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