defined order.</p>
				1329
				1330	<p>
				1331	The drawback of SetVector is that it requires twice as much space as a normal
				1332	set and has the sum of constant factors from the set-like container and the
				1333	sequential container that it uses. Use it only if you need to iterate over
				1334	the elements in a deterministic order. SetVector is also expensive to delete
				1335	elements out of (linear time), unless you use it's "pop_back" method, which is
				1336	faster.
				1337	</p>
				1338
				1339	<p>SetVector is an adapter class that defaults to using std::vector and std::set
				1340	for the underlying containers, so it is quite expensive. However,
				1341	<tt>"llvm/ADT/SetVector.h"</tt> also provides a SmallSetVector class, which
				1342	defaults to using a SmallVector and SmallSet of a specified size. If you use
				1343	this, and if your sets are dynamically smaller than N, you will save a lot of
				1344	heap traffic.</p>
				1345
				1346	</div>
				1347
				1348	<!-- _______________________________________________________________________ -->
				1349	<div class="doc_subsubsection">
				1350	<a name="dss_uniquevector">"llvm/ADT/UniqueVector.h"</a>
				1351	</div>
				1352
				1353	<div class="doc_text">
				1354
				1355	<p>
				1356	UniqueVector is similar to <a href="#dss_setvector">SetVector</a>, but it
				1357	retains a unique ID for each element inserted into the set. It internally
				1358	contains a map and a vector, and it assigns a unique ID for each value inserted
				1359	into the set.</p>
				1360
				1361	<p>UniqueVector is very expensive: its cost is the sum of the cost of
				1362	maintaining both the map and vector, it has high complexity, high constant
				1363	factors, and produces a lot of malloc traffic. It should be avoided.</p>
				1364
				1365	</div>
				1366
				1367
				1368	<!-- _______________________________________________________________________ -->
				1369	<div class="doc_subsubsection">
				1370	<a name="dss_otherset">Other Set-Like Container Options</a>
				1371	</div>
				1372
				1373	<div class="doc_text">
				1374
				1375	<p>
				1376	The STL provides several other options, such as std::multiset and the various
Chris Lattner	86a63d0	2009-03-09 05:20:45 +0000	[diff] [blame]	1377	"hash_set" like containers (whether from C++ TR1 or from the SGI library). We
				1378	never use hash_set and unordered_set because they are generally very expensive
				1379	(each insertion requires a malloc) and very non-portable.
				1380	</p>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1381
				1382	<p>std::multiset is useful if you're not interested in elimination of
				1383	duplicates, but has all the drawbacks of std::set. A sorted vector (where you
				1384	don't delete duplicate entries) or some other approach is almost always
				1385	better.</p>
				1386
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1387	</div>
				1388
				1389	<!-- ======================================================================= -->
				1390	<div class="doc_subsection">
				1391	<a name="ds_map">Map-Like Containers (std::map, DenseMap, etc)</a>
				1392	</div>
				1393
				1394	<div class="doc_text">
				1395	Map-like containers are useful when you want to associate data to a key. As
				1396	usual, there are a lot of different ways to do this. :)
				1397	</div>
				1398
				1399	<!-- _______________________________________________________________________ -->
				1400	<div class="doc_subsubsection">
				1401	<a name="dss_sortedvectormap">A sorted 'vector'</a>
				1402	</div>
				1403
				1404	<div class="doc_text">
				1405
				1406	<p>
				1407	If your usage pattern follows a strict insert-then-query approach, you can
				1408	trivially use the same approach as <a href="#dss_sortedvectorset">sorted vectors
				1409	for set-like containers</a>. The only difference is that your query function
				1410	(which uses std::lower_bound to get efficient log(n) lookup) should only compare
				1411	the key, not both the key and value. This yields the same advantages as sorted
				1412	vectors for sets.
				1413	</p>
				1414	</div>
				1415
				1416	<!-- _______________________________________________________________________ -->
				1417	<div class="doc_subsubsection">
				1418	<a name="dss_stringmap">"llvm/ADT/StringMap.h"</a>
				1419	</div>
				1420
				1421	<div class="doc_text">
				1422
				1423	<p>
				1424	Strings are commonly used as keys in maps, and they are difficult to support
				1425	efficiently: they are variable length, inefficient to hash and compare when
				1426	long, expensive to copy, etc. StringMap is a specialized container designed to
				1427	cope with these issues. It supports mapping an arbitrary range of bytes to an
				1428	arbitrary other object.</p>
				1429
				1430	<p>The StringMap implementation uses a quadratically-probed hash table, where
				1431	the buckets store a pointer to the heap allocated entries (and some other
				1432	stuff). The entries in the map must be heap allocated because the strings are
				1433	variable length. The string data (key) and the element object (value) are
				1434	stored in the same allocation with the string data immediately after the element
				1435	object. This container guarantees the "<tt>(char*)(&Value+1)</tt>" points
				1436	to the key string for a value.</p>
				1437
				1438	<p>The StringMap is very fast for several reasons: quadratic probing is very
				1439	cache efficient for lookups, the hash value of strings in buckets is not
				1440	recomputed when lookup up an element, StringMap rarely has to touch the
				1441	memory for unrelated objects when looking up a value (even when hash collisions
				1442	happen), hash table growth does not recompute the hash values for strings
				1443	already in the table, and each pair in the map is store in a single allocation
				1444	(the string data is stored in the same allocation as the Value of a pair).</p>
				1445
				1446	<p>StringMap also provides query methods that take byte ranges, so it only ever
				1447	copies a string if a value is inserted into the table.</p>
				1448	</div>
				1449
				1450	<!-- _______________________________________________________________________ -->
				1451	<div class="doc_subsubsection">
				1452	<a name="dss_indexedmap">"llvm/ADT/IndexedMap.h"</a>
				1453	</div>
				1454
				1455	<div class="doc_text">
				1456	<p>
				1457	IndexedMap is a specialized container for mapping small dense integers (or
				1458	values that can be mapped to small dense integers) to some other type. It is
				1459	internally implemented as a vector with a mapping function that maps the keys to
				1460	the dense integer range.
				1461	</p>
				1462
				1463	<p>
				1464	This is useful for cases like virtual registers in the LLVM code generator: they
				1465	have a dense mapping that is offset by a compile-time constant (the first
				1466	virtual register ID).</p>
				1467
				1468	</div>
				1469
				1470	<!-- _______________________________________________________________________ -->
				1471	<div class="doc_subsubsection">
				1472	<a name="dss_densemap">"llvm/ADT/DenseMap.h"</a>
				1473	</div>
				1474
				1475	<div class="doc_text">
				1476
				1477	<p>
				1478	DenseMap is a simple quadratically probed hash table. It excels at supporting
				1479	small keys and values: it uses a single allocation to hold all of the pairs that
				1480	are currently inserted in the map. DenseMap is a great way to map pointers to
				1481	pointers, or map other small types to each other.
				1482	</p>
				1483
				1484	<p>
				1485	There are several aspects of DenseMap that you should be aware of, however. The
				1486	iterators in a densemap are invalidated whenever an insertion occurs, unlike
				1487	map. Also, because DenseMap allocates space for a large number of key/value
				1488	pairs (it starts with 64 by default), it will waste a lot of space if your keys
				1489	or values are large. Finally, you must implement a partial specialization of
Chris Lattner	92eea07	2007-09-17 18:34:04 +0000	[diff] [blame]	1490	DenseMapInfo for the key that you want, if it isn't already supported. This
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1491	is required to tell DenseMap about two special marker values (which can never be
				1492	inserted into the map) that it needs internally.</p>
				1493
				1494	</div>
				1495
				1496	<!-- _______________________________________________________________________ -->
				1497	<div class="doc_subsubsection">
Jeffrey Yasskin	50e846d	2009-10-22 22:11:22 +0000	[diff] [blame]	1498	<a name="dss_valuemap">"llvm/ADT/ValueMap.h"</a>
				1499	</div>
				1500
				1501	<div class="doc_text">
				1502
				1503	<p>
				1504	ValueMap is a wrapper around a <a href="#dss_densemap">DenseMap</a> mapping
				1505	Value*s (or subclasses) to another type. When a Value is deleted or RAUW'ed,
				1506	ValueMap will update itself so the new version of the key is mapped to the same
				1507	value, just as if the key were a WeakVH. You can configure exactly how this
				1508	happens, and what else happens on these two events, by passing
				1509	a <code>Config</code> parameter to the ValueMap template.</p>
				1510
				1511	</div>
				1512
				1513	<!-- _______________________________________________________________________ -->
				1514	<div class="doc_subsubsection">
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1515	<a name="dss_map"><map></a>
				1516	</div>
				1517
				1518	<div class="doc_text">
				1519
				1520	<p>
				1521	std::map has similar characteristics to <a href="#dss_set">std::set</a>: it uses
				1522	a single allocation per pair inserted into the map, it offers log(n) lookup with
				1523	an extremely large constant factor, imposes a space penalty of 3 pointers per
				1524	pair in the map, etc.</p>
				1525
				1526	<p>std::map is most useful when your keys or values are very large, if you need
				1527	to iterate over the collection in sorted order, or if you need stable iterators
				1528	into the map (i.e. they don't get invalidated if an insertion or deletion of
				1529	another element takes place).</p>
				1530
				1531	</div>
				1532
				1533	<!-- _______________________________________________________________________ -->
				1534	<div class="doc_subsubsection">
				1535	<a name="dss_othermap">Other Map-Like Container Options</a>
				1536	</div>
				1537
				1538	<div class="doc_text">
				1539
				1540	<p>
				1541	The STL provides several other options, such as std::multimap and the various
Chris Lattner	86a63d0	2009-03-09 05:20:45 +0000	[diff] [blame]	1542	"hash_map" like containers (whether from C++ TR1 or from the SGI library). We
				1543	never use hash_set and unordered_set because they are generally very expensive
				1544	(each insertion requires a malloc) and very non-portable.</p>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1545
				1546	<p>std::multimap is useful if you want to map a key to multiple values, but has
				1547	all the drawbacks of std::map. A sorted vector or some other approach is almost
				1548	always better.</p>
				1549
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1550	</div>
				1551
Daniel Berlin	7ea44dc	2007-09-24 17:52:25 +0000	[diff] [blame]	1552	<!-- ======================================================================= -->
				1553	<div class="doc_subsection">
Chris Lattner	d8b95b7	2009-07-25 07:22:20 +0000	[diff] [blame]	1554	<a name="ds_string">String-like containers</a>
				1555	</div>
				1556
				1557	<div class="doc_text">
				1558
				1559	<p>
				1560	TODO: const char* vs stringref vs smallstring vs std::string. Describe twine,
				1561	xref to #string_apis.
				1562	</p>
				1563
				1564	</div>
				1565
				1566	<!-- ======================================================================= -->
				1567	<div class="doc_subsection">
Daniel Berlin	7ea44dc	2007-09-24 17:52:25 +0000	[diff] [blame]	1568	<a name="ds_bit">Bit storage containers (BitVector, SparseBitVector)</a>
				1569	</div>
				1570
				1571	<div class="doc_text">
Chris Lattner	62a4eae	2007-09-25 22:37:50 +0000	[diff] [blame]	1572	<p>Unlike the other containers, there are only two bit storage containers, and
				1573	choosing when to use each is relatively straightforward.</p>
				1574
				1575	<p>One additional option is
				1576	<tt>std::vector<bool></tt>: we discourage its use for two reasons 1) the
				1577	implementation in many common compilers (e.g. commonly available versions of
				1578	GCC) is extremely inefficient and 2) the C++ standards committee is likely to
				1579	deprecate this container and/or change it significantly somehow. In any case,
				1580	please don't use it.</p>
Daniel Berlin	7ea44dc	2007-09-24 17:52:25 +0000	[diff] [blame]	1581	</div>
				1582
				1583	<!-- _______________________________________________________________________ -->
				1584	<div class="doc_subsubsection">
				1585	<a name="dss_bitvector">BitVector</a>
				1586	</div>
				1587
				1588	<div class="doc_text">
Dan Gohman	c83a2b4	2010-01-05 18:24:00 +0000	[diff] [blame]	1589	<p> The BitVector container provides a dynamic size set of bits for manipulation.
Daniel Berlin	7ea44dc	2007-09-24 17:52:25 +0000	[diff] [blame]	1590	It supports individual bit setting/testing, as well as set operations. The set
				1591	operations take time O(size of bitvector), but operations are performed one word
				1592	at a time, instead of one bit at a time. This makes the BitVector very fast for
				1593	set operations compared to other containers. Use the BitVector when you expect
				1594	the number of set bits to be high (IE a dense set).
				1595	</p>
				1596	</div>
				1597
				1598	<!-- _______________________________________________________________________ -->
				1599	<div class="doc_subsubsection">
Dan Gohman	c83a2b4	2010-01-05 18:24:00 +0000	[diff] [blame]	1600	<a name="dss_smallbitvector">SmallBitVector</a>
				1601	</div>
				1602
				1603	<div class="doc_text">
				1604	<p> The SmallBitVector container provides the same interface as BitVector, but
				1605	it is optimized for the case where only a small number of bits, less than
				1606	25 or so, are needed. It also transparently supports larger bit counts, but
				1607	slightly less efficiently than a plain BitVector, so SmallBitVector should
				1608	only be used when larger counts are rare.
				1609	</p>
				1610
				1611	<p>
				1612	At this time, SmallBitVector does not support set operations (and, or, xor),
				1613	and its operator[] does not provide an assignable lvalue.
				1614	</p>
				1615	</div>
				1616
				1617	<!-- _______________________________________________________________________ -->
				1618	<div class="doc_subsubsection">
Daniel Berlin	7ea44dc	2007-09-24 17:52:25 +0000	[diff] [blame]	1619	<a name="dss_sparsebitvector">SparseBitVector</a>
				1620	</div>
				1621
				1622	<div class="doc_text">
				1623	<p> The SparseBitVector container is much like BitVector, with one major
				1624	difference: Only the bits that are set, are stored. This makes the
				1625	SparseBitVector much more space efficient than BitVector when the set is sparse,
				1626	as well as making set operations O(number of set bits) instead of O(size of
				1627	universe). The downside to the SparseBitVector is that setting and testing of random bits is O(N), and on large SparseBitVectors, this can be slower than BitVector. In our implementation, setting or testing bits in sorted order
				1628	(either forwards or reverse) is O(1) worst case. Testing and setting bits within 128 bits (depends on size) of the current bit is also O(1). As a general statement, testing/setting bits in a SparseBitVector is O(distance away from last set bit).
				1629	</p>
				1630	</div>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1631
				1632	<!-- *********************************************************************** -->
				1633	<div class="doc_section">
				1634	<a name="common">Helpful Hints for Common Operations</a>
				1635	</div>
				1636	<!-- *********************************************************************** -->
				1637
				1638	<div class="doc_text">
				1639
				1640	<p>This section describes how to perform some very simple transformations of
				1641	LLVM code. This is meant to give examples of common idioms used, showing the
				1642	practical side of LLVM transformations. <p> Because this is a "how-to" section,
				1643	you should also read about the main classes that you will be working with. The
				1644	<a href="#coreclasses">Core LLVM Class Hierarchy Reference</a> contains details
				1645	and descriptions of the main classes that you should know about.</p>
				1646
				1647	</div>
				1648
				1649	<!-- NOTE: this section should be heavy on example code -->
				1650	<!-- ======================================================================= -->
				1651	<div class="doc_subsection">
				1652	<a name="inspection">Basic Inspection and Traversal Routines</a>
				1653	</div>
				1654
				1655	<div class="doc_text">
				1656
				1657	<p>The LLVM compiler infrastructure have many different data structures that may
				1658	be traversed. Following the example of the C++ standard template library, the
				1659	techniques used to traverse these various data structures are all basically the
				1660	same. For a enumerable sequence of values, the <tt>XXXbegin()</tt> function (or
				1661	method) returns an iterator to the start of the sequence, the <tt>XXXend()</tt>
				1662	function returns an iterator pointing to one past the last valid element of the
				1663	sequence, and there is some <tt>XXXiterator</tt> data type that is common
				1664	between the two operations.</p>
				1665
				1666	<p>Because the pattern for iteration is common across many different aspects of
				1667	the program representation, the standard template library algorithms may be used
				1668	on them, and it is easier to remember how to iterate. First we show a few common
				1669	examples of the data structures that need to be traversed. Other data
				1670	structures are traversed in very similar ways.</p>
				1671
				1672	</div>
				1673
				1674	<!-- _______________________________________________________________________ -->
				1675	<div class="doc_subsubsection">
				1676	<a name="iterate_function">Iterating over the </a><a
				1677	href="#BasicBlock"><tt>BasicBlock</tt></a>s in a <a
				1678	href="#Function"><tt>Function</tt></a>
				1679	</div>
				1680
				1681	<div class="doc_text">
				1682
				1683	<p>It's quite common to have a <tt>Function</tt> instance that you'd like to
				1684	transform in some way; in particular, you'd like to manipulate its
				1685	<tt>BasicBlock</tt>s. To facilitate this, you'll need to iterate over all of
				1686	the <tt>BasicBlock</tt>s that constitute the <tt>Function</tt>. The following is
				1687	an example that prints the name of a <tt>BasicBlock</tt> and the number of
				1688	<tt>Instruction</tt>s it contains:</p>
				1689
				1690	<div class="doc_code">
				1691	<pre>
				1692	// <i>func is a pointer to a Function instance</i>
				1693	for (Function::iterator i = func->begin(), e = func->end(); i != e; ++i)
				1694	// <i>Print out the name of the basic block if it has one, and then the</i>
				1695	// <i>number of instructions that it contains</i>
Chris Lattner	599c392	2009-09-08 05:15:50 +0000	[diff] [blame]	1696	errs() << "Basic block (name=" << i->getName() << ") has "
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1697	<< i->size() << " instructions.\n";
				1698	</pre>
				1699	</div>
				1700
				1701	<p>Note that i can be used as if it were a pointer for the purposes of
				1702	invoking member functions of the <tt>Instruction</tt> class. This is
				1703	because the indirection operator is overloaded for the iterator
				1704	classes. In the above code, the expression <tt>i->size()</tt> is
				1705	exactly equivalent to <tt>(*i).size()</tt> just like you'd expect.</p>
				1706
				1707	</div>
				1708
				1709	<!-- _______________________________________________________________________ -->
				1710	<div class="doc_subsubsection">
				1711	<a name="iterate_basicblock">Iterating over the </a><a
				1712	href="#Instruction"><tt>Instruction</tt></a>s in a <a
				1713	href="#BasicBlock"><tt>BasicBlock</tt></a>
				1714	</div>
				1715
				1716	<div class="doc_text">
				1717
				1718	<p>Just like when dealing with <tt>BasicBlock</tt>s in <tt>Function</tt>s, it's
				1719	easy to iterate over the individual instructions that make up
				1720	<tt>BasicBlock</tt>s. Here's a code snippet that prints out each instruction in
				1721	a <tt>BasicBlock</tt>:</p>
				1722
				1723	<div class="doc_code">
				1724	<pre>
				1725	// <i>blk is a pointer to a BasicBlock instance</i>
				1726	for (BasicBlock::iterator i = blk->begin(), e = blk->end(); i != e; ++i)
				1727	// <i>The next statement works since operator<<(ostream&,...)</i>
				1728	// <i>is overloaded for Instruction&</i>
Chris Lattner	599c392	2009-09-08 05:15:50 +0000	[diff] [blame]	1729	errs() << *i << "\n";
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1730	</pre>
				1731	</div>
				1732
				1733	<p>However, this isn't really the best way to print out the contents of a
				1734	<tt>BasicBlock</tt>! Since the ostream operators are overloaded for virtually
				1735	anything you'll care about, you could have just invoked the print routine on the
Chris Lattner	599c392	2009-09-08 05:15:50 +0000	[diff] [blame]	1736	basic block itself: <tt>errs() << *blk << "\n";</tt>.</p>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1737
				1738	</div>
				1739
				1740	<!-- _______________________________________________________________________ -->
				1741	<div class="doc_subsubsection">
				1742	<a name="iterate_institer">Iterating over the </a><a
				1743	href="#Instruction"><tt>Instruction</tt></a>s in a <a
				1744	href="#Function"><tt>Function</tt></a>
				1745	</div>
				1746
				1747	<div class="doc_text">
				1748
				1749	<p>If you're finding that you commonly iterate over a <tt>Function</tt>'s
				1750	<tt>BasicBlock</tt>s and then that <tt>BasicBlock</tt>'s <tt>Instruction</tt>s,
				1751	<tt>InstIterator</tt> should be used instead. You'll need to include <a
				1752	href="/doxygen/InstIterator_8h-source.html"><tt>llvm/Support/InstIterator.h</tt></a>,
				1753	and then instantiate <tt>InstIterator</tt>s explicitly in your code. Here's a
				1754	small example that shows how to dump all instructions in a function to the standard error stream:<p>
				1755
				1756	<div class="doc_code">
				1757	<pre>
				1758	#include "<a href="/doxygen/InstIterator_8h-source.html">llvm/Support/InstIterator.h</a>"
				1759
				1760	// <i>F is a pointer to a Function instance</i>
Chris Lattner	fd62ee7	2008-06-04 18:20:42 +0000	[diff] [blame]	1761	for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
Chris Lattner	599c392	2009-09-08 05:15:50 +0000	[diff] [blame]	1762	errs() << *I << "\n";
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1763	</pre>
				1764	</div>
				1765
				1766	<p>Easy, isn't it? You can also use <tt>InstIterator</tt>s to fill a
				1767	work list with its initial contents. For example, if you wanted to
				1768	initialize a work list to contain all instructions in a <tt>Function</tt>
				1769	F, all you would need to do is something like:</p>
				1770
				1771	<div class="doc_code">
				1772	<pre>
				1773	std::set<Instruction*> worklist;
Chris Lattner	fd62ee7	2008-06-04 18:20:42 +0000	[diff] [blame]	1774	// or better yet, SmallPtrSet<Instruction*, 64> worklist;
				1775
				1776	for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
				1777	worklist.insert(&*I);
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1778	</pre>
				1779	</div>
				1780
				1781	<p>The STL set <tt>worklist</tt> would now contain all instructions in the
				1782	<tt>Function</tt> pointed to by F.</p>
				1783
				1784	</div>
				1785
				1786	<!-- _______________________________________________________________________ -->
				1787	<div class="doc_subsubsection">
				1788	<a name="iterate_convert">Turning an iterator into a class pointer (and
				1789	vice-versa)</a>
				1790	</div>
				1791
				1792	<div class="doc_text">
				1793
				1794	<p>Sometimes, it'll be useful to grab a reference (or pointer) to a class
				1795	instance when all you've got at hand is an iterator. Well, extracting
				1796	a reference or a pointer from an iterator is very straight-forward.
				1797	Assuming that <tt>i</tt> is a <tt>BasicBlock::iterator</tt> and <tt>j</tt>
				1798	is a <tt>BasicBlock::const_iterator</tt>:</p>
				1799
				1800	<div class="doc_code">
				1801	<pre>
				1802	Instruction& inst = *i; // <i>Grab reference to instruction reference</i>
				1803	Instruction* pinst = &*i; // <i>Grab pointer to instruction reference</i>
				1804	const Instruction& inst = *j;
				1805	</pre>
				1806	</div>
				1807
				1808	<p>However, the iterators you'll be working with in the LLVM framework are
				1809	special: they will automatically convert to a ptr-to-instance type whenever they
				1810	need to. Instead of dereferencing the iterator and then taking the address of
				1811	the result, you can simply assign the iterator to the proper pointer type and
				1812	you get the dereference and address-of operation as a result of the assignment
				1813	(behind the scenes, this is a result of overloading casting mechanisms). Thus
				1814	the last line of the last example,</p>
				1815
				1816	<div class="doc_code">
				1817	<pre>
Chris Lattner	0665e1f	2008-01-03 16:56:04 +0000	[diff] [blame]	1818	Instruction pinst = &i;
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1819	</pre>
				1820	</div>
				1821
				1822	<p>is semantically equivalent to</p>
				1823
				1824	<div class="doc_code">
				1825	<pre>
Chris Lattner	0665e1f	2008-01-03 16:56:04 +0000	[diff] [blame]	1826	Instruction *pinst = i;
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1827	</pre>
				1828	</div>
				1829
				1830	<p>It's also possible to turn a class pointer into the corresponding iterator,
				1831	and this is a constant time operation (very efficient). The following code
				1832	snippet illustrates use of the conversion constructors provided by LLVM
				1833	iterators. By using these, you can explicitly grab the iterator of something
				1834	without actually obtaining it via iteration over some structure:</p>
				1835
				1836	<div class="doc_code">
				1837	<pre>
				1838	void printNextInstruction(Instruction* inst) {
				1839	BasicBlock::iterator it(inst);
				1840	++it; // <i>After this line, it refers to the instruction after *inst</i>
Chris Lattner	599c392	2009-09-08 05:15:50 +0000	[diff] [blame]	1841	if (it != inst->getParent()->end()) errs() << *it << "\n";
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1842	}
				1843	</pre>
				1844	</div>
				1845
Dan Gohman	2ece7fb	2010-03-26 19:39:05 +0000	[diff] [blame]	1846	<p>Unfortunately, these implicit conversions come at a cost; they prevent
				1847	these iterators from conforming to standard iterator conventions, and thus
Dan Gohman	166a023	2010-03-26 19:51:14 +0000	[diff] [blame]	1848	from being usable with standard algorithms and containers. For example, they
				1849	prevent the following code, where <tt>B</tt> is a <tt>BasicBlock</tt>,
Dan Gohman	2ece7fb	2010-03-26 19:39:05 +0000	[diff] [blame]	1850	from compiling:</p>
				1851
				1852	<div class="doc_code">
				1853	<pre>
				1854	llvm::SmallVector<llvm::Instruction *, 16>(B->begin(), B->end());
				1855	</pre>
				1856	</div>
				1857
				1858	<p>Because of this, these implicit conversions may be removed some day,
Dan Gohman	166a023	2010-03-26 19:51:14 +0000	[diff] [blame]	1859	and <tt>operator*</tt> changed to return a pointer instead of a reference.</p>
Dan Gohman	2ece7fb	2010-03-26 19:39:05 +0000	[diff] [blame]	1860
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1861	</div>
				1862
				1863	<!--_______________________________________________________________________-->
				1864	<div class="doc_subsubsection">
				1865	<a name="iterate_complex">Finding call sites: a slightly more complex
				1866	example</a>
				1867	</div>
				1868
				1869	<div class="doc_text">
				1870
				1871	<p>Say that you're writing a FunctionPass and would like to count all the
				1872	locations in the entire module (that is, across every <tt>Function</tt>) where a
				1873	certain function (i.e., some <tt>Function</tt>*) is already in scope. As you'll
				1874	learn later, you may want to use an <tt>InstVisitor</tt> to accomplish this in a
				1875	much more straight-forward manner, but this example will allow us to explore how
				1876	you'd do it if you didn't have <tt>InstVisitor</tt> around. In pseudo-code, this
				1877	is what we want to do:</p>
				1878
				1879	<div class="doc_code">
				1880	<pre>
				1881	initialize callCounter to zero
				1882	for each Function f in the Module
				1883	for each BasicBlock b in f
				1884	for each Instruction i in b
				1885	if (i is a CallInst and calls the given function)
				1886	increment callCounter
				1887	</pre>
				1888	</div>
				1889
				1890	<p>And the actual code is (remember, because we're writing a
				1891	<tt>FunctionPass</tt>, our <tt>FunctionPass</tt>-derived class simply has to
				1892	override the <tt>runOnFunction</tt> method):</p>
				1893
				1894	<div class="doc_code">
				1895	<pre>
				1896	Function* targetFunc = ...;
				1897
				1898	class OurFunctionPass : public FunctionPass {
				1899	public:
				1900	OurFunctionPass(): callCounter(0) { }
				1901
				1902	virtual runOnFunction(Function& F) {
				1903	for (Function::iterator b = F.begin(), be = F.end(); b != be; ++b) {
Eric Christopher	5fbecf7	2008-11-08 08:20:49 +0000	[diff] [blame]	1904	for (BasicBlock::iterator i = b->begin(), ie = b->end(); i != ie; ++i) {
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1905	if (<a href="#CallInst">CallInst</a>* callInst = <a href="#isa">dyn_cast</a><<a
				1906	href="#CallInst">CallInst</a>>(&*i)) {
				1907	// <i>We know we've encountered a call instruction, so we</i>
				1908	// <i>need to determine if it's a call to the</i>
Chris Lattner	0665e1f	2008-01-03 16:56:04 +0000	[diff] [blame]	1909	// <i>function pointed to by m_func or not.</i>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1910	if (callInst->getCalledFunction() == targetFunc)
				1911	++callCounter;
				1912	}
				1913	}
				1914	}
				1915	}
				1916
				1917	private:
Chris Lattner	0665e1f	2008-01-03 16:56:04 +0000	[diff] [blame]	1918	unsigned callCounter;
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1919	};
				1920	</pre>
				1921	</div>
				1922
				1923	</div>
				1924
				1925	<!--_______________________________________________________________________-->
				1926	<div class="doc_subsubsection">
				1927	<a name="calls_and_invokes">Treating calls and invokes the same way</a>
				1928	</div>
				1929
				1930	<div class="doc_text">
				1931
				1932	<p>You may have noticed that the previous example was a bit oversimplified in
				1933	that it did not deal with call sites generated by 'invoke' instructions. In
				1934	this, and in other situations, you may find that you want to treat
				1935	<tt>CallInst</tt>s and <tt>InvokeInst</tt>s the same way, even though their
				1936	most-specific common base class is <tt>Instruction</tt>, which includes lots of
				1937	less closely-related things. For these cases, LLVM provides a handy wrapper
				1938	class called <a
				1939	href="http://llvm.org/doxygen/classllvm_1_1CallSite.html"><tt>CallSite</tt></a>.
				1940	It is essentially a wrapper around an <tt>Instruction</tt> pointer, with some
				1941	methods that provide functionality common to <tt>CallInst</tt>s and
				1942	<tt>InvokeInst</tt>s.</p>
				1943
				1944	<p>This class has "value semantics": it should be passed by value, not by
				1945	reference and it should not be dynamically allocated or deallocated using
				1946	<tt>operator new</tt> or <tt>operator delete</tt>. It is efficiently copyable,
				1947	assignable and constructable, with costs equivalents to that of a bare pointer.
				1948	If you look at its definition, it has only a single pointer member.</p>
				1949
				1950	</div>
				1951
				1952	<!--_______________________________________________________________________-->
				1953	<div class="doc_subsubsection">
				1954	<a name="iterate_chains">Iterating over def-use & use-def chains</a>
				1955	</div>
				1956
				1957	<div class="doc_text">
				1958
				1959	<p>Frequently, we might have an instance of the <a
				1960	href="/doxygen/classllvm_1_1Value.html">Value Class</a> and we want to
				1961	determine which <tt>User</tt>s use the <tt>Value</tt>. The list of all
				1962	<tt>User</tt>s of a particular <tt>Value</tt> is called a <i>def-use</i> chain.
				1963	For example, let's say we have a <tt>Function*</tt> named <tt>F</tt> to a
				1964	particular function <tt>foo</tt>. Finding all of the instructions that
				1965	<i>use</i> <tt>foo</tt> is as simple as iterating over the <i>def-use</i> chain
				1966	of <tt>F</tt>:</p>
				1967
				1968	<div class="doc_code">
				1969	<pre>
Chris Lattner	0665e1f	2008-01-03 16:56:04 +0000	[diff] [blame]	1970	Function *F = ...;
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1971
				1972	for (Value::use_iterator i = F->use_begin(), e = F->use_end(); i != e; ++i)
				1973	if (Instruction Inst = dyn_cast<Instruction>(i)) {
Chris Lattner	599c392	2009-09-08 05:15:50 +0000	[diff] [blame]	1974	errs() << "F is used in instruction:\n";
				1975	errs() << *Inst << "\n";
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1976	}
				1977	</pre>
Gabor Greif	b114a80	2010-03-26 19:35:48 +0000	[diff] [blame]	1978	</div>
				1979
Gabor Greif	221b837	2010-03-26 19:40:38 +0000	[diff] [blame]	1980	<p>Note that dereferencing a <tt>Value::use_iterator</tt> is not a very cheap
Gabor Greif	0029985	2010-03-26 19:30:47 +0000	[diff] [blame]	1981	operation. Instead of performing <tt>*i</tt> above several times, consider
Gabor Greif	221b837	2010-03-26 19:40:38 +0000	[diff] [blame]	1982	doing it only once in the loop body and reusing its result.</p>
Gabor Greif	0029985	2010-03-26 19:30:47 +0000	[diff] [blame]	1983
Gabor Greif	d555350	2010-03-26 19:04:42 +0000	[diff] [blame]	1984	<p>Alternatively, it's common to have an instance of the <a
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1985	href="/doxygen/classllvm_1_1User.html">User Class</a> and need to know what
				1986	<tt>Value</tt>s are used by it. The list of all <tt>Value</tt>s used by a
				1987	<tt>User</tt> is known as a <i>use-def</i> chain. Instances of class
				1988	<tt>Instruction</tt> are common <tt>User</tt>s, so we might want to iterate over
				1989	all of the values that a particular instruction uses (that is, the operands of
				1990	the particular <tt>Instruction</tt>):</p>
				1991
				1992	<div class="doc_code">
				1993	<pre>
Chris Lattner	0665e1f	2008-01-03 16:56:04 +0000	[diff] [blame]	1994	Instruction *pi = ...;
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1995
				1996	for (User::op_iterator i = pi->op_begin(), e = pi->op_end(); i != e; ++i) {
Chris Lattner	0665e1f	2008-01-03 16:56:04 +0000	[diff] [blame]	1997	Value v = i;
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	1998	// <i>...</i>
				1999	}
				2000	</pre>
				2001	</div>
				2002
Gabor Greif	0029985	2010-03-26 19:30:47 +0000	[diff] [blame]	2003	<p>Declaring objects as <tt>const</tt> is an important tool of enforcing
Gabor Greif	221b837	2010-03-26 19:40:38 +0000	[diff] [blame]	2004	mutation free algorithms (such as analyses, etc.). For this purpose above
Gabor Greif	0029985	2010-03-26 19:30:47 +0000	[diff] [blame]	2005	iterators come in constant flavors as <tt>Value::const_use_iterator</tt>
				2006	and <tt>Value::const_op_iterator</tt>. They automatically arise when
				2007	calling <tt>use/op_begin()</tt> on <tt>const Value*</tt>s or
				2008	<tt>const User*</tt>s respectively. Upon dereferencing, they return
Gabor Greif	221b837	2010-03-26 19:40:38 +0000	[diff] [blame]	2009	<tt>const Use*</tt>s. Otherwise the above patterns remain unchanged.</p>
				2010
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	2011	</div>
				2012
Chris Lattner	0665e1f	2008-01-03 16:56:04 +0000	[diff] [blame]	2013	<!--_______________________________________________________________________-->
				2014	<div class="doc_subsubsection">
				2015	<a name="iterate_preds">Iterating over predecessors &
				2016	successors of blocks</a>
				2017	</div>
				2018
				2019	<div class="doc_text">
				2020
				2021	<p>Iterating over the predecessors and successors of a block is quite easy
				2022	with the routines defined in <tt>"llvm/Support/CFG.h"</tt>. Just use code like
				2023	this to iterate over all predecessors of BB:</p>
				2024
				2025	<div class="doc_code">
				2026	<pre>
				2027	#include "llvm/Support/CFG.h"
				2028	BasicBlock *BB = ...;
				2029
				2030	for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
				2031	BasicBlock Pred = PI;
				2032	// <i>...</i>
				2033	}
				2034	</pre>
				2035	</div>
				2036
				2037	<p>Similarly, to iterate over successors use
				2038	succ_iterator/succ_begin/succ_end.</p>
				2039
				2040	</div>
				2041
				2042
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	2043	<!-- ======================================================================= -->
				2044	<div class="doc_subsection">
				2045	<a name="simplechanges">Making simple changes</a>
				2046	</div>
				2047
				2048	<div class="doc_text">
				2049
				2050	<p>There are some primitive transformation operations present in the LLVM
				2051	infrastructure that are worth knowing about. When performing
				2052	transformations, it's fairly common to manipulate the contents of basic
				2053	blocks. This section describes some of the common methods for doing so
				2054	and gives example code.</p>
				2055
				2056	</div>
				2057
				2058	<!--_______________________________________________________________________-->
				2059	<div class="doc_subsubsection">
				2060	<a name="schanges_creating">Creating and inserting new
				2061	<tt>Instruction</tt>s</a>
				2062	</div>
				2063
				2064	<div class="doc_text">
				2065
				2066	<p><i>Instantiating Instructions</i></p>
				2067
				2068	<p>Creation of <tt>Instruction</tt>s is straight-forward: simply call the
				2069	constructor for the kind of instruction to instantiate and provide the necessary
				2070	parameters. For example, an <tt>AllocaInst</tt> only <i>requires</i> a
				2071	(const-ptr-to) <tt>Type</tt>. Thus:</p>
				2072
				2073	<div class="doc_code">
				2074	<pre>
Nick Lewycky	c004e5b	2007-12-03 01:52:52 +0000	[diff] [blame]	2075	AllocaInst* ai = new AllocaInst(Type::Int32Ty);
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	2076	</pre>
				2077	</div>
				2078
				2079	<p>will create an <tt>AllocaInst</tt> instance that represents the allocation of
				2080	one integer in the current stack frame, at run time. Each <tt>Instruction</tt>
				2081	subclass is likely to have varying default parameters which change the semantics
				2082	of the instruction, so refer to the <a
				2083	href="/doxygen/classllvm_1_1Instruction.html">doxygen documentation for the subclass of
				2084	Instruction</a> that you're interested in instantiating.</p>
				2085
				2086	<p><i>Naming values</i></p>
				2087
				2088	<p>It is very useful to name the values of instructions when you're able to, as
				2089	this facilitates the debugging of your transformations. If you end up looking
				2090	at generated LLVM machine code, you definitely want to have logical names
				2091	associated with the results of instructions! By supplying a value for the
				2092	<tt>Name</tt> (default) parameter of the <tt>Instruction</tt> constructor, you
				2093	associate a logical name with the result of the instruction's execution at
				2094	run time. For example, say that I'm writing a transformation that dynamically
				2095	allocates space for an integer on the stack, and that integer is going to be
				2096	used as some kind of index by some other code. To accomplish this, I place an
				2097	<tt>AllocaInst</tt> at the first point in the first <tt>BasicBlock</tt> of some
				2098	<tt>Function</tt>, and I'm intending to use it within the same
				2099	<tt>Function</tt>. I might do:</p>
				2100
				2101	<div class="doc_code">
				2102	<pre>
Nick Lewycky	c004e5b	2007-12-03 01:52:52 +0000	[diff] [blame]	2103	AllocaInst* pa = new AllocaInst(Type::Int32Ty, 0, "indexLoc");
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	2104	</pre>
				2105	</div>
				2106
				2107	<p>where <tt>indexLoc</tt> is now the logical name of the instruction's
				2108	execution value, which is a pointer to an integer on the run time stack.</p>
				2109
				2110	<p><i>Inserting instructions</i></p>
				2111
				2112	<p>There are essentially two ways to insert an <tt>Instruction</tt>
				2113	into an existing sequence of instructions that form a <tt>BasicBlock</tt>:</p>
				2114
				2115	<ul>
				2116	<li>Insertion into an explicit instruction list
				2117
				2118	<p>Given a <tt>BasicBlock* pb</tt>, an <tt>Instruction* pi</tt> within that
				2119	<tt>BasicBlock</tt>, and a newly-created instruction we wish to insert
				2120	before <tt>*pi</tt>, we do the following: </p>
				2121
				2122	<div class="doc_code">
				2123	<pre>
				2124	BasicBlock *pb = ...;
				2125	Instruction *pi = ...;
				2126	Instruction *newInst = new Instruction(...);
				2127
				2128	pb->getInstList().insert(pi, newInst); // <i>Inserts newInst before pi in pb</i>
				2129	</pre>
				2130	</div>
				2131
				2132	<p>Appending to the end of a <tt>BasicBlock</tt> is so common that
				2133	the <tt>Instruction</tt> class and <tt>Instruction</tt>-derived
				2134	classes provide constructors which take a pointer to a
				2135	<tt>BasicBlock</tt> to be appended to. For example code that
				2136	looked like: </p>
				2137
				2138	<div class="doc_code">
				2139	<pre>
				2140	BasicBlock *pb = ...;
				2141	Instruction *newInst = new Instruction(...);
				2142
				2143	pb->getInstList().push_back(newInst); // <i>Appends newInst to pb</i>
				2144	</pre>
				2145	</div>
				2146
				2147	<p>becomes: </p>
				2148
				2149	<div class="doc_code">
				2150	<pre>
				2151	BasicBlock *pb = ...;
				2152	Instruction *newInst = new Instruction(..., pb);
				2153	</pre>
				2154	</div>
				2155
				2156	<p>which is much cleaner, especially if you are creating
				2157	long instruction streams.</p></li>
				2158
				2159	<li>Insertion into an implicit instruction list
				2160
				2161	<p><tt>Instruction</tt> instances that are already in <tt>BasicBlock</tt>s
				2162	are implicitly associated with an existing instruction list: the instruction
				2163	list of the enclosing basic block. Thus, we could have accomplished the same
				2164	thing as the above code without being given a <tt>BasicBlock</tt> by doing:
				2165	</p>
				2166
				2167	<div class="doc_code">
				2168	<pre>
				2169	Instruction *pi = ...;
				2170	Instruction *newInst = new Instruction(...);
				2171
				2172	pi->getParent()->getInstList().insert(pi, newInst);
				2173	</pre>
				2174	</div>
				2175
				2176	<p>In fact, this sequence of steps occurs so frequently that the
				2177	<tt>Instruction</tt> class and <tt>Instruction</tt>-derived classes provide
				2178	constructors which take (as a default parameter) a pointer to an
				2179	<tt>Instruction</tt> which the newly-created <tt>Instruction</tt> should
				2180	precede. That is, <tt>Instruction</tt> constructors are capable of
				2181	inserting the newly-created instance into the <tt>BasicBlock</tt> of a
				2182	provided instruction, immediately before that instruction. Using an
				2183	<tt>Instruction</tt> constructor with a <tt>insertBefore</tt> (default)
				2184	parameter, the above code becomes:</p>
				2185
				2186	<div class="doc_code">
				2187	<pre>
				2188	Instruction* pi = ...;
				2189	Instruction* newInst = new Instruction(..., pi);
				2190	</pre>
				2191	</div>
				2192
				2193	<p>which is much cleaner, especially if you're creating a lot of
				2194	instructions and adding them to <tt>BasicBlock</tt>s.</p></li>
				2195	</ul>
				2196
				2197	</div>
				2198
				2199	<!--_______________________________________________________________________-->
				2200	<div class="doc_subsubsection">
				2201	<a name="schanges_deleting">Deleting <tt>Instruction</tt>s</a>
				2202	</div>
				2203
				2204	<div class="doc_text">
				2205
				2206	<p>Deleting an instruction from an existing sequence of instructions that form a
				2207	<a href="#BasicBlock"><tt>BasicBlock</tt></a> is very straight-forward. First,
				2208	you must have a pointer to the instruction that you wish to delete. Second, you
				2209	need to obtain the pointer to that instruction's basic block. You use the
				2210	pointer to the basic block to get its list of instructions and then use the
				2211	erase function to remove your instruction. For example:</p>
				2212
				2213	<div class="doc_code">
				2214	<pre>
				2215	<a href="#Instruction">Instruction</a> *I = .. ;
Chris Lattner	3db8f77	2008-02-15 22:57:17 +0000	[diff] [blame]	2216	I->eraseFromParent();
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	2217	</pre>
				2218	</div>
				2219
				2220	</div>
				2221
				2222	<!--_______________________________________________________________________-->
				2223	<div class="doc_subsubsection">
				2224	<a name="schanges_replacing">Replacing an <tt>Instruction</tt> with another
				2225	<tt>Value</tt></a>
				2226	</div>
				2227
				2228	<div class="doc_text">
				2229
				2230	<p><i>Replacing individual instructions</i></p>
				2231
				2232	<p>Including "<a href="/doxygen/BasicBlockUtils_8h-source.html">llvm/Transforms/Utils/BasicBlockUtils.h</a>"
				2233	permits use of two very useful replace functions: <tt>ReplaceInstWithValue</tt>
				2234	and <tt>ReplaceInstWithInst</tt>.</p>
				2235
				2236	<h4><a name="schanges_deleting">Deleting <tt>Instruction</tt>s</a></h4>
				2237
				2238	<ul>
				2239	<li><tt>ReplaceInstWithValue</tt>
				2240
Nick Lewycky	48d4b03	2008-09-15 06:31:52 +0000	[diff] [blame]	2241	<p>This function replaces all uses of a given instruction with a value,
				2242	and then removes the original instruction. The following example
				2243	illustrates the replacement of the result of a particular
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	2244	<tt>AllocaInst</tt> that allocates memory for a single integer with a null
				2245	pointer to an integer.</p>
				2246
				2247	<div class="doc_code">
				2248	<pre>
				2249	AllocaInst* instToReplace = ...;
				2250	BasicBlock::iterator ii(instToReplace);
				2251
				2252	ReplaceInstWithValue(instToReplace->getParent()->getInstList(), ii,
Daniel Dunbar	8ce7962	2008-10-03 22:17:25 +0000	[diff] [blame]	2253	Constant::getNullValue(PointerType::getUnqual(Type::Int32Ty)));
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	2254	</pre></div></li>
				2255
				2256	<li><tt>ReplaceInstWithInst</tt>
				2257
				2258	<p>This function replaces a particular instruction with another
Nick Lewycky	48d4b03	2008-09-15 06:31:52 +0000	[diff] [blame]	2259	instruction, inserting the new instruction into the basic block at the
				2260	location where the old instruction was, and replacing any uses of the old
				2261	instruction with the new instruction. The following example illustrates
				2262	the replacement of one <tt>AllocaInst</tt> with another.</p>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	2263
				2264	<div class="doc_code">
				2265	<pre>
				2266	AllocaInst* instToReplace = ...;
				2267	BasicBlock::iterator ii(instToReplace);
				2268
				2269	ReplaceInstWithInst(instToReplace->getParent()->getInstList(), ii,
Nick Lewycky	c004e5b	2007-12-03 01:52:52 +0000	[diff] [blame]	2270	new AllocaInst(Type::Int32Ty, 0, "ptrToReplacedInt"));
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	2271	</pre></div></li>
				2272	</ul>
				2273
				2274	<p><i>Replacing multiple uses of <tt>User</tt>s and <tt>Value</tt>s</i></p>
				2275
				2276	<p>You can use <tt>Value::replaceAllUsesWith</tt> and
				2277	<tt>User::replaceUsesOfWith</tt> to change more than one use at a time. See the
				2278	doxygen documentation for the <a href="/doxygen/classllvm_1_1Value.html">Value Class</a>
				2279	and <a href="/doxygen/classllvm_1_1User.html">User Class</a>, respectively, for more
				2280	information.</p>
				2281
				2282	<!-- Value::replaceAllUsesWith User::replaceUsesOfWith Point out:
				2283	include/llvm/Transforms/Utils/ especially BasicBlockUtils.h with:
				2284	ReplaceInstWithValue, ReplaceInstWithInst -->
				2285
				2286	</div>
				2287
				2288	<!--_______________________________________________________________________-->
				2289	<div class="doc_subsubsection">
				2290	<a name="schanges_deletingGV">Deleting <tt>GlobalVariable</tt>s</a>
				2291	</div>
				2292
				2293	<div class="doc_text">
				2294
				2295	<p>Deleting a global variable from a module is just as easy as deleting an
				2296	Instruction. First, you must have a pointer to the global variable that you wish
				2297	to delete. You use this pointer to erase it from its parent, the module.
				2298	For example:</p>
				2299
				2300	<div class="doc_code">
				2301	<pre>
				2302	<a href="#GlobalVariable">GlobalVariable</a> *GV = .. ;
				2303
				2304	GV->eraseFromParent();
				2305	</pre>
				2306	</div>
				2307
				2308	</div>
				2309
Jeffrey Yasskin	7ebb6ac	2009-04-30 22:33:41 +0000	[diff] [blame]	2310	<!-- ======================================================================= -->
				2311	<div class="doc_subsection">
				2312	<a name="create_types">How to Create Types</a>
				2313	</div>
				2314
				2315	<div class="doc_text">
				2316
				2317	<p>In generating IR, you may need some complex types. If you know these types
Misha Brukman	0672513	2009-05-01 20:40:51 +0000	[diff] [blame]	2318	statically, you can use <tt>TypeBuilder<...>::get()</tt>, defined
Jeffrey Yasskin	7ebb6ac	2009-04-30 22:33:41 +0000	[diff] [blame]	2319	in <tt>llvm/Support/TypeBuilder.h</tt>, to retrieve them. <tt>TypeBuilder</tt>
				2320	has two forms depending on whether you're building types for cross-compilation
Misha Brukman	0672513	2009-05-01 20:40:51 +0000	[diff] [blame]	2321	or native library use. <tt>TypeBuilder<T, true></tt> requires
Jeffrey Yasskin	7ebb6ac	2009-04-30 22:33:41 +0000	[diff] [blame]	2322	that <tt>T</tt> be independent of the host environment, meaning that it's built
				2323	out of types from
				2324	the <a href="/doxygen/namespacellvm_1_1types.html"><tt>llvm::types</tt></a>
				2325	namespace and pointers, functions, arrays, etc. built of
Misha Brukman	0672513	2009-05-01 20:40:51 +0000	[diff] [blame]	2326	those. <tt>TypeBuilder<T, false></tt> additionally allows native C types
Jeffrey Yasskin	7ebb6ac	2009-04-30 22:33:41 +0000	[diff] [blame]	2327	whose size may depend on the host compiler. For example,</p>
				2328
				2329	<div class="doc_code">
				2330	<pre>
Misha Brukman	0672513	2009-05-01 20:40:51 +0000	[diff] [blame]	2331	FunctionType ft = TypeBuilder<types::i<8>(types::i<32>), true>::get();
Jeffrey Yasskin	7ebb6ac	2009-04-30 22:33:41 +0000	[diff] [blame]	2332	</pre>
				2333	</div>
				2334
				2335	<p>is easier to read and write than the equivalent</p>
				2336
				2337	<div class="doc_code">
				2338	<pre>
Owen Anderson	60f8757	2009-06-16 17:40:28 +0000	[diff] [blame]	2339	std::vector<const Type*> params;
Jeffrey Yasskin	7ebb6ac	2009-04-30 22:33:41 +0000	[diff] [blame]	2340	params.push_back(PointerType::getUnqual(Type::Int32Ty));
				2341	FunctionType *ft = FunctionType::get(Type::Int8Ty, params, false);
				2342	</pre>
				2343	</div>
				2344
				2345	<p>See the <a href="/doxygen/TypeBuilder_8h-source.html#l00001">class
				2346	comment</a> for more details.</p>
				2347
				2348	</div>
				2349
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	2350	<!-- *********************************************************************** -->
				2351	<div class="doc_section">
Owen Anderson	e8370c0	2009-06-16 01:17:16 +0000	[diff] [blame]	2352	<a name="threading">Threads and LLVM</a>
				2353	</div>
				2354	<!-- *********************************************************************** -->
				2355
				2356	<div class="doc_text">
				2357	<p>
				2358	This section describes the interaction of the LLVM APIs with multithreading,
				2359	both on the part of client applications, and in the JIT, in the hosted
				2360	application.
				2361	</p>
				2362
				2363	<p>
				2364	Note that LLVM's support for multithreading is still relatively young. Up
				2365	through version 2.5, the execution of threaded hosted applications was
				2366	supported, but not threaded client access to the APIs. While this use case is
				2367	now supported, clients <em>must</em> adhere to the guidelines specified below to
				2368	ensure proper operation in multithreaded mode.
				2369	</p>
				2370
				2371	<p>
				2372	Note that, on Unix-like platforms, LLVM requires the presence of GCC's atomic
				2373	intrinsics in order to support threaded operation. If you need a
				2374	multhreading-capable LLVM on a platform without a suitably modern system
				2375	compiler, consider compiling LLVM and LLVM-GCC in single-threaded mode, and
				2376	using the resultant compiler to build a copy of LLVM with multithreading
				2377	support.
				2378	</p>
				2379	</div>
				2380
				2381	<!-- ======================================================================= -->
				2382	<div class="doc_subsection">
Owen Anderson	b418690	2009-06-16 18:04:19 +0000	[diff] [blame]	2383	<a name="startmultithreaded">Entering and Exiting Multithreaded Mode</a>
Owen Anderson	e8370c0	2009-06-16 01:17:16 +0000	[diff] [blame]	2384	</div>
				2385
				2386	<div class="doc_text">
				2387
				2388	<p>
				2389	In order to properly protect its internal data structures while avoiding
Owen Anderson	b418690	2009-06-16 18:04:19 +0000	[diff] [blame]	2390	excessive locking overhead in the single-threaded case, the LLVM must intialize
				2391	certain data structures necessary to provide guards around its internals. To do
				2392	so, the client program must invoke <tt>llvm_start_multithreaded()</tt> before
				2393	making any concurrent LLVM API calls. To subsequently tear down these
				2394	structures, use the <tt>llvm_stop_multithreaded()</tt> call. You can also use
				2395	the <tt>llvm_is_multithreaded()</tt> call to check the status of multithreaded
				2396	mode.
Owen Anderson	e8370c0	2009-06-16 01:17:16 +0000	[diff] [blame]	2397	</p>
				2398
				2399	<p>
Owen Anderson	b418690	2009-06-16 18:04:19 +0000	[diff] [blame]	2400	Note that both of these calls must be made <em>in isolation</em>. That is to
				2401	say that no other LLVM API calls may be executing at any time during the
				2402	execution of <tt>llvm_start_multithreaded()</tt> or <tt>llvm_stop_multithreaded
				2403	</tt>. It's is the client's responsibility to enforce this isolation.
				2404	</p>
				2405
				2406	<p>
				2407	The return value of <tt>llvm_start_multithreaded()</tt> indicates the success or
				2408	failure of the initialization. Failure typically indicates that your copy of
				2409	LLVM was built without multithreading support, typically because GCC atomic
				2410	intrinsics were not found in your system compiler. In this case, the LLVM API
				2411	will not be safe for concurrent calls. However, it <em>will</em> be safe for
Jeffrey Yasskin	4071514	2010-01-29 19:10:38 +0000	[diff] [blame]	2412	hosting threaded applications in the JIT, though <a href="#jitthreading">care
				2413	must be taken</a> to ensure that side exits and the like do not accidentally
				2414	result in concurrent LLVM API calls.
Owen Anderson	e8370c0	2009-06-16 01:17:16 +0000	[diff] [blame]	2415	</p>
				2416	</div>
				2417
				2418	<!-- ======================================================================= -->
				2419	<div class="doc_subsection">
				2420	<a name="shutdown">Ending Execution with <tt>llvm_shutdown()</tt></a>
				2421	</div>
				2422
				2423	<div class="doc_text">
				2424	<p>
				2425	When you are done using the LLVM APIs, you should call <tt>llvm_shutdown()</tt>
Owen Anderson	b418690	2009-06-16 18:04:19 +0000	[diff] [blame]	2426	to deallocate memory used for internal structures. This will also invoke
				2427	<tt>llvm_stop_multithreaded()</tt> if LLVM is operating in multithreaded mode.
				2428	As such, <tt>llvm_shutdown()</tt> requires the same isolation guarantees as
				2429	<tt>llvm_stop_multithreaded()</tt>.
Owen Anderson	e8370c0	2009-06-16 01:17:16 +0000	[diff] [blame]	2430	</p>
				2431
				2432	<p>
				2433	Note that, if you use scope-based shutdown, you can use the
				2434	<tt>llvm_shutdown_obj</tt> class, which calls <tt>llvm_shutdown()</tt> in its
				2435	destructor.
				2436	</div>
				2437
				2438	<!-- ======================================================================= -->
				2439	<div class="doc_subsection">
				2440	<a name="managedstatic">Lazy Initialization with <tt>ManagedStatic</tt></a>
				2441	</div>
				2442
				2443	<div class="doc_text">
				2444	<p>
				2445	<tt>ManagedStatic</tt> is a utility class in LLVM used to implement static
				2446	initialization of static resources, such as the global type tables. Before the
				2447	invocation of <tt>llvm_shutdown()</tt>, it implements a simple lazy
				2448	initialization scheme. Once <tt>llvm_start_multithreaded()</tt> returns,
				2449	however, it uses double-checked locking to implement thread-safe lazy
				2450	initialization.
				2451	</p>
				2452
				2453	<p>
				2454	Note that, because no other threads are allowed to issue LLVM API calls before
				2455	<tt>llvm_start_multithreaded()</tt> returns, it is possible to have
				2456	<tt>ManagedStatic</tt>s of <tt>llvm::sys::Mutex</tt>s.
				2457	</p>
Owen Anderson	b418690	2009-06-16 18:04:19 +0000	[diff] [blame]	2458
				2459	<p>
				2460	The <tt>llvm_acquire_global_lock()</tt> and <tt>llvm_release_global_lock</tt>
				2461	APIs provide access to the global lock used to implement the double-checked
				2462	locking for lazy initialization. These should only be used internally to LLVM,
				2463	and only if you know what you're doing!
				2464	</p>
Owen Anderson	e8370c0	2009-06-16 01:17:16 +0000	[diff] [blame]	2465	</div>
				2466
Owen Anderson	3f2da0e	2009-08-19 17:58:52 +0000	[diff] [blame]	2467	<!-- ======================================================================= -->
				2468	<div class="doc_subsection">
				2469	<a name="llvmcontext">Achieving Isolation with <tt>LLVMContext</tt></a>
				2470	</div>
				2471
				2472	<div class="doc_text">
				2473	<p>
				2474	<tt>LLVMContext</tt> is an opaque class in the LLVM API which clients can use
				2475	to operate multiple, isolated instances of LLVM concurrently within the same
				2476	address space. For instance, in a hypothetical compile-server, the compilation
				2477	of an individual translation unit is conceptually independent from all the
				2478	others, and it would be desirable to be able to compile incoming translation
				2479	units concurrently on independent server threads. Fortunately,
				2480	<tt>LLVMContext</tt> exists to enable just this kind of scenario!
				2481	</p>
				2482
				2483	<p>
				2484	Conceptually, <tt>LLVMContext</tt> provides isolation. Every LLVM entity
				2485	(<tt>Module</tt>s, <tt>Value</tt>s, <tt>Type</tt>s, <tt>Constant</tt>s, etc.)
Chris Lattner	9ca98ce	2009-08-20 03:10:14 +0000	[diff] [blame]	2486	in LLVM's in-memory IR belongs to an <tt>LLVMContext</tt>. Entities in
Owen Anderson	3f2da0e	2009-08-19 17:58:52 +0000	[diff] [blame]	2487	different contexts <em>cannot</em> interact with each other: <tt>Module</tt>s in
				2488	different contexts cannot be linked together, <tt>Function</tt>s cannot be added
				2489	to <tt>Module</tt>s in different contexts, etc. What this means is that is is
				2490	safe to compile on multiple threads simultaneously, as long as no two threads
				2491	operate on entities within the same context.
				2492	</p>
				2493
				2494	<p>
				2495	In practice, very few places in the API require the explicit specification of a
				2496	<tt>LLVMContext</tt>, other than the <tt>Type</tt> creation/lookup APIs.
				2497	Because every <tt>Type</tt> carries a reference to its owning context, most
				2498	other entities can determine what context they belong to by looking at their
				2499	own <tt>Type</tt>. If you are adding new entities to LLVM IR, please try to
				2500	maintain this interface design.
				2501	</p>
				2502
				2503	<p>
				2504	For clients that do <em>not</em> require the benefits of isolation, LLVM
				2505	provides a convenience API <tt>getGlobalContext()</tt>. This returns a global,
				2506	lazily initialized <tt>LLVMContext</tt> that may be used in situations where
				2507	isolation is not a concern.
				2508	</p>
				2509	</div>
				2510
Jeffrey Yasskin	4071514	2010-01-29 19:10:38 +0000	[diff] [blame]	2511	<!-- ======================================================================= -->
				2512	<div class="doc_subsection">
				2513	<a name="jitthreading">Threads and the JIT</a>
				2514	</div>
				2515
				2516	<div class="doc_text">
				2517	<p>
				2518	LLVM's "eager" JIT compiler is safe to use in threaded programs. Multiple
				2519	threads can call <tt>ExecutionEngine::getPointerToFunction()</tt> or
				2520	<tt>ExecutionEngine::runFunction()</tt> concurrently, and multiple threads can
				2521	run code output by the JIT concurrently. The user must still ensure that only
				2522	one thread accesses IR in a given <tt>LLVMContext</tt> while another thread
				2523	might be modifying it. One way to do that is to always hold the JIT lock while
				2524	accessing IR outside the JIT (the JIT <em>modifies</em> the IR by adding
				2525	<tt>CallbackVH</tt>s). Another way is to only
				2526	call <tt>getPointerToFunction()</tt> from the <tt>LLVMContext</tt>'s thread.
				2527	</p>
				2528
				2529	<p>When the JIT is configured to compile lazily (using
				2530	<tt>ExecutionEngine::DisableLazyCompilation(false)</tt>), there is currently a
				2531	<a href="http://llvm.org/bugs/show_bug.cgi?id=5184">race condition</a> in
				2532	updating call sites after a function is lazily-jitted. It's still possible to
				2533	use the lazy JIT in a threaded program if you ensure that only one thread at a
				2534	time can call any particular lazy stub and that the JIT lock guards any IR
				2535	access, but we suggest using only the eager JIT in threaded programs.
				2536	</p>
				2537	</div>
				2538
Owen Anderson	e8370c0	2009-06-16 01:17:16 +0000	[diff] [blame]	2539	<!-- *********************************************************************** -->
				2540	<div class="doc_section">
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	2541	<a name="advanced">Advanced Topics</a>
				2542	</div>
				2543	<!-- *********************************************************************** -->
				2544
				2545	<div class="doc_text">
				2546	<p>
				2547	This section describes some of the advanced or obscure API's that most clients
				2548	do not need to be aware of. These API's tend manage the inner workings of the
				2549	LLVM system, and only need to be accessed in unusual circumstances.
				2550	</p>
				2551	</div>
				2552
				2553	<!-- ======================================================================= -->
				2554	<div class="doc_subsection">
				2555	<a name="TypeResolve">LLVM Type Resolution</a>
				2556	</div>
				2557
				2558	<div class="doc_text">
				2559
				2560	<p>
				2561	The LLVM type system has a very simple goal: allow clients to compare types for
				2562	structural equality with a simple pointer comparison (aka a shallow compare).
				2563	This goal makes clients much simpler and faster, and is used throughout the LLVM
				2564	system.
				2565	</p>
				2566
				2567	<p>
				2568	Unfortunately achieving this goal is not a simple matter. In particular,
				2569	recursive types and late resolution of opaque types makes the situation very
				2570	difficult to handle. Fortunately, for the most part, our implementation makes
				2571	most clients able to be completely unaware of the nasty internal details. The
				2572	primary case where clients are exposed to the inner workings of it are when
				2573	building a recursive type. In addition to this case, the LLVM bitcode reader,
				2574	assembly parser, and linker also have to be aware of the inner workings of this
				2575	system.
				2576	</p>
				2577
				2578	<p>
				2579	For our purposes below, we need three concepts. First, an "Opaque Type" is
				2580	exactly as defined in the <a href="LangRef.html#t_opaque">language
				2581	reference</a>. Second an "Abstract Type" is any type which includes an
				2582	opaque type as part of its type graph (for example "<tt>{ opaque, i32 }</tt>").
				2583	Third, a concrete type is a type that is not an abstract type (e.g. "<tt>{ i32,
				2584	float }</tt>").
				2585	</p>
				2586
				2587	</div>
				2588
				2589	<!-- ______________________________________________________________________ -->
				2590	<div class="doc_subsubsection">
				2591	<a name="BuildRecType">Basic Recursive Type Construction</a>
				2592	</div>
				2593
				2594	<div class="doc_text">
				2595
				2596	<p>
				2597	Because the most common question is "how do I build a recursive type with LLVM",
				2598	we answer it now and explain it as we go. Here we include enough to cause this
				2599	to be emitted to an output .ll file:
				2600	</p>
				2601
				2602	<div class="doc_code">
				2603	<pre>
				2604	%mylist = type { %mylist*, i32 }
				2605	</pre>
				2606	</div>
				2607
				2608	<p>
				2609	To build this, use the following LLVM APIs:
				2610	</p>
				2611
				2612	<div class="doc_code">
				2613	<pre>
				2614	// <i>Create the initial outer struct</i>
				2615	<a href="#PATypeHolder">PATypeHolder</a> StructTy = OpaqueType::get();
				2616	std::vector<const Type*> Elts;
Daniel Dunbar	8ce7962	2008-10-03 22:17:25 +0000	[diff] [blame]	2617	Elts.push_back(PointerType::getUnqual(StructTy));
Nick Lewycky	c004e5b	2007-12-03 01:52:52 +0000	[diff] [blame]	2618	Elts.push_back(Type::Int32Ty);
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	2619	StructType *NewSTy = StructType::get(Elts);
				2620
				2621	// <i>At this point, NewSTy = "{ opaque*, i32 }". Tell VMCore that</i>
				2622	// <i>the struct and the opaque type are actually the same.</i>
				2623	cast<OpaqueType>(StructTy.get())-><a href="#refineAbstractTypeTo">refineAbstractTypeTo</a>(NewSTy);
				2624
				2625	// <i>NewSTy is potentially invalidated, but StructTy (a <a href="#PATypeHolder">PATypeHolder</a>) is</i>
				2626	// <i>kept up-to-date</i>
				2627	NewSTy = cast<StructType>(StructTy.get());
				2628
				2629	// <i>Add a name for the type to the module symbol table (optional)</i>
				2630	MyModule->addTypeName("mylist", NewSTy);
				2631	</pre>
				2632	</div>
				2633
				2634	<p>
				2635	This code shows the basic approach used to build recursive types: build a
				2636	non-recursive type using 'opaque', then use type unification to close the cycle.
				2637	The type unification step is performed by the <tt><a
				2638	href="#refineAbstractTypeTo">refineAbstractTypeTo</a></tt> method, which is
				2639	described next. After that, we describe the <a
				2640	href="#PATypeHolder">PATypeHolder class</a>.
				2641	</p>
				2642
				2643	</div>
				2644
				2645	<!-- ______________________________________________________________________ -->
				2646	<div class="doc_subsubsection">
				2647	<a name="refineAbstractTypeTo">The <tt>refineAbstractTypeTo</tt> method</a>
				2648	</div>
				2649
				2650	<div class="doc_text">
				2651	<p>
				2652	The <tt>refineAbstractTypeTo</tt> method starts the type unification process.
				2653	While this method is actually a member of the DerivedType class, it is most
				2654	often used on OpaqueType instances. Type unification is actually a recursive
				2655	process. After unification, types can become structurally isomorphic to
				2656	existing types, and all duplicates are deleted (to preserve pointer equality).
				2657	</p>
				2658
				2659	<p>
				2660	In the example above, the OpaqueType object is definitely deleted.
				2661	Additionally, if there is an "{ \2*, i32}" type already created in the system,
				2662	the pointer and struct type created are <b>also</b> deleted. Obviously whenever
				2663	a type is deleted, any "Type*" pointers in the program are invalidated. As
				2664	such, it is safest to avoid having <i>any</i> "Type*" pointers to abstract types
				2665	live across a call to <tt>refineAbstractTypeTo</tt> (note that non-abstract
				2666	types can never move or be deleted). To deal with this, the <a
				2667	href="#PATypeHolder">PATypeHolder</a> class is used to maintain a stable
				2668	reference to a possibly refined type, and the <a
				2669	href="#AbstractTypeUser">AbstractTypeUser</a> class is used to update more
				2670	complex datastructures.
				2671	</p>
				2672
				2673	</div>
				2674
				2675	<!-- ______________________________________________________________________ -->
				2676	<div class="doc_subsubsection">
				2677	<a name="PATypeHolder">The PATypeHolder Class</a>
				2678	</div>
				2679
				2680	<div class="doc_text">
				2681	<p>
				2682	PATypeHolder is a form of a "smart pointer" for Type objects. When VMCore
				2683	happily goes about nuking types that become isomorphic to existing types, it
				2684	automatically updates all PATypeHolder objects to point to the new type. In the
				2685	example above, this allows the code to maintain a pointer to the resultant
				2686	resolved recursive type, even though the Type*'s are potentially invalidated.
				2687	</p>
				2688
				2689	<p>
				2690	PATypeHolder is an extremely light-weight object that uses a lazy union-find
				2691	implementation to update pointers. For example the pointer from a Value to its
				2692	Type is maintained by PATypeHolder objects.
				2693	</p>
				2694
				2695	</div>
				2696
				2697	<!-- ______________________________________________________________________ -->
				2698	<div class="doc_subsubsection">
				2699	<a name="AbstractTypeUser">The AbstractTypeUser Class</a>
				2700	</div>
				2701
				2702	<div class="doc_text">
				2703
				2704	<p>
				2705	Some data structures need more to perform more complex updates when types get
				2706	resolved. To support this, a class can derive from the AbstractTypeUser class.
				2707	This class
				2708	allows it to get callbacks when certain types are resolved. To register to get
				2709	callbacks for a particular type, the DerivedType::{add/remove}AbstractTypeUser
				2710	methods can be called on a type. Note that these methods only work for <i>
				2711	abstract</i> types. Concrete types (those that do not include any opaque
				2712	objects) can never be refined.
				2713	</p>
				2714	</div>
				2715
				2716
				2717	<!-- ======================================================================= -->
				2718	<div class="doc_subsection">
				2719	<a name="SymbolTable">The <tt>ValueSymbolTable</tt> and
				2720	<tt>TypeSymbolTable</tt> classes</a>
				2721	</div>
				2722
				2723	<div class="doc_text">
				2724	<p>The <tt><a href="http://llvm.org/doxygen/classllvm_1_1ValueSymbolTable.html">
				2725	ValueSymbolTable</a></tt> class provides a symbol table that the <a
				2726	href="#Function"><tt>Function</tt></a> and <a href="#Module">
				2727	<tt>Module</tt></a> classes use for naming value definitions. The symbol table
				2728	can provide a name for any <a href="#Value"><tt>Value</tt></a>.
				2729	The <tt><a href="http://llvm.org/doxygen/classllvm_1_1TypeSymbolTable.html">
				2730	TypeSymbolTable</a></tt> class is used by the <tt>Module</tt> class to store
				2731	names for types.</p>
				2732
				2733	<p>Note that the <tt>SymbolTable</tt> class should not be directly accessed
				2734	by most clients. It should only be used when iteration over the symbol table
				2735	names themselves are required, which is very special purpose. Note that not
				2736	all LLVM
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2737	<tt><a href="#Value">Value</a></tt>s have names, and those without names (i.e. they have
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	2738	an empty name) do not exist in the symbol table.
				2739	</p>
				2740
				2741	<p>These symbol tables support iteration over the values/types in the symbol
				2742	table with <tt>begin/end/iterator</tt> and supports querying to see if a
				2743	specific name is in the symbol table (with <tt>lookup</tt>). The
				2744	<tt>ValueSymbolTable</tt> class exposes no public mutator methods, instead,
				2745	simply call <tt>setName</tt> on a value, which will autoinsert it into the
				2746	appropriate symbol table. For types, use the Module::addTypeName method to
				2747	insert entries into the symbol table.</p>
				2748
				2749	</div>
				2750
				2751
				2752
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2753	<!-- ======================================================================= -->
				2754	<div class="doc_subsection">
				2755	<a name="UserLayout">The <tt>User</tt> and owned <tt>Use</tt> classes' memory layout</a>
				2756	</div>
				2757
				2758	<div class="doc_text">
				2759	<p>The <tt><a href="http://llvm.org/doxygen/classllvm_1_1User.html">
Gabor Greif	50626fc	2009-01-05 16:05:32 +0000	[diff] [blame]	2760	User</a></tt> class provides a basis for expressing the ownership of <tt>User</tt>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2761	towards other <tt><a href="http://llvm.org/doxygen/classllvm_1_1Value.html">
				2762	Value</a></tt>s. The <tt><a href="http://llvm.org/doxygen/classllvm_1_1Use.html">
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2763	Use</a></tt> helper class is employed to do the bookkeeping and to facilitate <i>O(1)</i>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2764	addition and removal.</p>
				2765
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2766	<!-- ______________________________________________________________________ -->
				2767	<div class="doc_subsubsection">
Gabor Greif	50626fc	2009-01-05 16:05:32 +0000	[diff] [blame]	2768	<a name="Use2User">Interaction and relationship between <tt>User</tt> and <tt>Use</tt> objects</a>
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2769	</div>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2770
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2771	<div class="doc_text">
				2772	<p>
				2773	A subclass of <tt>User</tt> can choose between incorporating its <tt>Use</tt> objects
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2774	or refer to them out-of-line by means of a pointer. A mixed variant
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2775	(some <tt>Use</tt>s inline others hung off) is impractical and breaks the invariant
				2776	that the <tt>Use</tt> objects belonging to the same <tt>User</tt> form a contiguous array.
				2777	</p>
				2778	</div>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2779
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2780	<p>
				2781	We have 2 different layouts in the <tt>User</tt> (sub)classes:
				2782	<ul>
				2783	<li><p>Layout a)
				2784	The <tt>Use</tt> object(s) are inside (resp. at fixed offset) of the <tt>User</tt>
				2785	object and there are a fixed number of them.</p>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2786
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2787	<li><p>Layout b)
				2788	The <tt>Use</tt> object(s) are referenced by a pointer to an
				2789	array from the <tt>User</tt> object and there may be a variable
				2790	number of them.</p>
				2791	</ul>
				2792	<p>
Gabor Greif	e247e36	2008-06-25 00:10:22 +0000	[diff] [blame]	2793	As of v2.4 each layout still possesses a direct pointer to the
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2794	start of the array of <tt>Use</tt>s. Though not mandatory for layout a),
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2795	we stick to this redundancy for the sake of simplicity.
Gabor Greif	e247e36	2008-06-25 00:10:22 +0000	[diff] [blame]	2796	The <tt>User</tt> object also stores the number of <tt>Use</tt> objects it
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2797	has. (Theoretically this information can also be calculated
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2798	given the scheme presented below.)</p>
				2799	<p>
				2800	Special forms of allocation operators (<tt>operator new</tt>)
Gabor Greif	e247e36	2008-06-25 00:10:22 +0000	[diff] [blame]	2801	enforce the following memory layouts:</p>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2802
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2803	<ul>
Gabor Greif	e247e36	2008-06-25 00:10:22 +0000	[diff] [blame]	2804	<li><p>Layout a) is modelled by prepending the <tt>User</tt> object by the <tt>Use[]</tt> array.</p>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2805
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2806	<pre>
				2807	...---.---.---.---.-------...
				2808	\| P \| P \| P \| P \| User
				2809	'''---'---'---'---'-------'''
				2810	</pre>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2811
Gabor Greif	e247e36	2008-06-25 00:10:22 +0000	[diff] [blame]	2812	<li><p>Layout b) is modelled by pointing at the <tt>Use[]</tt> array.</p>
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2813	<pre>
				2814	.-------...
				2815	\| User
				2816	'-------'''
				2817	\|
				2818	v
				2819	.---.---.---.---...
				2820	\| P \| P \| P \| P \|
				2821	'---'---'---'---'''
				2822	</pre>
				2823	</ul>
				2824	<i>(In the above figures '<tt>P</tt>' stands for the <tt>Use**</tt> that
				2825	is stored in each <tt>Use</tt> object in the member <tt>Use::Prev</tt>)</i>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2826
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2827	<!-- ______________________________________________________________________ -->
				2828	<div class="doc_subsubsection">
Gabor Greif	50626fc	2009-01-05 16:05:32 +0000	[diff] [blame]	2829	<a name="Waymarking">The waymarking algorithm</a>
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2830	</div>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2831
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2832	<div class="doc_text">
				2833	<p>
Gabor Greif	e247e36	2008-06-25 00:10:22 +0000	[diff] [blame]	2834	Since the <tt>Use</tt> objects are deprived of the direct (back)pointer to
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2835	their <tt>User</tt> objects, there must be a fast and exact method to
				2836	recover it. This is accomplished by the following scheme:</p>
				2837	</div>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2838
Gabor Greif	e247e36	2008-06-25 00:10:22 +0000	[diff] [blame]	2839	A bit-encoding in the 2 LSBits (least significant bits) of the <tt>Use::Prev</tt> allows to find the
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2840	start of the <tt>User</tt> object:
				2841	<ul>
				2842	<li><tt>00</tt> —> binary digit 0</li>
				2843	<li><tt>01</tt> —> binary digit 1</li>
				2844	<li><tt>10</tt> —> stop and calculate (<tt>s</tt>)</li>
				2845	<li><tt>11</tt> —> full stop (<tt>S</tt>)</li>
				2846	</ul>
				2847	<p>
				2848	Given a <tt>Use*</tt>, all we have to do is to walk till we get
				2849	a stop and we either have a <tt>User</tt> immediately behind or
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2850	we have to walk to the next stop picking up digits
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2851	and calculating the offset:</p>
				2852	<pre>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2853	.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.----------------
				2854	\| 1 \| s \| 1 \| 0 \| 1 \| 0 \| s \| 1 \| 1 \| 0 \| s \| 1 \| 1 \| s \| 1 \| S \| User (or User*)
				2855	'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'----------------
				2856	\|+15 \|+10 \|+6 \|+3 \|+1
				2857	\| \| \| \| \|__>
				2858	\| \| \| \|__________>
				2859	\| \| \|______________________>
				2860	\| \|______________________________________>
				2861	\|__________________________________________________________>
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2862	</pre>
				2863	<p>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2864	Only the significant number of bits need to be stored between the
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2865	stops, so that the <i>worst case is 20 memory accesses</i> when there are
				2866	1000 <tt>Use</tt> objects associated with a <tt>User</tt>.</p>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2867
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2868	<!-- ______________________________________________________________________ -->
				2869	<div class="doc_subsubsection">
Gabor Greif	50626fc	2009-01-05 16:05:32 +0000	[diff] [blame]	2870	<a name="ReferenceImpl">Reference implementation</a>
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2871	</div>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2872
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2873	<div class="doc_text">
				2874	<p>
				2875	The following literate Haskell fragment demonstrates the concept:</p>
				2876	</div>
				2877
				2878	<div class="doc_code">
				2879	<pre>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2880	> import Test.QuickCheck
				2881	>
				2882	> digits :: Int -> [Char] -> [Char]
				2883	> digits 0 acc = '0' : acc
				2884	> digits 1 acc = '1' : acc
				2885	> digits n acc = digits (n `div` 2) $ digits (n `mod` 2) acc
				2886	>
				2887	> dist :: Int -> [Char] -> [Char]
				2888	> dist 0 [] = ['S']
				2889	> dist 0 acc = acc
				2890	> dist 1 acc = let r = dist 0 acc in 's' : digits (length r) r
				2891	> dist n acc = dist (n - 1) $ dist 1 acc
				2892	>
				2893	> takeLast n ss = reverse $ take n $ reverse ss
				2894	>
				2895	> test = takeLast 40 $ dist 20 []
				2896	>
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2897	</pre>
				2898	</div>
				2899	<p>
				2900	Printing <test> gives: <tt>"1s100000s11010s10100s1111s1010s110s11s1S"</tt></p>
				2901	<p>
				2902	The reverse algorithm computes the length of the string just by examining
				2903	a certain prefix:</p>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2904
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2905	<div class="doc_code">
				2906	<pre>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2907	> pref :: [Char] -> Int
				2908	> pref "S" = 1
				2909	> pref ('s':'1':rest) = decode 2 1 rest
				2910	> pref (_:rest) = 1 + pref rest
				2911	>
				2912	> decode walk acc ('0':rest) = decode (walk + 1) (acc * 2) rest
				2913	> decode walk acc ('1':rest) = decode (walk + 1) (acc * 2 + 1) rest
				2914	> decode walk acc _ = walk + acc
				2915	>
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2916	</pre>
				2917	</div>
				2918	<p>
				2919	Now, as expected, printing <pref test> gives <tt>40</tt>.</p>
				2920	<p>
				2921	We can <i>quickCheck</i> this with following property:</p>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2922
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2923	<div class="doc_code">
				2924	<pre>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2925	> testcase = dist 2000 []
				2926	> testcaseLength = length testcase
				2927	>
				2928	> identityProp n = n > 0 && n <= testcaseLength ==> length arr == pref arr
				2929	> where arr = takeLast n testcase
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2930	>
				2931	</pre>
				2932	</div>
				2933	<p>
				2934	As expected <quickCheck identityProp> gives:</p>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2935
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2936	<pre>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2937	*Main> quickCheck identityProp
				2938	OK, passed 100 tests.
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2939	</pre>
				2940	<p>
				2941	Let's be a bit more exhaustive:</p>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2942
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2943	<div class="doc_code">
				2944	<pre>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2945	>
				2946	> deepCheck p = check (defaultConfig { configMaxTest = 500 }) p
				2947	>
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2948	</pre>
				2949	</div>
				2950	<p>
				2951	And here is the result of <deepCheck identityProp>:</p>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2952
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2953	<pre>
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2954	*Main> deepCheck identityProp
				2955	OK, passed 500 tests.
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2956	</pre>
				2957
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2958	<!-- ______________________________________________________________________ -->
				2959	<div class="doc_subsubsection">
Gabor Greif	50626fc	2009-01-05 16:05:32 +0000	[diff] [blame]	2960	<a name="Tagging">Tagging considerations</a>
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2961	</div>
				2962
				2963	<p>
				2964	To maintain the invariant that the 2 LSBits of each <tt>Use**</tt> in <tt>Use</tt>
				2965	never change after being set up, setters of <tt>Use::Prev</tt> must re-tag the
				2966	new <tt>Use**</tt> on every modification. Accordingly getters must strip the
				2967	tag bits.</p>
				2968	<p>
Gabor Greif	e247e36	2008-06-25 00:10:22 +0000	[diff] [blame]	2969	For layout b) instead of the <tt>User</tt> we find a pointer (<tt>User*</tt> with LSBit set).
				2970	Following this pointer brings us to the <tt>User</tt>. A portable trick ensures
				2971	that the first bytes of <tt>User</tt> (if interpreted as a pointer) never has
Gabor Greif	50626fc	2009-01-05 16:05:32 +0000	[diff] [blame]	2972	the LSBit set. (Portability is relying on the fact that all known compilers place the
				2973	<tt>vptr</tt> in the first word of the instances.)</p>
Gabor Greif	93b462b	2008-06-18 13:44:57 +0000	[diff] [blame]	2974
Gabor Greif	92e8776	2008-06-16 21:06:12 +0000	[diff] [blame]	2975	</div>
				2976
				2977	<!-- *********************************************************************** -->
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	2978	<div class="doc_section">
				2979	<a name="coreclasses">The Core LLVM Class Hierarchy Reference </a>
				2980	</div>
				2981	<!-- *********************************************************************** -->
				2982
				2983	<div class="doc_text">
				2984	<p><tt>#include "<a href="/doxygen/Type_8h-source.html">llvm/Type.h</a>"</tt>
				2985	<br>doxygen info: <a href="/doxygen/classllvm_1_1Type.html">Type Class</a></p>
				2986
				2987	<p>The Core LLVM classes are the primary means of representing the program
				2988	being inspected or transformed. The core LLVM classes are defined in
				2989	header files in the <tt>include/llvm/</tt> directory, and implemented in
				2990	the <tt>lib/VMCore</tt> directory.</p>
				2991
				2992	</div>
				2993
				2994	<!-- ======================================================================= -->
				2995	<div class="doc_subsection">
				2996	<a name="Type">The <tt>Type</tt> class and Derived Types</a>
				2997	</div>
				2998
				2999	<div class="doc_text">
				3000
				3001	<p><tt>Type</tt> is a superclass of all type classes. Every <tt>Value</tt> has
				3002	a <tt>Type</tt>. <tt>Type</tt> cannot be instantiated directly but only
				3003	through its subclasses. Certain primitive types (<tt>VoidType</tt>,
				3004	<tt>LabelType</tt>, <tt>FloatType</tt> and <tt>DoubleType</tt>) have hidden
				3005	subclasses. They are hidden because they offer no useful functionality beyond
				3006	what the <tt>Type</tt> class offers except to distinguish themselves from
				3007	other subclasses of <tt>Type</tt>.</p>
				3008	<p>All other types are subclasses of <tt>DerivedType</tt>. Types can be
				3009	named, but this is not a requirement. There exists exactly
				3010	one instance of a given shape at any one time. This allows type equality to
				3011	be performed with address equality of the Type Instance. That is, given two
				3012	<tt>Type*</tt> values, the types are identical if the pointers are identical.
				3013	</p>
				3014	</div>
				3015
				3016	<!-- _______________________________________________________________________ -->
				3017	<div class="doc_subsubsection">
Gabor Greif	50626fc	2009-01-05 16:05:32 +0000	[diff] [blame]	3018	<a name="m_Type">Important Public Methods</a>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	3019	</div>
				3020
				3021	<div class="doc_text">
				3022
				3023	<ul>
Duncan Sands	e92dee1	2010-02-15 16:12:20 +0000	[diff] [blame]	3024	<li><tt>bool isIntegerTy() const</tt>: Returns true for any integer type.</li>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	3025
Duncan Sands	e92dee1	2010-02-15 16:12:20 +0000	[diff] [blame]	3026	<li><tt>bool isFloatingPointTy()</tt>: Return true if this is one of the five
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	3027	floating point types.</li>
				3028
				3029	<li><tt>bool isAbstract()</tt>: Return true if the type is abstract (contains
				3030	an OpaqueType anywhere in its definition).</li>
				3031
				3032	<li><tt>bool isSized()</tt>: Return true if the type has known size. Things
				3033	that don't have a size are abstract types, labels and void.</li>
				3034
				3035	</ul>
				3036	</div>
				3037
				3038	<!-- _______________________________________________________________________ -->
				3039	<div class="doc_subsubsection">
Gabor Greif	50626fc	2009-01-05 16:05:32 +0000	[diff] [blame]	3040	<a name="derivedtypes">Important Derived Types</a>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	3041	</div>
				3042	<div class="doc_text">
				3043	<dl>
				3044	<dt><tt>IntegerType</tt></dt>
				3045	<dd>Subclass of DerivedType that represents integer types of any bit width.
				3046	Any bit width between <tt>IntegerType::MIN_INT_BITS</tt> (1) and
				3047	<tt>IntegerType::MAX_INT_BITS</tt> (~8 million) can be represented.
				3048	<ul>
				3049	<li><tt>static const IntegerType* get(unsigned NumBits)</tt>: get an integer
				3050	type of a specific bit width.</li>
				3051	<li><tt>unsigned getBitWidth() const</tt>: Get the bit width of an integer
				3052	type.</li>
				3053	</ul>
				3054	</dd>
				3055	<dt><tt>SequentialType</tt></dt>
				3056	<dd>This is subclassed by ArrayType and PointerType
				3057	<ul>
				3058	<li><tt>const Type * getElementType() const</tt>: Returns the type of each
				3059	of the elements in the sequential type. </li>
				3060	</ul>
				3061	</dd>
				3062	<dt><tt>ArrayType</tt></dt>
				3063	<dd>This is a subclass of SequentialType and defines the interface for array
				3064	types.
				3065	<ul>
				3066	<li><tt>unsigned getNumElements() const</tt>: Returns the number of
				3067	elements in the array. </li>
				3068	</ul>
				3069	</dd>
				3070	<dt><tt>PointerType</tt></dt>
				3071	<dd>Subclass of SequentialType for pointer types.</dd>
				3072	<dt><tt>VectorType</tt></dt>
				3073	<dd>Subclass of SequentialType for vector types. A
				3074	vector type is similar to an ArrayType but is distinguished because it is
Benjamin Kramer	5fb9d7e	2009-10-12 14:46:08 +0000	[diff] [blame]	3075	a first class type whereas ArrayType is not. Vector types are used for
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	3076	vector operations and are usually small vectors of of an integer or floating
				3077	point type.</dd>
				3078	<dt><tt>StructType</tt></dt>
				3079	<dd>Subclass of DerivedTypes for struct types.</dd>
				3080	<dt><tt><a name="FunctionType">FunctionType</a></tt></dt>
				3081	<dd>Subclass of DerivedTypes for function types.
				3082	<ul>
Dan Gohman	41dc51c	2010-03-30 20:04:57 +0000	[diff] [blame]	3083	<li><tt>bool isVarArg() const</tt>: Returns true if it's a vararg
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	3084	function</li>
				3085	<li><tt> const Type * getReturnType() const</tt>: Returns the
				3086	return type of the function.</li>
				3087	<li><tt>const Type * getParamType (unsigned i)</tt>: Returns
				3088	the type of the ith parameter.</li>
				3089	<li><tt> const unsigned getNumParams() const</tt>: Returns the
				3090	number of formal parameters.</li>
				3091	</ul>
				3092	</dd>
				3093	<dt><tt>OpaqueType</tt></dt>
				3094	<dd>Sublcass of DerivedType for abstract types. This class
				3095	defines no content and is used as a placeholder for some other type. Note
				3096	that OpaqueType is used (temporarily) during type resolution for forward
				3097	references of types. Once the referenced type is resolved, the OpaqueType
				3098	is replaced with the actual type. OpaqueType can also be used for data
				3099	abstraction. At link time opaque types can be resolved to actual types
				3100	of the same name.</dd>
				3101	</dl>
				3102	</div>
				3103
				3104
				3105
				3106	<!-- ======================================================================= -->
				3107	<div class="doc_subsection">
				3108	<a name="Module">The <tt>Module</tt> class</a>
				3109	</div>
				3110
				3111	<div class="doc_text">
				3112
				3113	<p><tt>#include "<a
				3114	href="/doxygen/Module_8h-source.html">llvm/Module.h</a>"</tt><br> doxygen info:
				3115	<a href="/doxygen/classllvm_1_1Module.html">Module Class</a></p>
				3116
				3117	<p>The <tt>Module</tt> class represents the top level structure present in LLVM
				3118	programs. An LLVM module is effectively either a translation unit of the
				3119	original program or a combination of several translation units merged by the
				3120	linker. The <tt>Module</tt> class keeps track of a list of <a
				3121	href="#Function"><tt>Function</tt></a>s, a list of <a
				3122	href="#GlobalVariable"><tt>GlobalVariable</tt></a>s, and a <a
				3123	href="#SymbolTable"><tt>SymbolTable</tt></a>. Additionally, it contains a few
				3124	helpful member functions that try to make common operations easy.</p>
				3125
				3126	</div>
				3127
				3128	<!-- _______________________________________________________________________ -->
				3129	<div class="doc_subsubsection">
				3130	<a name="m_Module">Important Public Members of the <tt>Module</tt> class</a>
				3131	</div>
				3132
				3133	<div class="doc_text">
				3134
				3135	<ul>
				3136	<li><tt>Module::Module(std::string name = "")</tt></li>
				3137	</ul>
				3138
				3139	<p>Constructing a <a href="#Module">Module</a> is easy. You can optionally
				3140	provide a name for it (probably based on the name of the translation unit).</p>
				3141
				3142	<ul>
				3143	<li><tt>Module::iterator</tt> - Typedef for function list iterator<br>
				3144	<tt>Module::const_iterator</tt> - Typedef for const_iterator.<br>
				3145
				3146	<tt>begin()</tt>, <tt>end()</tt>
				3147	<tt>size()</tt>, <tt>empty()</tt>
				3148
				3149	<p>These are forwarding methods that make it easy to access the contents of
				3150	a <tt>Module</tt> object's <a href="#Function"><tt>Function</tt></a>
				3151	list.</p></li>
				3152
				3153	<li><tt>Module::FunctionListType &getFunctionList()</tt>
				3154
				3155	<p> Returns the list of <a href="#Function"><tt>Function</tt></a>s. This is
				3156	necessary to use when you need to update the list or perform a complex
				3157	action that doesn't have a forwarding method.</p>
				3158
				3159	<p><!-- Global Variable --></p></li>
				3160	</ul>
				3161
				3162	<hr>
				3163
				3164	<ul>
				3165	<li><tt>Module::global_iterator</tt> - Typedef for global variable list iterator<br>
				3166
				3167	<tt>Module::const_global_iterator</tt> - Typedef for const_iterator.<br>
				3168
				3169	<tt>global_begin()</tt>, <tt>global_end()</tt>
				3170	<tt>global_size()</tt>, <tt>global_empty()</tt>
				3171
				3172	<p> These are forwarding methods that make it easy to access the contents of
				3173	a <tt>Module</tt> object's <a
				3174	href="#GlobalVariable"><tt>GlobalVariable</tt></a> list.</p></li>
				3175
				3176	<li><tt>Module::GlobalListType &getGlobalList()</tt>
				3177
				3178	<p>Returns the list of <a
				3179	href="#GlobalVariable"><tt>GlobalVariable</tt></a>s. This is necessary to
				3180	use when you need to update the list or perform a complex action that
				3181	doesn't have a forwarding method.</p>
				3182
				3183	<p><!-- Symbol table stuff --> </p></li>
				3184	</ul>
				3185
				3186	<hr>
				3187
				3188	<ul>
				3189	<li><tt><a href="#SymbolTable">SymbolTable</a> *getSymbolTable()</tt>
				3190
				3191	<p>Return a reference to the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
				3192	for this <tt>Module</tt>.</p>
				3193
				3194	<p><!-- Convenience methods --></p></li>
				3195	</ul>
				3196
				3197	<hr>
				3198
				3199	<ul>
				3200	<li><tt><a href="#Function">Function</a> *getFunction(const std::string
				3201	&Name, const <a href="#FunctionType">FunctionType</a> *Ty)</tt>
				3202
				3203	<p>Look up the specified function in the <tt>Module</tt> <a
				3204	href="#SymbolTable"><tt>SymbolTable</tt></a>. If it does not exist, return
				3205	<tt>null</tt>.</p></li>
				3206
				3207	<li><tt><a href="#Function">Function</a> *getOrInsertFunction(const
				3208	std::string &Name, const <a href="#FunctionType">FunctionType</a> *T)</tt>
				3209
				3210	<p>Look up the specified function in the <tt>Module</tt> <a
				3211	href="#SymbolTable"><tt>SymbolTable</tt></a>. If it does not exist, add an
				3212	external declaration for the function and return it.</p></li>
				3213
				3214	<li><tt>std::string getTypeName(const <a href="#Type">Type</a> *Ty)</tt>
				3215
				3216	<p>If there is at least one entry in the <a
				3217	href="#SymbolTable"><tt>SymbolTable</tt></a> for the specified <a
				3218	href="#Type"><tt>Type</tt></a>, return it. Otherwise return the empty
				3219	string.</p></li>
				3220
				3221	<li><tt>bool addTypeName(const std::string &Name, const <a
				3222	href="#Type">Type</a> *Ty)</tt>
				3223
				3224	<p>Insert an entry in the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
				3225	mapping <tt>Name</tt> to <tt>Ty</tt>. If there is already an entry for this
				3226	name, true is returned and the <a
				3227	href="#SymbolTable"><tt>SymbolTable</tt></a> is not modified.</p></li>
				3228	</ul>
				3229
				3230	</div>
				3231
				3232
				3233	<!-- ======================================================================= -->
				3234	<div class="doc_subsection">
				3235	<a name="Value">The <tt>Value</tt> class</a>
				3236	</div>
				3237
				3238	<div class="doc_text">
				3239
				3240	<p><tt>#include "<a href="/doxygen/Value_8h-source.html">llvm/Value.h</a>"</tt>
				3241	<br>
				3242	doxygen info: <a href="/doxygen/classllvm_1_1Value.html">Value Class</a></p>
				3243
				3244	<p>The <tt>Value</tt> class is the most important class in the LLVM Source
				3245	base. It represents a typed value that may be used (among other things) as an
				3246	operand to an instruction. There are many different types of <tt>Value</tt>s,
				3247	such as <a href="#Constant"><tt>Constant</tt></a>s,<a
				3248	href="#Argument"><tt>Argument</tt></a>s. Even <a
				3249	href="#Instruction"><tt>Instruction</tt></a>s and <a
				3250	href="#Function"><tt>Function</tt></a>s are <tt>Value</tt>s.</p>
				3251
				3252	<p>A particular <tt>Value</tt> may be used many times in the LLVM representation
				3253	for a program. For example, an incoming argument to a function (represented
				3254	with an instance of the <a href="#Argument">Argument</a> class) is "used" by
				3255	every instruction in the function that references the argument. To keep track
				3256	of this relationship, the <tt>Value</tt> class keeps a list of all of the <a
				3257	href="#User"><tt>User</tt></a>s that is using it (the <a
				3258	href="#User"><tt>User</tt></a> class is a base class for all nodes in the LLVM
				3259	graph that can refer to <tt>Value</tt>s). This use list is how LLVM represents
				3260	def-use information in the program, and is accessible through the <tt>use_</tt>*
				3261	methods, shown below.</p>
				3262
				3263	<p>Because LLVM is a typed representation, every LLVM <tt>Value</tt> is typed,
				3264	and this <a href="#Type">Type</a> is available through the <tt>getType()</tt>
				3265	method. In addition, all LLVM values can be named. The "name" of the
				3266	<tt>Value</tt> is a symbolic string printed in the LLVM code:</p>
				3267
				3268	<div class="doc_code">
				3269	<pre>
				3270	%<b>foo</b> = add i32 1, 2
				3271	</pre>
				3272	</div>
				3273
				3274	<p><a name="nameWarning">The name of this instruction is "foo".</a> <b>NOTE</b>
				3275	that the name of any value may be missing (an empty string), so names should
				3276	<b>ONLY</b> be used for debugging (making the source code easier to read,
				3277	debugging printouts), they should not be used to keep track of values or map
				3278	between them. For this purpose, use a <tt>std::map</tt> of pointers to the
				3279	<tt>Value</tt> itself instead.</p>
				3280
				3281	<p>One important aspect of LLVM is that there is no distinction between an SSA
				3282	variable and the operation that produces it. Because of this, any reference to
				3283	the value produced by an instruction (or the value available as an incoming
				3284	argument, for example) is represented as a direct pointer to the instance of
				3285	the class that
				3286	represents this value. Although this may take some getting used to, it
				3287	simplifies the representation and makes it easier to manipulate.</p>
				3288
				3289	</div>
				3290
				3291	<!-- _______________________________________________________________________ -->
				3292	<div class="doc_subsubsection">
				3293	<a name="m_Value">Important Public Members of the <tt>Value</tt> class</a>
				3294	</div>
				3295
				3296	<div class="doc_text">
				3297
				3298	<ul>
				3299	<li><tt>Value::use_iterator</tt> - Typedef for iterator over the
				3300	use-list<br>
Gabor Greif	4c1ffad	2010-03-26 19:59:25 +0000	[diff] [blame]	3301	<tt>Value::const_use_iterator</tt> - Typedef for const_iterator over
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	3302	the use-list<br>
				3303	<tt>unsigned use_size()</tt> - Returns the number of users of the
				3304	value.<br>
				3305	<tt>bool use_empty()</tt> - Returns true if there are no users.<br>
				3306	<tt>use_iterator use_begin()</tt> - Get an iterator to the start of
				3307	the use-list.<br>
				3308	<tt>use_iterator use_end()</tt> - Get an iterator to the end of the
				3309	use-list.<br>
				3310	<tt><a href="#User">User</a> *use_back()</tt> - Returns the last
				3311	element in the list.
				3312	<p> These methods are the interface to access the def-use
				3313	information in LLVM. As with all other iterators in LLVM, the naming
				3314	conventions follow the conventions defined by the <a href="#stl">STL</a>.</p>
				3315	</li>
				3316	<li><tt><a href="#Type">Type</a> *getType() const</tt>
				3317	<p>This method returns the Type of the Value.</p>
				3318	</li>
				3319	<li><tt>bool hasName() const</tt><br>
				3320	<tt>std::string getName() const</tt><br>
				3321	<tt>void setName(const std::string &Name)</tt>
				3322	<p> This family of methods is used to access and assign a name to a <tt>Value</tt>,
				3323	be aware of the <a href="#nameWarning">precaution above</a>.</p>
				3324	</li>
				3325	<li><tt>void replaceAllUsesWith(Value *V)</tt>
				3326
				3327	<p>This method traverses the use list of a <tt>Value</tt> changing all <a
				3328	href="#User"><tt>User</tt>s</a> of the current value to refer to
				3329	"<tt>V</tt>" instead. For example, if you detect that an instruction always
				3330	produces a constant value (for example through constant folding), you can
				3331	replace all uses of the instruction with the constant like this:</p>
				3332
				3333	<div class="doc_code">
				3334	<pre>
				3335	Inst->replaceAllUsesWith(ConstVal);
				3336	</pre>
				3337	</div>
				3338
				3339	</ul>
				3340
				3341	</div>
				3342
				3343	<!-- ======================================================================= -->
				3344	<div class="doc_subsection">
				3345	<a name="User">The <tt>User</tt> class</a>
				3346	</div>
				3347
				3348	<div class="doc_text">
				3349
				3350	<p>
				3351	<tt>#include "<a href="/doxygen/User_8h-source.html">llvm/User.h</a>"</tt><br>
				3352	doxygen info: <a href="/doxygen/classllvm_1_1User.html">User Class</a><br>
				3353	Superclass: <a href="#Value"><tt>Value</tt></a></p>
				3354
				3355	<p>The <tt>User</tt> class is the common base class of all LLVM nodes that may
				3356	refer to <a href="#Value"><tt>Value</tt></a>s. It exposes a list of "Operands"
				3357	that are all of the <a href="#Value"><tt>Value</tt></a>s that the User is
				3358	referring to. The <tt>User</tt> class itself is a subclass of
				3359	<tt>Value</tt>.</p>
				3360
				3361	<p>The operands of a <tt>User</tt> point directly to the LLVM <a
				3362	href="#Value"><tt>Value</tt></a> that it refers to. Because LLVM uses Static
				3363	Single Assignment (SSA) form, there can only be one definition referred to,
				3364	allowing this direct connection. This connection provides the use-def
				3365	information in LLVM.</p>
				3366
				3367	</div>
				3368
				3369	<!-- _______________________________________________________________________ -->
				3370	<div class="doc_subsubsection">
				3371	<a name="m_User">Important Public Members of the <tt>User</tt> class</a>
				3372	</div>
				3373
				3374	<div class="doc_text">
				3375
				3376	<p>The <tt>User</tt> class exposes the operand list in two ways: through
				3377	an index access interface and through an iterator based interface.</p>
				3378
				3379	<ul>
				3380	<li><tt>Value *getOperand(unsigned i)</tt><br>
				3381	<tt>unsigned getNumOperands()</tt>
				3382	<p> These two methods expose the operands of the <tt>User</tt> in a
				3383	convenient form for direct access.</p></li>
				3384
				3385	<li><tt>User::op_iterator</tt> - Typedef for iterator over the operand
				3386	list<br>
				3387	<tt>op_iterator op_begin()</tt> - Get an iterator to the start of
				3388	the operand list.<br>
				3389	<tt>op_iterator op_end()</tt> - Get an iterator to the end of the
				3390	operand list.
				3391	<p> Together, these methods make up the iterator based interface to
				3392	the operands of a <tt>User</tt>.</p></li>
				3393	</ul>
				3394
				3395	</div>
				3396
				3397	<!-- ======================================================================= -->
				3398	<div class="doc_subsection">
				3399	<a name="Instruction">The <tt>Instruction</tt> class</a>
				3400	</div>
				3401
				3402	<div class="doc_text">
				3403
				3404	<p><tt>#include "</tt><tt><a
				3405	href="/doxygen/Instruction_8h-source.html">llvm/Instruction.h</a>"</tt><br>
				3406	doxygen info: <a href="/doxygen/classllvm_1_1Instruction.html">Instruction Class</a><br>
				3407	Superclasses: <a href="#User"><tt>User</tt></a>, <a
				3408	href="#Value"><tt>Value</tt></a></p>
				3409
				3410	<p>The <tt>Instruction</tt> class is the common base class for all LLVM
				3411	instructions. It provides only a few methods, but is a very commonly used
				3412	class. The primary data tracked by the <tt>Instruction</tt> class itself is the
				3413	opcode (instruction type) and the parent <a
				3414	href="#BasicBlock"><tt>BasicBlock</tt></a> the <tt>Instruction</tt> is embedded
				3415	into. To represent a specific type of instruction, one of many subclasses of
				3416	<tt>Instruction</tt> are used.</p>
				3417
				3418	<p> Because the <tt>Instruction</tt> class subclasses the <a
				3419	href="#User"><tt>User</tt></a> class, its operands can be accessed in the same
				3420	way as for other <a href="#User"><tt>User</tt></a>s (with the
				3421	<tt>getOperand()</tt>/<tt>getNumOperands()</tt> and
				3422	<tt>op_begin()</tt>/<tt>op_end()</tt> methods).</p> <p> An important file for
				3423	the <tt>Instruction</tt> class is the <tt>llvm/Instruction.def</tt> file. This
				3424	file contains some meta-data about the various different types of instructions
				3425	in LLVM. It describes the enum values that are used as opcodes (for example
				3426	<tt>Instruction::Add</tt> and <tt>Instruction::ICmp</tt>), as well as the
				3427	concrete sub-classes of <tt>Instruction</tt> that implement the instruction (for
				3428	example <tt><a href="#BinaryOperator">BinaryOperator</a></tt> and <tt><a
				3429	href="#CmpInst">CmpInst</a></tt>). Unfortunately, the use of macros in
				3430	this file confuses doxygen, so these enum values don't show up correctly in the
				3431	<a href="/doxygen/classllvm_1_1Instruction.html">doxygen output</a>.</p>
				3432
				3433	</div>
				3434
				3435	<!-- _______________________________________________________________________ -->
				3436	<div class="doc_subsubsection">
				3437	<a name="s_Instruction">Important Subclasses of the <tt>Instruction</tt>
				3438	class</a>
				3439	</div>
				3440	<div class="doc_text">
				3441	<ul>
				3442	<li><tt><a name="BinaryOperator">BinaryOperator</a></tt>
				3443	<p>This subclasses represents all two operand instructions whose operands
				3444	must be the same type, except for the comparison instructions.</p></li>
				3445	<li><tt><a name="CastInst">CastInst</a></tt>
				3446	<p>This subclass is the parent of the 12 casting instructions. It provides
				3447	common operations on cast instructions.</p>
				3448	<li><tt><a name="CmpInst">CmpInst</a></tt>
				3449	<p>This subclass respresents the two comparison instructions,
				3450	<a href="LangRef.html#i_icmp">ICmpInst</a> (integer opreands), and
				3451	<a href="LangRef.html#i_fcmp">FCmpInst</a> (floating point operands).</p>
				3452	<li><tt><a name="TerminatorInst">TerminatorInst</a></tt>
				3453	<p>This subclass is the parent of all terminator instructions (those which
				3454	can terminate a block).</p>
				3455	</ul>
				3456	</div>
				3457
				3458	<!-- _______________________________________________________________________ -->
				3459	<div class="doc_subsubsection">
				3460	<a name="m_Instruction">Important Public Members of the <tt>Instruction</tt>
				3461	class</a>
				3462	</div>
				3463
				3464	<div class="doc_text">
				3465
				3466	<ul>
				3467	<li><tt><a href="#BasicBlock">BasicBlock</a> *getParent()</tt>
				3468	<p>Returns the <a href="#BasicBlock"><tt>BasicBlock</tt></a> that
				3469	this <tt>Instruction</tt> is embedded into.</p></li>
				3470	<li><tt>bool mayWriteToMemory()</tt>
				3471	<p>Returns true if the instruction writes to memory, i.e. it is a
				3472	<tt>call</tt>,<tt>free</tt>,<tt>invoke</tt>, or <tt>store</tt>.</p></li>
				3473	<li><tt>unsigned getOpcode()</tt>
				3474	<p>Returns the opcode for the <tt>Instruction</tt>.</p></li>
				3475	<li><tt><a href="#Instruction">Instruction</a> *clone() const</tt>
				3476	<p>Returns another instance of the specified instruction, identical
				3477	in all ways to the original except that the instruction has no parent
				3478	(ie it's not embedded into a <a href="#BasicBlock"><tt>BasicBlock</tt></a>),
				3479	and it has no name</p></li>
				3480	</ul>
				3481
				3482	</div>
				3483
				3484	<!-- ======================================================================= -->
				3485	<div class="doc_subsection">
				3486	<a name="Constant">The <tt>Constant</tt> class and subclasses</a>
				3487	</div>
				3488
				3489	<div class="doc_text">
				3490
				3491	<p>Constant represents a base class for different types of constants. It
				3492	is subclassed by ConstantInt, ConstantArray, etc. for representing
				3493	the various types of Constants. <a href="#GlobalValue">GlobalValue</a> is also
				3494	a subclass, which represents the address of a global variable or function.
				3495	</p>
				3496
				3497	</div>
				3498
				3499	<!-- _______________________________________________________________________ -->
				3500	<div class="doc_subsubsection">Important Subclasses of Constant </div>
				3501	<div class="doc_text">
				3502	<ul>
				3503	<li>ConstantInt : This subclass of Constant represents an integer constant of
				3504	any width.
				3505	<ul>
				3506	<li><tt>const APInt& getValue() const</tt>: Returns the underlying
				3507	value of this constant, an APInt value.</li>
				3508	<li><tt>int64_t getSExtValue() const</tt>: Converts the underlying APInt
				3509	value to an int64_t via sign extension. If the value (not the bit width)
				3510	of the APInt is too large to fit in an int64_t, an assertion will result.
				3511	For this reason, use of this method is discouraged.</li>
				3512	<li><tt>uint64_t getZExtValue() const</tt>: Converts the underlying APInt
				3513	value to a uint64_t via zero extension. IF the value (not the bit width)
				3514	of the APInt is too large to fit in a uint64_t, an assertion will result.
				3515	For this reason, use of this method is discouraged.</li>
				3516	<li><tt>static ConstantInt* get(const APInt& Val)</tt>: Returns the
				3517	ConstantInt object that represents the value provided by <tt>Val</tt>.
				3518	The type is implied as the IntegerType that corresponds to the bit width
				3519	of <tt>Val</tt>.</li>
				3520	<li><tt>static ConstantInt* get(const Type *Ty, uint64_t Val)</tt>:
				3521	Returns the ConstantInt object that represents the value provided by
				3522	<tt>Val</tt> for integer type <tt>Ty</tt>.</li>
				3523	</ul>
				3524	</li>
				3525	<li>ConstantFP : This class represents a floating point constant.
				3526	<ul>
				3527	<li><tt>double getValue() const</tt>: Returns the underlying value of
				3528	this constant. </li>
				3529	</ul>
				3530	</li>
				3531	<li>ConstantArray : This represents a constant array.
				3532	<ul>
				3533	<li><tt>const std::vector<Use> &getValues() const</tt>: Returns
				3534	a vector of component constants that makeup this array. </li>
				3535	</ul>
				3536	</li>
				3537	<li>ConstantStruct : This represents a constant struct.
				3538	<ul>
				3539	<li><tt>const std::vector<Use> &getValues() const</tt>: Returns
				3540	a vector of component constants that makeup this array. </li>
				3541	</ul>
				3542	</li>
				3543	<li>GlobalValue : This represents either a global variable or a function. In
				3544	either case, the value is a constant fixed address (after linking).
				3545	</li>
				3546	</ul>
				3547	</div>
				3548
				3549
				3550	<!-- ======================================================================= -->
				3551	<div class="doc_subsection">
				3552	<a name="GlobalValue">The <tt>GlobalValue</tt> class</a>
				3553	</div>
				3554
				3555	<div class="doc_text">
				3556
				3557	<p><tt>#include "<a
				3558	href="/doxygen/GlobalValue_8h-source.html">llvm/GlobalValue.h</a>"</tt><br>
				3559	doxygen info: <a href="/doxygen/classllvm_1_1GlobalValue.html">GlobalValue
				3560	Class</a><br>
				3561	Superclasses: <a href="#Constant"><tt>Constant</tt></a>,
				3562	<a href="#User"><tt>User</tt></a>, <a href="#Value"><tt>Value</tt></a></p>
				3563
				3564	<p>Global values (<a href="#GlobalVariable"><tt>GlobalVariable</tt></a>s or <a
				3565	href="#Function"><tt>Function</tt></a>s) are the only LLVM values that are
				3566	visible in the bodies of all <a href="#Function"><tt>Function</tt></a>s.
				3567	Because they are visible at global scope, they are also subject to linking with
				3568	other globals defined in different translation units. To control the linking
				3569	process, <tt>GlobalValue</tt>s know their linkage rules. Specifically,
				3570	<tt>GlobalValue</tt>s know whether they have internal or external linkage, as
				3571	defined by the <tt>LinkageTypes</tt> enumeration.</p>
				3572
				3573	<p>If a <tt>GlobalValue</tt> has internal linkage (equivalent to being
				3574	<tt>static</tt> in C), it is not visible to code outside the current translation
				3575	unit, and does not participate in linking. If it has external linkage, it is
				3576	visible to external code, and does participate in linking. In addition to
				3577	linkage information, <tt>GlobalValue</tt>s keep track of which <a
				3578	href="#Module"><tt>Module</tt></a> they are currently part of.</p>
				3579
				3580	<p>Because <tt>GlobalValue</tt>s are memory objects, they are always referred to
				3581	by their <b>address</b>. As such, the <a href="#Type"><tt>Type</tt></a> of a
				3582	global is always a pointer to its contents. It is important to remember this
				3583	when using the <tt>GetElementPtrInst</tt> instruction because this pointer must
				3584	be dereferenced first. For example, if you have a <tt>GlobalVariable</tt> (a
				3585	subclass of <tt>GlobalValue)</tt> that is an array of 24 ints, type <tt>[24 x
				3586	i32]</tt>, then the <tt>GlobalVariable</tt> is a pointer to that array. Although
				3587	the address of the first element of this array and the value of the
				3588	<tt>GlobalVariable</tt> are the same, they have different types. The
				3589	<tt>GlobalVariable</tt>'s type is <tt>[24 x i32]</tt>. The first element's type
				3590	is <tt>i32.</tt> Because of this, accessing a global value requires you to
				3591	dereference the pointer with <tt>GetElementPtrInst</tt> first, then its elements
				3592	can be accessed. This is explained in the <a href="LangRef.html#globalvars">LLVM
				3593	Language Reference Manual</a>.</p>
				3594
				3595	</div>
				3596
				3597	<!-- _______________________________________________________________________ -->
				3598	<div class="doc_subsubsection">
				3599	<a name="m_GlobalValue">Important Public Members of the <tt>GlobalValue</tt>
				3600	class</a>
				3601	</div>
				3602
				3603	<div class="doc_text">
				3604
				3605	<ul>
				3606	<li><tt>bool hasInternalLinkage() const</tt><br>
				3607	<tt>bool hasExternalLinkage() const</tt><br>
				3608	<tt>void setInternalLinkage(bool HasInternalLinkage)</tt>
				3609	<p> These methods manipulate the linkage characteristics of the <tt>GlobalValue</tt>.</p>
				3610	<p> </p>
				3611	</li>
				3612	<li><tt><a href="#Module">Module</a> *getParent()</tt>
				3613	<p> This returns the <a href="#Module"><tt>Module</tt></a> that the
				3614	GlobalValue is currently embedded into.</p></li>
				3615	</ul>
				3616
				3617	</div>
				3618
				3619	<!-- ======================================================================= -->
				3620	<div class="doc_subsection">
				3621	<a name="Function">The <tt>Function</tt> class</a>
				3622	</div>
				3623
				3624	<div class="doc_text">
				3625
				3626	<p><tt>#include "<a
				3627	href="/doxygen/Function_8h-source.html">llvm/Function.h</a>"</tt><br> doxygen
				3628	info: <a href="/doxygen/classllvm_1_1Function.html">Function Class</a><br>
				3629	Superclasses: <a href="#GlobalValue"><tt>GlobalValue</tt></a>,
				3630	<a href="#Constant"><tt>Constant</tt></a>,
				3631	<a href="#User"><tt>User</tt></a>,
				3632	<a href="#Value"><tt>Value</tt></a></p>
				3633
				3634	<p>The <tt>Function</tt> class represents a single procedure in LLVM. It is
Edwin Török	b26e279	2009-10-12 13:37:29 +0000	[diff] [blame]	3635	actually one of the more complex classes in the LLVM hierarchy because it must
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	3636	keep track of a large amount of data. The <tt>Function</tt> class keeps track
				3637	of a list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s, a list of formal
				3638	<a href="#Argument"><tt>Argument</tt></a>s, and a
				3639	<a href="#SymbolTable"><tt>SymbolTable</tt></a>.</p>
				3640
				3641	<p>The list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s is the most
				3642	commonly used part of <tt>Function</tt> objects. The list imposes an implicit
				3643	ordering of the blocks in the function, which indicate how the code will be
Benjamin Kramer	5fb9d7e	2009-10-12 14:46:08 +0000	[diff] [blame]	3644	laid out by the backend. Additionally, the first <a
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	3645	href="#BasicBlock"><tt>BasicBlock</tt></a> is the implicit entry node for the
				3646	<tt>Function</tt>. It is not legal in LLVM to explicitly branch to this initial
				3647	block. There are no implicit exit nodes, and in fact there may be multiple exit
				3648	nodes from a single <tt>Function</tt>. If the <a
				3649	href="#BasicBlock"><tt>BasicBlock</tt></a> list is empty, this indicates that
				3650	the <tt>Function</tt> is actually a function declaration: the actual body of the
				3651	function hasn't been linked in yet.</p>
				3652
				3653	<p>In addition to a list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s, the
				3654	<tt>Function</tt> class also keeps track of the list of formal <a
				3655	href="#Argument"><tt>Argument</tt></a>s that the function receives. This
				3656	container manages the lifetime of the <a href="#Argument"><tt>Argument</tt></a>
				3657	nodes, just like the <a href="#BasicBlock"><tt>BasicBlock</tt></a> list does for
				3658	the <a href="#BasicBlock"><tt>BasicBlock</tt></a>s.</p>
				3659
				3660	<p>The <a href="#SymbolTable"><tt>SymbolTable</tt></a> is a very rarely used
				3661	LLVM feature that is only used when you have to look up a value by name. Aside
				3662	from that, the <a href="#SymbolTable"><tt>SymbolTable</tt></a> is used
				3663	internally to make sure that there are not conflicts between the names of <a
				3664	href="#Instruction"><tt>Instruction</tt></a>s, <a
				3665	href="#BasicBlock"><tt>BasicBlock</tt></a>s, or <a
				3666	href="#Argument"><tt>Argument</tt></a>s in the function body.</p>
				3667
				3668	<p>Note that <tt>Function</tt> is a <a href="#GlobalValue">GlobalValue</a>
				3669	and therefore also a <a href="#Constant">Constant</a>. The value of the function
				3670	is its address (after linking) which is guaranteed to be constant.</p>
				3671	</div>
				3672
				3673	<!-- _______________________________________________________________________ -->
				3674	<div class="doc_subsubsection">
				3675	<a name="m_Function">Important Public Members of the <tt>Function</tt>
				3676	class</a>
				3677	</div>
				3678
				3679	<div class="doc_text">
				3680
				3681	<ul>
				3682	<li><tt>Function(const </tt><tt><a href="#FunctionType">FunctionType</a>
				3683	Ty, LinkageTypes Linkage, const std::string &N = "", Module Parent = 0)</tt>
				3684
				3685	<p>Constructor used when you need to create new <tt>Function</tt>s to add
				3686	the the program. The constructor must specify the type of the function to
				3687	create and what type of linkage the function should have. The <a
				3688	href="#FunctionType"><tt>FunctionType</tt></a> argument
				3689	specifies the formal arguments and return value for the function. The same
				3690	<a href="#FunctionType"><tt>FunctionType</tt></a> value can be used to
				3691	create multiple functions. The <tt>Parent</tt> argument specifies the Module
				3692	in which the function is defined. If this argument is provided, the function
				3693	will automatically be inserted into that module's list of
				3694	functions.</p></li>
				3695
Chris Lattner	5e70957	2008-11-25 18:34:50 +0000	[diff] [blame]	3696	<li><tt>bool isDeclaration()</tt>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	3697
				3698	<p>Return whether or not the <tt>Function</tt> has a body defined. If the
				3699	function is "external", it does not have a body, and thus must be resolved
				3700	by linking with a function defined in a different translation unit.</p></li>
				3701
				3702	<li><tt>Function::iterator</tt> - Typedef for basic block list iterator<br>
				3703	<tt>Function::const_iterator</tt> - Typedef for const_iterator.<br>
				3704
				3705	<tt>begin()</tt>, <tt>end()</tt>
				3706	<tt>size()</tt>, <tt>empty()</tt>
				3707
				3708	<p>These are forwarding methods that make it easy to access the contents of
				3709	a <tt>Function</tt> object's <a href="#BasicBlock"><tt>BasicBlock</tt></a>
				3710	list.</p></li>
				3711
				3712	<li><tt>Function::BasicBlockListType &getBasicBlockList()</tt>
				3713
				3714	<p>Returns the list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s. This
				3715	is necessary to use when you need to update the list or perform a complex
				3716	action that doesn't have a forwarding method.</p></li>
				3717
				3718	<li><tt>Function::arg_iterator</tt> - Typedef for the argument list
				3719	iterator<br>
				3720	<tt>Function::const_arg_iterator</tt> - Typedef for const_iterator.<br>
				3721
				3722	<tt>arg_begin()</tt>, <tt>arg_end()</tt>
				3723	<tt>arg_size()</tt>, <tt>arg_empty()</tt>
				3724
				3725	<p>These are forwarding methods that make it easy to access the contents of
				3726	a <tt>Function</tt> object's <a href="#Argument"><tt>Argument</tt></a>
				3727	list.</p></li>
				3728
				3729	<li><tt>Function::ArgumentListType &getArgumentList()</tt>
				3730
				3731	<p>Returns the list of <a href="#Argument"><tt>Argument</tt></a>s. This is
				3732	necessary to use when you need to update the list or perform a complex
				3733	action that doesn't have a forwarding method.</p></li>
				3734
				3735	<li><tt><a href="#BasicBlock">BasicBlock</a> &getEntryBlock()</tt>
				3736
				3737	<p>Returns the entry <a href="#BasicBlock"><tt>BasicBlock</tt></a> for the
				3738	function. Because the entry block for the function is always the first
				3739	block, this returns the first block of the <tt>Function</tt>.</p></li>
				3740
				3741	<li><tt><a href="#Type">Type</a> *getReturnType()</tt><br>
				3742	<tt><a href="#FunctionType">FunctionType</a> *getFunctionType()</tt>
				3743
				3744	<p>This traverses the <a href="#Type"><tt>Type</tt></a> of the
				3745	<tt>Function</tt> and returns the return type of the function, or the <a
				3746	href="#FunctionType"><tt>FunctionType</tt></a> of the actual
				3747	function.</p></li>
				3748
				3749	<li><tt><a href="#SymbolTable">SymbolTable</a> *getSymbolTable()</tt>
				3750
				3751	<p> Return a pointer to the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
				3752	for this <tt>Function</tt>.</p></li>
				3753	</ul>
				3754
				3755	</div>
				3756
				3757	<!-- ======================================================================= -->
				3758	<div class="doc_subsection">
				3759	<a name="GlobalVariable">The <tt>GlobalVariable</tt> class</a>
				3760	</div>
				3761
				3762	<div class="doc_text">
				3763
				3764	<p><tt>#include "<a
				3765	href="/doxygen/GlobalVariable_8h-source.html">llvm/GlobalVariable.h</a>"</tt>
				3766	<br>
				3767	doxygen info: <a href="/doxygen/classllvm_1_1GlobalVariable.html">GlobalVariable
				3768	Class</a><br>
				3769	Superclasses: <a href="#GlobalValue"><tt>GlobalValue</tt></a>,
				3770	<a href="#Constant"><tt>Constant</tt></a>,
				3771	<a href="#User"><tt>User</tt></a>,
				3772	<a href="#Value"><tt>Value</tt></a></p>
				3773
Benjamin Kramer	5fb9d7e	2009-10-12 14:46:08 +0000	[diff] [blame]	3774	<p>Global variables are represented with the (surprise surprise)
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	3775	<tt>GlobalVariable</tt> class. Like functions, <tt>GlobalVariable</tt>s are also
				3776	subclasses of <a href="#GlobalValue"><tt>GlobalValue</tt></a>, and as such are
				3777	always referenced by their address (global values must live in memory, so their
				3778	"name" refers to their constant address). See
				3779	<a href="#GlobalValue"><tt>GlobalValue</tt></a> for more on this. Global
				3780	variables may have an initial value (which must be a
				3781	<a href="#Constant"><tt>Constant</tt></a>), and if they have an initializer,
				3782	they may be marked as "constant" themselves (indicating that their contents
				3783	never change at runtime).</p>
				3784	</div>
				3785
				3786	<!-- _______________________________________________________________________ -->
				3787	<div class="doc_subsubsection">
				3788	<a name="m_GlobalVariable">Important Public Members of the
				3789	<tt>GlobalVariable</tt> class</a>
				3790	</div>
				3791
				3792	<div class="doc_text">
				3793
				3794	<ul>
				3795	<li><tt>GlobalVariable(const </tt><tt><a href="#Type">Type</a> *Ty, bool
				3796	isConstant, LinkageTypes& Linkage, <a href="#Constant">Constant</a>
				3797	Initializer = 0, const std::string &Name = "", Module Parent = 0)</tt>
				3798
				3799	<p>Create a new global variable of the specified type. If
				3800	<tt>isConstant</tt> is true then the global variable will be marked as
				3801	unchanging for the program. The Linkage parameter specifies the type of
Duncan Sands	19d161f	2009-03-07 15:45:40 +0000	[diff] [blame]	3802	linkage (internal, external, weak, linkonce, appending) for the variable.
				3803	If the linkage is InternalLinkage, WeakAnyLinkage, WeakODRLinkage,
				3804	LinkOnceAnyLinkage or LinkOnceODRLinkage,  then the resultant
				3805	global variable will have internal linkage. AppendingLinkage concatenates
				3806	together all instances (in different translation units) of the variable
				3807	into a single variable but is only applicable to arrays.  See
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	3808	the <a href="LangRef.html#modulestructure">LLVM Language Reference</a> for
				3809	further details on linkage types. Optionally an initializer, a name, and the
				3810	module to put the variable into may be specified for the global variable as
				3811	well.</p></li>
				3812
				3813	<li><tt>bool isConstant() const</tt>
				3814
				3815	<p>Returns true if this is a global variable that is known not to
				3816	be modified at runtime.</p></li>
				3817
				3818	<li><tt>bool hasInitializer()</tt>
				3819
				3820	<p>Returns true if this <tt>GlobalVariable</tt> has an intializer.</p></li>
				3821
				3822	<li><tt><a href="#Constant">Constant</a> *getInitializer()</tt>
				3823
Benjamin Kramer	5fb9d7e	2009-10-12 14:46:08 +0000	[diff] [blame]	3824	<p>Returns the initial value for a <tt>GlobalVariable</tt>. It is not legal
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	3825	to call this method if there is no initializer.</p></li>
				3826	</ul>
				3827
				3828	</div>
				3829
				3830
				3831	<!-- ======================================================================= -->
				3832	<div class="doc_subsection">
				3833	<a name="BasicBlock">The <tt>BasicBlock</tt> class</a>
				3834	</div>
				3835
				3836	<div class="doc_text">
				3837
				3838	<p><tt>#include "<a
				3839	href="/doxygen/BasicBlock_8h-source.html">llvm/BasicBlock.h</a>"</tt><br>
Stefanus Du Toit	f017c85	2009-06-17 21:12:26 +0000	[diff] [blame]	3840	doxygen info: <a href="/doxygen/classllvm_1_1BasicBlock.html">BasicBlock
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	3841	Class</a><br>
				3842	Superclass: <a href="#Value"><tt>Value</tt></a></p>
				3843
				3844	<p>This class represents a single entry multiple exit section of the code,
				3845	commonly known as a basic block by the compiler community. The
				3846	<tt>BasicBlock</tt> class maintains a list of <a
				3847	href="#Instruction"><tt>Instruction</tt></a>s, which form the body of the block.
				3848	Matching the language definition, the last element of this list of instructions
				3849	is always a terminator instruction (a subclass of the <a
				3850	href="#TerminatorInst"><tt>TerminatorInst</tt></a> class).</p>
				3851
				3852	<p>In addition to tracking the list of instructions that make up the block, the
				3853	<tt>BasicBlock</tt> class also keeps track of the <a
				3854	href="#Function"><tt>Function</tt></a> that it is embedded into.</p>
				3855
				3856	<p>Note that <tt>BasicBlock</tt>s themselves are <a
				3857	href="#Value"><tt>Value</tt></a>s, because they are referenced by instructions
				3858	like branches and can go in the switch tables. <tt>BasicBlock</tt>s have type
				3859	<tt>label</tt>.</p>
				3860
				3861	</div>
				3862
				3863	<!-- _______________________________________________________________________ -->
				3864	<div class="doc_subsubsection">
				3865	<a name="m_BasicBlock">Important Public Members of the <tt>BasicBlock</tt>
				3866	class</a>
				3867	</div>
				3868
				3869	<div class="doc_text">
				3870	<ul>
				3871
				3872	<li><tt>BasicBlock(const std::string &Name = "", </tt><tt><a
				3873	href="#Function">Function</a> *Parent = 0)</tt>
				3874
				3875	<p>The <tt>BasicBlock</tt> constructor is used to create new basic blocks for
				3876	insertion into a function. The constructor optionally takes a name for the new
				3877	block, and a <a href="#Function"><tt>Function</tt></a> to insert it into. If
				3878	the <tt>Parent</tt> parameter is specified, the new <tt>BasicBlock</tt> is
				3879	automatically inserted at the end of the specified <a
				3880	href="#Function"><tt>Function</tt></a>, if not specified, the BasicBlock must be
				3881	manually inserted into the <a href="#Function"><tt>Function</tt></a>.</p></li>
				3882
				3883	<li><tt>BasicBlock::iterator</tt> - Typedef for instruction list iterator<br>
				3884	<tt>BasicBlock::const_iterator</tt> - Typedef for const_iterator.<br>
				3885	<tt>begin()</tt>, <tt>end()</tt>, <tt>front()</tt>, <tt>back()</tt>,
				3886	<tt>size()</tt>, <tt>empty()</tt>
				3887	STL-style functions for accessing the instruction list.
				3888
				3889	<p>These methods and typedefs are forwarding functions that have the same
				3890	semantics as the standard library methods of the same names. These methods
				3891	expose the underlying instruction list of a basic block in a way that is easy to
				3892	manipulate. To get the full complement of container operations (including
				3893	operations to update the list), you must use the <tt>getInstList()</tt>
				3894	method.</p></li>
				3895
				3896	<li><tt>BasicBlock::InstListType &getInstList()</tt>
				3897
				3898	<p>This method is used to get access to the underlying container that actually
				3899	holds the Instructions. This method must be used when there isn't a forwarding
				3900	function in the <tt>BasicBlock</tt> class for the operation that you would like
				3901	to perform. Because there are no forwarding functions for "updating"
				3902	operations, you need to use this if you want to update the contents of a
				3903	<tt>BasicBlock</tt>.</p></li>
				3904
				3905	<li><tt><a href="#Function">Function</a> *getParent()</tt>
				3906
				3907	<p> Returns a pointer to <a href="#Function"><tt>Function</tt></a> the block is
				3908	embedded into, or a null pointer if it is homeless.</p></li>
				3909
				3910	<li><tt><a href="#TerminatorInst">TerminatorInst</a> *getTerminator()</tt>
				3911
				3912	<p> Returns a pointer to the terminator instruction that appears at the end of
				3913	the <tt>BasicBlock</tt>. If there is no terminator instruction, or if the last
				3914	instruction in the block is not a terminator, then a null pointer is
				3915	returned.</p></li>
				3916
				3917	</ul>
				3918
				3919	</div>
				3920
				3921
				3922	<!-- ======================================================================= -->
				3923	<div class="doc_subsection">
				3924	<a name="Argument">The <tt>Argument</tt> class</a>
				3925	</div>
				3926
				3927	<div class="doc_text">
				3928
				3929	<p>This subclass of Value defines the interface for incoming formal
				3930	arguments to a function. A Function maintains a list of its formal
				3931	arguments. An argument has a pointer to the parent Function.</p>
				3932
				3933	</div>
				3934
				3935	<!-- *********************************************************************** -->
				3936	<hr>
				3937	<address>
				3938	<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman	947321d	2008-12-11 17:34:48 +0000	[diff] [blame]	3939	src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	3940	<a href="http://validator.w3.org/check/referer"><img
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Dan Gohman	f17a25c	2007-07-18 16:29:46 +0000	[diff] [blame]	3942
				3943	<a href="mailto:dhurjati@cs.uiuc.edu">Dinakar Dhurjati</a> and
				3944	<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
				3945	<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
				3946	Last modified: $Date$
				3947	</address>
				3948
				3949	</body>
				3950	</html>