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				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>
				32	<li><a href="#string_apis">Passing strings (the <tt>StringRef</tt>
				33	and <tt>Twine</tt> classes)</a>
				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>
				38	</li>
				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>
				65	<li><a href="#dss_ilist">llvm/ADT/ilist.h</a></li>
				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>
				73	<li><a href="#dss_denseset">"llvm/ADT/DenseSet.h"</a></li>
				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_valuemap">"llvm/ADT/ValueMap.h"</a></li>
				87	<li><a href="#dss_map"><map></a></li>
				88	<li><a href="#dss_othermap">Other Map-Like Container Options</a></li>
				89	</ul></li>
				90	<li><a href="#ds_string">String-like containers</a>
				91	<!--<ul>
				92	todo
				93	</ul>--></li>
				94	<li><a href="#ds_bit">BitVector-like containers</a>
				95	<ul>
				96	<li><a href="#dss_bitvector">A dense bitvector</a></li>
				97	<li><a href="#dss_smallbitvector">A "small" dense bitvector</a></li>
				98	<li><a href="#dss_sparsebitvector">A sparse bitvector</a></li>
				99	</ul></li>
				100	</ul>
				101	</li>
				102	<li><a href="#common">Helpful Hints for Common Operations</a>
				103	<ul>
				104	<li><a href="#inspection">Basic Inspection and Traversal Routines</a>
				105	<ul>
				106	<li><a href="#iterate_function">Iterating over the <tt>BasicBlock</tt>s
				107	in a <tt>Function</tt></a> </li>
				108	<li><a href="#iterate_basicblock">Iterating over the <tt>Instruction</tt>s
				109	in a <tt>BasicBlock</tt></a> </li>
				110	<li><a href="#iterate_institer">Iterating over the <tt>Instruction</tt>s
				111	in a <tt>Function</tt></a> </li>
				112	<li><a href="#iterate_convert">Turning an iterator into a
				113	class pointer</a> </li>
				114	<li><a href="#iterate_complex">Finding call sites: a more
				115	complex example</a> </li>
				116	<li><a href="#calls_and_invokes">Treating calls and invokes
				117	the same way</a> </li>
				118	<li><a href="#iterate_chains">Iterating over def-use &
				119	use-def chains</a> </li>
				120	<li><a href="#iterate_preds">Iterating over predecessors &
				121	successors of blocks</a></li>
				122	</ul>
				123	</li>
				124	<li><a href="#simplechanges">Making simple changes</a>
				125	<ul>
				126	<li><a href="#schanges_creating">Creating and inserting new
				127	<tt>Instruction</tt>s</a> </li>
				128	<li><a href="#schanges_deleting">Deleting <tt>Instruction</tt>s</a> </li>
				129	<li><a href="#schanges_replacing">Replacing an <tt>Instruction</tt>
				130	with another <tt>Value</tt></a> </li>
				131	<li><a href="#schanges_deletingGV">Deleting <tt>GlobalVariable</tt>s</a> </li>
				132	</ul>
				133	</li>
				134	<li><a href="#create_types">How to Create Types</a></li>
				135	<!--
				136	<li>Working with the Control Flow Graph
				137	<ul>
				138	<li>Accessing predecessors and successors of a <tt>BasicBlock</tt>
				139	<li>
				140	<li>
				141	</ul>
				142	-->
				143	</ul>
				144	</li>
				145
				146	<li><a href="#threading">Threads and LLVM</a>
				147	<ul>
				148	<li><a href="#startmultithreaded">Entering and Exiting Multithreaded Mode
				149	</a></li>
				150	<li><a href="#shutdown">Ending execution with <tt>llvm_shutdown()</tt></a></li>
				151	<li><a href="#managedstatic">Lazy initialization with <tt>ManagedStatic</tt></a></li>
				152	<li><a href="#llvmcontext">Achieving Isolation with <tt>LLVMContext</tt></a></li>
				153	<li><a href="#jitthreading">Threads and the JIT</a></li>
				154	</ul>
				155	</li>
				156
				157	<li><a href="#advanced">Advanced Topics</a>
				158	<ul>
				159	<li><a href="#TypeResolve">LLVM Type Resolution</a>
				160	<ul>
				161	<li><a href="#BuildRecType">Basic Recursive Type Construction</a></li>
				162	<li><a href="#refineAbstractTypeTo">The <tt>refineAbstractTypeTo</tt> method</a></li>
				163	<li><a href="#PATypeHolder">The PATypeHolder Class</a></li>
				164	<li><a href="#AbstractTypeUser">The AbstractTypeUser Class</a></li>
				165	</ul></li>
				166
				167	<li><a href="#SymbolTable">The <tt>ValueSymbolTable</tt> and <tt>TypeSymbolTable</tt> classes</a></li>
				168	<li><a href="#UserLayout">The <tt>User</tt> and owned <tt>Use</tt> classes' memory layout</a></li>
				169	</ul></li>
				170
				171	<li><a href="#coreclasses">The Core LLVM Class Hierarchy Reference</a>
				172	<ul>
				173	<li><a href="#Type">The <tt>Type</tt> class</a> </li>
				174	<li><a href="#Module">The <tt>Module</tt> class</a></li>
				175	<li><a href="#Value">The <tt>Value</tt> class</a>
				176	<ul>
				177	<li><a href="#User">The <tt>User</tt> class</a>
				178	<ul>
				179	<li><a href="#Instruction">The <tt>Instruction</tt> class</a></li>
				180	<li><a href="#Constant">The <tt>Constant</tt> class</a>
				181	<ul>
				182	<li><a href="#GlobalValue">The <tt>GlobalValue</tt> class</a>
				183	<ul>
				184	<li><a href="#Function">The <tt>Function</tt> class</a></li>
				185	<li><a href="#GlobalVariable">The <tt>GlobalVariable</tt> class</a></li>
				186	</ul>
				187	</li>
				188	</ul>
				189	</li>
				190	</ul>
				191	</li>
				192	<li><a href="#BasicBlock">The <tt>BasicBlock</tt> class</a></li>
				193	<li><a href="#Argument">The <tt>Argument</tt> class</a></li>
				194	</ul>
				195	</li>
				196	</ul>
				197	</li>
				198	</ol>
				199
				200	<div class="doc_author">
				201	<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>,
				202	<a href="mailto:dhurjati@cs.uiuc.edu">Dinakar Dhurjati</a>,
				203	<a href="mailto:ggreif@gmail.com">Gabor Greif</a>,
				204	<a href="mailto:jstanley@cs.uiuc.edu">Joel Stanley</a>,
				205	<a href="mailto:rspencer@x10sys.com">Reid Spencer</a> and
				206	<a href="mailto:owen@apple.com">Owen Anderson</a></p>
				207	</div>
				208
				209	<!-- *********************************************************************** -->
				210	<div class="doc_section">
				211	<a name="introduction">Introduction </a>
				212	</div>
				213	<!-- *********************************************************************** -->
				214
				215	<div class="doc_text">
				216
				217	<p>This document is meant to highlight some of the important classes and
				218	interfaces available in the LLVM source-base. This manual is not
				219	intended to explain what LLVM is, how it works, and what LLVM code looks
				220	like. It assumes that you know the basics of LLVM and are interested
				221	in writing transformations or otherwise analyzing or manipulating the
				222	code.</p>
				223
				224	<p>This document should get you oriented so that you can find your
				225	way in the continuously growing source code that makes up the LLVM
				226	infrastructure. Note that this manual is not intended to serve as a
				227	replacement for reading the source code, so if you think there should be
				228	a method in one of these classes to do something, but it's not listed,
				229	check the source. Links to the <a href="/doxygen/">doxygen</a> sources
				230	are provided to make this as easy as possible.</p>
				231
				232	<p>The first section of this document describes general information that is
				233	useful to know when working in the LLVM infrastructure, and the second describes
				234	the Core LLVM classes. In the future this manual will be extended with
				235	information describing how to use extension libraries, such as dominator
				236	information, CFG traversal routines, and useful utilities like the <tt><a
				237	href="/doxygen/InstVisitor_8h-source.html">InstVisitor</a></tt> template.</p>
				238
				239	</div>
				240
				241	<!-- *********************************************************************** -->
				242	<div class="doc_section">
				243	<a name="general">General Information</a>
				244	</div>
				245	<!-- *********************************************************************** -->
				246
				247	<div class="doc_text">
				248
				249	<p>This section contains general information that is useful if you are working
				250	in the LLVM source-base, but that isn't specific to any particular API.</p>
				251
				252	</div>
				253
				254	<!-- ======================================================================= -->
				255	<div class="doc_subsection">
				256	<a name="stl">The C++ Standard Template Library</a>
				257	</div>
				258
				259	<div class="doc_text">
				260
				261	<p>LLVM makes heavy use of the C++ Standard Template Library (STL),
				262	perhaps much more than you are used to, or have seen before. Because of
				263	this, you might want to do a little background reading in the
				264	techniques used and capabilities of the library. There are many good
				265	pages that discuss the STL, and several books on the subject that you
				266	can get, so it will not be discussed in this document.</p>
				267
				268	<p>Here are some useful links:</p>
				269
				270	<ol>
				271
				272	<li><a href="http://www.dinkumware.com/refxcpp.html">Dinkumware C++ Library
				273	reference</a> - an excellent reference for the STL and other parts of the
				274	standard C++ library.</li>
				275
				276	<li><a href="http://www.tempest-sw.com/cpp/">C++ In a Nutshell</a> - This is an
				277	O'Reilly book in the making. It has a decent Standard Library
				278	Reference that rivals Dinkumware's, and is unfortunately no longer free since the
				279	book has been published.</li>
				280
				281	<li><a href="http://www.parashift.com/c++-faq-lite/">C++ Frequently Asked
				282	Questions</a></li>
				283
				284	<li><a href="http://www.sgi.com/tech/stl/">SGI's STL Programmer's Guide</a> -
				285	Contains a useful <a
				286	href="http://www.sgi.com/tech/stl/stl_introduction.html">Introduction to the
				287	STL</a>.</li>
				288
				289	<li><a href="http://www.research.att.com/%7Ebs/C++.html">Bjarne Stroustrup's C++
				290	Page</a></li>
				291
				292	<li><a href="http://64.78.49.204/">
				293	Bruce Eckel's Thinking in C++, 2nd ed. Volume 2 Revision 4.0 (even better, get
				294	the book).</a></li>
				295
				296	</ol>
				297
				298	<p>You are also encouraged to take a look at the <a
				299	href="CodingStandards.html">LLVM Coding Standards</a> guide which focuses on how
				300	to write maintainable code more than where to put your curly braces.</p>
				301
				302	</div>
				303
				304	<!-- ======================================================================= -->
				305	<div class="doc_subsection">
				306	<a name="stl">Other useful references</a>
				307	</div>
				308
				309	<div class="doc_text">
				310
				311	<ol>
				312	<li><a href="http://www.psc.edu/%7Esemke/cvs_branches.html">CVS
				313	Branch and Tag Primer</a></li>
				314	<li><a href="http://www.fortran-2000.com/ArnaudRecipes/sharedlib.html">Using
				315	static and shared libraries across platforms</a></li>
				316	</ol>
				317
				318	</div>
				319
				320	<!-- *********************************************************************** -->
				321	<div class="doc_section">
				322	<a name="apis">Important and useful LLVM APIs</a>
				323	</div>
				324	<!-- *********************************************************************** -->
				325
				326	<div class="doc_text">
				327
				328	<p>Here we highlight some LLVM APIs that are generally useful and good to
				329	know about when writing transformations.</p>
				330
				331	</div>
				332
				333	<!-- ======================================================================= -->
				334	<div class="doc_subsection">
				335	<a name="isa">The <tt>isa<></tt>, <tt>cast<></tt> and
				336	<tt>dyn_cast<></tt> templates</a>
				337	</div>
				338
				339	<div class="doc_text">
				340
				341	<p>The LLVM source-base makes extensive use of a custom form of RTTI.
				342	These templates have many similarities to the C++ <tt>dynamic_cast<></tt>
				343	operator, but they don't have some drawbacks (primarily stemming from
				344	the fact that <tt>dynamic_cast<></tt> only works on classes that
				345	have a v-table). Because they are used so often, you must know what they
				346	do and how they work. All of these templates are defined in the <a
				347	href="/doxygen/Casting_8h-source.html"><tt>llvm/Support/Casting.h</tt></a>
				348	file (note that you very rarely have to include this file directly).</p>
				349
				350	<dl>
				351	<dt><tt>isa<></tt>: </dt>
				352
				353	<dd><p>The <tt>isa<></tt> operator works exactly like the Java
				354	"<tt>instanceof</tt>" operator. It returns true or false depending on whether
				355	a reference or pointer points to an instance of the specified class. This can
				356	be very useful for constraint checking of various sorts (example below).</p>
				357	</dd>
				358
				359	<dt><tt>cast<></tt>: </dt>
				360
				361	<dd><p>The <tt>cast<></tt> operator is a "checked cast" operation. It
				362	converts a pointer or reference from a base class to a derived class, causing
				363	an assertion failure if it is not really an instance of the right type. This
				364	should be used in cases where you have some information that makes you believe
				365	that something is of the right type. An example of the <tt>isa<></tt>
				366	and <tt>cast<></tt> template is:</p>
				367
				368	<div class="doc_code">
				369	<pre>
				370	static bool isLoopInvariant(const <a href="#Value">Value</a> V, const Loop L) {
				371	if (isa<<a href="#Constant">Constant</a>>(V) \|\| isa<<a href="#Argument">Argument</a>>(V) \|\| isa<<a href="#GlobalValue">GlobalValue</a>>(V))
				372	return true;
				373
				374	// <i>Otherwise, it must be an instruction...</i>
				375	return !L->contains(cast<<a href="#Instruction">Instruction</a>>(V)->getParent());
				376	}
				377	</pre>
				378	</div>
				379
				380	<p>Note that you should <b>not</b> use an <tt>isa<></tt> test followed
				381	by a <tt>cast<></tt>, for that use the <tt>dyn_cast<></tt>
				382	operator.</p>
				383
				384	</dd>
				385
				386	<dt><tt>dyn_cast<></tt>:</dt>
				387
				388	<dd><p>The <tt>dyn_cast<></tt> operator is a "checking cast" operation.
				389	It checks to see if the operand is of the specified type, and if so, returns a
				390	pointer to it (this operator does not work with references). If the operand is
				391	not of the correct type, a null pointer is returned. Thus, this works very
				392	much like the <tt>dynamic_cast<></tt> operator in C++, and should be
				393	used in the same circumstances. Typically, the <tt>dyn_cast<></tt>
				394	operator is used in an <tt>if</tt> statement or some other flow control
				395	statement like this:</p>
				396
				397	<div class="doc_code">
				398	<pre>
				399	if (<a href="#AllocationInst">AllocationInst</a> *AI = dyn_cast<<a href="#AllocationInst">AllocationInst</a>>(Val)) {
				400	// <i>...</i>
				401	}
				402	</pre>
				403	</div>
				404
				405	<p>This form of the <tt>if</tt> statement effectively combines together a call
				406	to <tt>isa<></tt> and a call to <tt>cast<></tt> into one
				407	statement, which is very convenient.</p>
				408
				409	<p>Note that the <tt>dyn_cast<></tt> operator, like C++'s
				410	<tt>dynamic_cast<></tt> or Java's <tt>instanceof</tt> operator, can be
				411	abused. In particular, you should not use big chained <tt>if/then/else</tt>
				412	blocks to check for lots of different variants of classes. If you find
				413	yourself wanting to do this, it is much cleaner and more efficient to use the
				414	<tt>InstVisitor</tt> class to dispatch over the instruction type directly.</p>
				415
				416	</dd>
				417
				418	<dt><tt>cast_or_null<></tt>: </dt>
				419
				420	<dd><p>The <tt>cast_or_null<></tt> operator works just like the
				421	<tt>cast<></tt> operator, except that it allows for a null pointer as an
				422	argument (which it then propagates). This can sometimes be useful, allowing
				423	you to combine several null checks into one.</p></dd>
				424
				425	<dt><tt>dyn_cast_or_null<></tt>: </dt>
				426
				427	<dd><p>The <tt>dyn_cast_or_null<></tt> operator works just like the
				428	<tt>dyn_cast<></tt> operator, except that it allows for a null pointer
				429	as an argument (which it then propagates). This can sometimes be useful,
				430	allowing you to combine several null checks into one.</p></dd>
				431
				432	</dl>
				433
				434	<p>These five templates can be used with any classes, whether they have a
				435	v-table or not. To add support for these templates, you simply need to add
				436	<tt>classof</tt> static methods to the class you are interested casting
				437	to. Describing this is currently outside the scope of this document, but there
				438	are lots of examples in the LLVM source base.</p>
				439
				440	</div>
				441
				442
				443	<!-- ======================================================================= -->
				444	<div class="doc_subsection">
				445	<a name="string_apis">Passing strings (the <tt>StringRef</tt>
				446	and <tt>Twine</tt> classes)</a>
				447	</div>
				448
				449	<div class="doc_text">
				450
				451	<p>Although LLVM generally does not do much string manipulation, we do have
				452	several important APIs which take strings. Two important examples are the
				453	Value class -- which has names for instructions, functions, etc. -- and the
				454	StringMap class which is used extensively in LLVM and Clang.</p>
				455
				456	<p>These are generic classes, and they need to be able to accept strings which
				457	may have embedded null characters. Therefore, they cannot simply take
				458	a <tt>const char *</tt>, and taking a <tt>const std::string&</tt> requires
				459	clients to perform a heap allocation which is usually unnecessary. Instead,
				460	many LLVM APIs use a <tt>const StringRef&</tt> or a <tt>const
				461	Twine&</tt> for passing strings efficiently.</p>
				462
				463	</div>
				464
				465	<!-- _______________________________________________________________________ -->
				466	<div class="doc_subsubsection">
				467	<a name="StringRef">The <tt>StringRef</tt> class</a>
				468	</div>
				469
				470	<div class="doc_text">
				471
				472	<p>The <tt>StringRef</tt> data type represents a reference to a constant string
				473	(a character array and a length) and supports the common operations available
				474	on <tt>std:string</tt>, but does not require heap allocation.</p>
				475
				476	<p>It can be implicitly constructed using a C style null-terminated string,
				477	an <tt>std::string</tt>, or explicitly with a character pointer and length.
				478	For example, the <tt>StringRef</tt> find function is declared as:</p>
				479
				480	<div class="doc_code">
				481	iterator find(const StringRef &Key);
				482	</div>
				483
				484	<p>and clients can call it using any one of:</p>
				485
				486	<div class="doc_code">
				487	<pre>
				488	Map.find("foo"); <i>// Lookup "foo"</i>
				489	Map.find(std::string("bar")); <i>// Lookup "bar"</i>
				490	Map.find(StringRef("\0baz", 4)); <i>// Lookup "\0baz"</i>
				491	</pre>
				492	</div>
				493
				494	<p>Similarly, APIs which need to return a string may return a <tt>StringRef</tt>
				495	instance, which can be used directly or converted to an <tt>std::string</tt>
				496	using the <tt>str</tt> member function. See
				497	"<tt><a href="/doxygen/classllvm_1_1StringRef_8h-source.html">llvm/ADT/StringRef.h</a></tt>"
				498	for more information.</p>
				499
				500	<p>You should rarely use the <tt>StringRef</tt> class directly, because it contains
				501	pointers to external memory it is not generally safe to store an instance of the
				502	class (unless you know that the external storage will not be freed).</p>
				503
				504	</div>
				505
				506	<!-- _______________________________________________________________________ -->
				507	<div class="doc_subsubsection">
				508	<a name="Twine">The <tt>Twine</tt> class</a>
				509	</div>
				510
				511	<div class="doc_text">
				512
				513	<p>The <tt>Twine</tt> class is an efficient way for APIs to accept concatenated
				514	strings. For example, a common LLVM paradigm is to name one instruction based on
				515	the name of another instruction with a suffix, for example:</p>
				516
				517	<div class="doc_code">
				518	<pre>
				519	New = CmpInst::Create(<i>...</i>, SO->getName() + ".cmp");
				520	</pre>
				521	</div>
				522
				523	<p>The <tt>Twine</tt> class is effectively a
				524	lightweight <a href="http://en.wikipedia.org/wiki/Rope_(computer_science)">rope</a>
				525	which points to temporary (stack allocated) objects. Twines can be implicitly
				526	constructed as the result of the plus operator applied to strings (i.e., a C
				527	strings, an <tt>std::string</tt>, or a <tt>StringRef</tt>). The twine delays the
				528	actual concatentation of strings until it is actually required, at which point
				529	it can be efficiently rendered directly into a character array. This avoids
				530	unnecessary heap allocation involved in constructing the temporary results of
				531	string concatenation. See
				532	"<tt><a href="/doxygen/classllvm_1_1Twine_8h-source.html">llvm/ADT/Twine.h</a></tt>"
				533	for more information.</p>
				534
				535	<p>As with a <tt>StringRef</tt>, <tt>Twine</tt> objects point to external memory
				536	and should almost never be stored or mentioned directly. They are intended
				537	solely for use when defining a function which should be able to efficiently
				538	accept concatenated strings.</p>
				539
				540	</div>
				541
				542
				543	<!-- ======================================================================= -->
				544	<div class="doc_subsection">
				545	<a name="DEBUG">The <tt>DEBUG()</tt> macro and <tt>-debug</tt> option</a>
				546	</div>
				547
				548	<div class="doc_text">
				549
				550	<p>Often when working on your pass you will put a bunch of debugging printouts
				551	and other code into your pass. After you get it working, you want to remove
				552	it, but you may need it again in the future (to work out new bugs that you run
				553	across).</p>
				554
				555	<p> Naturally, because of this, you don't want to delete the debug printouts,
				556	but you don't want them to always be noisy. A standard compromise is to comment
				557	them out, allowing you to enable them if you need them in the future.</p>
				558
				559	<p>The "<tt><a href="/doxygen/Debug_8h-source.html">llvm/Support/Debug.h</a></tt>"
				560	file provides a macro named <tt>DEBUG()</tt> that is a much nicer solution to
				561	this problem. Basically, you can put arbitrary code into the argument of the
				562	<tt>DEBUG</tt> macro, and it is only executed if '<tt>opt</tt>' (or any other
				563	tool) is run with the '<tt>-debug</tt>' command line argument:</p>
				564
				565	<div class="doc_code">
				566	<pre>
				567	DEBUG(errs() << "I am here!\n");
				568	</pre>
				569	</div>
				570
				571	<p>Then you can run your pass like this:</p>
				572
				573	<div class="doc_code">
				574	<pre>
				575	$ opt < a.bc > /dev/null -mypass
				576	<i><no output></i>
				577	$ opt < a.bc > /dev/null -mypass -debug
				578	I am here!
				579	</pre>
				580	</div>
				581
				582	<p>Using the <tt>DEBUG()</tt> macro instead of a home-brewed solution allows you
				583	to not have to create "yet another" command line option for the debug output for
				584	your pass. Note that <tt>DEBUG()</tt> macros are disabled for optimized builds,
				585	so they do not cause a performance impact at all (for the same reason, they
				586	should also not contain side-effects!).</p>
				587
				588	<p>One additional nice thing about the <tt>DEBUG()</tt> macro is that you can
				589	enable or disable it directly in gdb. Just use "<tt>set DebugFlag=0</tt>" or
				590	"<tt>set DebugFlag=1</tt>" from the gdb if the program is running. If the
				591	program hasn't been started yet, you can always just run it with
				592	<tt>-debug</tt>.</p>
				593
				594	</div>
				595
				596	<!-- _______________________________________________________________________ -->
				597	<div class="doc_subsubsection">
				598	<a name="DEBUG_TYPE">Fine grained debug info with <tt>DEBUG_TYPE</tt> and
				599	the <tt>-debug-only</tt> option</a>
				600	</div>
				601
				602	<div class="doc_text">
				603
				604	<p>Sometimes you may find yourself in a situation where enabling <tt>-debug</tt>
				605	just turns on <b>too much</b> information (such as when working on the code
				606	generator). If you want to enable debug information with more fine-grained
				607	control, you define the <tt>DEBUG_TYPE</tt> macro and the <tt>-debug</tt> only
				608	option as follows:</p>
				609
				610	<div class="doc_code">
				611	<pre>
				612	#undef DEBUG_TYPE
				613	DEBUG(errs() << "No debug type\n");
				614	#define DEBUG_TYPE "foo"
				615	DEBUG(errs() << "'foo' debug type\n");
				616	#undef DEBUG_TYPE
				617	#define DEBUG_TYPE "bar"
				618	DEBUG(errs() << "'bar' debug type\n"));
				619	#undef DEBUG_TYPE
				620	#define DEBUG_TYPE ""
				621	DEBUG(errs() << "No debug type (2)\n");
				622	</pre>
				623	</div>
				624
				625	<p>Then you can run your pass like this:</p>
				626
				627	<div class="doc_code">
				628	<pre>
				629	$ opt < a.bc > /dev/null -mypass
				630	<i><no output></i>
				631	$ opt < a.bc > /dev/null -mypass -debug
				632	No debug type
				633	'foo' debug type
				634	'bar' debug type
				635	No debug type (2)
				636	$ opt < a.bc > /dev/null -mypass -debug-only=foo
				637	'foo' debug type
				638	$ opt < a.bc > /dev/null -mypass -debug-only=bar
				639	'bar' debug type
				640	</pre>
				641	</div>
				642
				643	<p>Of course, in practice, you should only set <tt>DEBUG_TYPE</tt> at the top of
				644	a file, to specify the debug type for the entire module (if you do this before
				645	you <tt>#include "llvm/Support/Debug.h"</tt>, you don't have to insert the ugly
				646	<tt>#undef</tt>'s). Also, you should use names more meaningful than "foo" and
				647	"bar", because there is no system in place to ensure that names do not
				648	conflict. If two different modules use the same string, they will all be turned
				649	on when the name is specified. This allows, for example, all debug information
				650	for instruction scheduling to be enabled with <tt>-debug-type=InstrSched</tt>,
				651	even if the source lives in multiple files.</p>
				652
				653	<p>The <tt>DEBUG_WITH_TYPE</tt> macro is also available for situations where you
				654	would like to set <tt>DEBUG_TYPE</tt>, but only for one specific <tt>DEBUG</tt>
				655	statement. It takes an additional first parameter, which is the type to use. For
				656	example, the preceding example could be written as:</p>
				657
				658
				659	<div class="doc_code">
				660	<pre>
				661	DEBUG_WITH_TYPE("", errs() << "No debug type\n");
				662	DEBUG_WITH_TYPE("foo", errs() << "'foo' debug type\n");
				663	DEBUG_WITH_TYPE("bar", errs() << "'bar' debug type\n"));
				664	DEBUG_WITH_TYPE("", errs() << "No debug type (2)\n");
				665	</pre>
				666	</div>
				667
				668	</div>
				669
				670	<!-- ======================================================================= -->
				671	<div class="doc_subsection">
				672	<a name="Statistic">The <tt>Statistic</tt> class & <tt>-stats</tt>
				673	option</a>
				674	</div>
				675
				676	<div class="doc_text">
				677
				678	<p>The "<tt><a
				679	href="/doxygen/Statistic_8h-source.html">llvm/ADT/Statistic.h</a></tt>" file
				680	provides a class named <tt>Statistic</tt> that is used as a unified way to
				681	keep track of what the LLVM compiler is doing and how effective various
				682	optimizations are. It is useful to see what optimizations are contributing to
				683	making a particular program run faster.</p>
				684
				685	<p>Often you may run your pass on some big program, and you're interested to see
				686	how many times it makes a certain transformation. Although you can do this with
				687	hand inspection, or some ad-hoc method, this is a real pain and not very useful
				688	for big programs. Using the <tt>Statistic</tt> class makes it very easy to
				689	keep track of this information, and the calculated information is presented in a
				690	uniform manner with the rest of the passes being executed.</p>
				691
				692	<p>There are many examples of <tt>Statistic</tt> uses, but the basics of using
				693	it are as follows:</p>
				694
				695	<ol>
				696	<li><p>Define your statistic like this:</p>
				697
				698	<div class="doc_code">
				699	<pre>
				700	#define <a href="#DEBUG_TYPE">DEBUG_TYPE</a> "mypassname" <i>// This goes before any #includes.</i>
				701	STATISTIC(NumXForms, "The # of times I did stuff");
				702	</pre>
				703	</div>
				704
				705	<p>The <tt>STATISTIC</tt> macro defines a static variable, whose name is
				706	specified by the first argument. The pass name is taken from the DEBUG_TYPE
				707	macro, and the description is taken from the second argument. The variable
				708	defined ("NumXForms" in this case) acts like an unsigned integer.</p></li>
				709
				710	<li><p>Whenever you make a transformation, bump the counter:</p>
				711
				712	<div class="doc_code">
				713	<pre>
				714	++NumXForms; // <i>I did stuff!</i>
				715	</pre>
				716	</div>
				717
				718	</li>
				719	</ol>
				720
				721	<p>That's all you have to do. To get '<tt>opt</tt>' to print out the
				722	statistics gathered, use the '<tt>-stats</tt>' option:</p>
				723
				724	<div class="doc_code">
				725	<pre>
				726	$ opt -stats -mypassname < program.bc > /dev/null
				727	<i>... statistics output ...</i>
				728	</pre>
				729	</div>
				730
				731	<p> When running <tt>opt</tt> on a C file from the SPEC benchmark
				732	suite, it gives a report that looks like this:</p>
				733
				734	<div class="doc_code">
				735	<pre>
				736	7646 bitcodewriter - Number of normal instructions
				737	725 bitcodewriter - Number of oversized instructions
				738	129996 bitcodewriter - Number of bitcode bytes written
				739	2817 raise - Number of insts DCEd or constprop'd
				740	3213 raise - Number of cast-of-self removed
				741	5046 raise - Number of expression trees converted
				742	75 raise - Number of other getelementptr's formed
				743	138 raise - Number of load/store peepholes
				744	42 deadtypeelim - Number of unused typenames removed from symtab
				745	392 funcresolve - Number of varargs functions resolved
				746	27 globaldce - Number of global variables removed
				747	2 adce - Number of basic blocks removed
				748	134 cee - Number of branches revectored
				749	49 cee - Number of setcc instruction eliminated
				750	532 gcse - Number of loads removed
				751	2919 gcse - Number of instructions removed
				752	86 indvars - Number of canonical indvars added
				753	87 indvars - Number of aux indvars removed
				754	25 instcombine - Number of dead inst eliminate
				755	434 instcombine - Number of insts combined
				756	248 licm - Number of load insts hoisted
				757	1298 licm - Number of insts hoisted to a loop pre-header
				758	3 licm - Number of insts hoisted to multiple loop preds (bad, no loop pre-header)
				759	75 mem2reg - Number of alloca's promoted
				760	1444 cfgsimplify - Number of blocks simplified
				761	</pre>
				762	</div>
				763
				764	<p>Obviously, with so many optimizations, having a unified framework for this
				765	stuff is very nice. Making your pass fit well into the framework makes it more
				766	maintainable and useful.</p>
				767
				768	</div>
				769
				770	<!-- ======================================================================= -->
				771	<div class="doc_subsection">
				772	<a name="ViewGraph">Viewing graphs while debugging code</a>
				773	</div>
				774
				775	<div class="doc_text">
				776
				777	<p>Several of the important data structures in LLVM are graphs: for example
				778	CFGs made out of LLVM <a href="#BasicBlock">BasicBlock</a>s, CFGs made out of
				779	LLVM <a href="CodeGenerator.html#machinebasicblock">MachineBasicBlock</a>s, and
				780	<a href="CodeGenerator.html#selectiondag_intro">Instruction Selection
				781	DAGs</a>. In many cases, while debugging various parts of the compiler, it is
				782	nice to instantly visualize these graphs.</p>
				783
				784	<p>LLVM provides several callbacks that are available in a debug build to do
				785	exactly that. If you call the <tt>Function::viewCFG()</tt> method, for example,
				786	the current LLVM tool will pop up a window containing the CFG for the function
				787	where each basic block is a node in the graph, and each node contains the
				788	instructions in the block. Similarly, there also exists
				789	<tt>Function::viewCFGOnly()</tt> (does not include the instructions), the
				790	<tt>MachineFunction::viewCFG()</tt> and <tt>MachineFunction::viewCFGOnly()</tt>,
				791	and the <tt>SelectionDAG::viewGraph()</tt> methods. Within GDB, for example,
				792	you can usually use something like <tt>call DAG.viewGraph()</tt> to pop
				793	up a window. Alternatively, you can sprinkle calls to these functions in your
				794	code in places you want to debug.</p>
				795
				796	<p>Getting this to work requires a small amount of configuration. On Unix
				797	systems with X11, install the <a href="http://www.graphviz.org">graphviz</a>
				798	toolkit, and make sure 'dot' and 'gv' are in your path. If you are running on
				799	Mac OS/X, download and install the Mac OS/X <a
				800	href="http://www.pixelglow.com/graphviz/">Graphviz program</a>, and add
				801	<tt>/Applications/Graphviz.app/Contents/MacOS/</tt> (or wherever you install
				802	it) to your path. Once in your system and path are set up, rerun the LLVM
				803	configure script and rebuild LLVM to enable this functionality.</p>
				804
				805	<p><tt>SelectionDAG</tt> has been extended to make it easier to locate
				806	<i>interesting</i> nodes in large complex graphs. From gdb, if you
				807	<tt>call DAG.setGraphColor(<i>node</i>, "<i>color</i>")</tt>, then the
				808	next <tt>call DAG.viewGraph()</tt> would highlight the node in the
				809	specified color (choices of colors can be found at <a
				810	href="http://www.graphviz.org/doc/info/colors.html">colors</a>.) More
				811	complex node attributes can be provided with <tt>call
				812	DAG.setGraphAttrs(<i>node</i>, "<i>attributes</i>")</tt> (choices can be
				813	found at <a href="http://www.graphviz.org/doc/info/attrs.html">Graph
				814	Attributes</a>.) If you want to restart and clear all the current graph
				815	attributes, then you can <tt>call DAG.clearGraphAttrs()</tt>. </p>
				816
				817	</div>
				818
				819	<!-- *********************************************************************** -->
				820	<div class="doc_section">
				821	<a name="datastructure">Picking the Right Data Structure for a Task</a>
				822	</div>
				823	<!-- *********************************************************************** -->
				824
				825	<div class="doc_text">
				826
				827	<p>LLVM has a plethora of data structures in the <tt>llvm/ADT/</tt> directory,
				828	and we commonly use STL data structures. This section describes the trade-offs
				829	you should consider when you pick one.</p>
				830
				831	<p>
				832	The first step is a choose your own adventure: do you want a sequential
				833	container, a set-like container, or a map-like container? The most important
				834	thing when choosing a container is the algorithmic properties of how you plan to
				835	access the container. Based on that, you should use:</p>
				836
				837	<ul>
				838	<li>a <a href="#ds_map">map-like</a> container if you need efficient look-up
				839	of an value based on another value. Map-like containers also support
				840	efficient queries for containment (whether a key is in the map). Map-like
				841	containers generally do not support efficient reverse mapping (values to
				842	keys). If you need that, use two maps. Some map-like containers also
				843	support efficient iteration through the keys in sorted order. Map-like
				844	containers are the most expensive sort, only use them if you need one of
				845	these capabilities.</li>
				846
				847	<li>a <a href="#ds_set">set-like</a> container if you need to put a bunch of
				848	stuff into a container that automatically eliminates duplicates. Some
				849	set-like containers support efficient iteration through the elements in
				850	sorted order. Set-like containers are more expensive than sequential
				851	containers.
				852	</li>
				853
				854	<li>a <a href="#ds_sequential">sequential</a> container provides
				855	the most efficient way to add elements and keeps track of the order they are
				856	added to the collection. They permit duplicates and support efficient
				857	iteration, but do not support efficient look-up based on a key.
				858	</li>
				859
				860	<li>a <a href="#ds_string">string</a> container is a specialized sequential
				861	container or reference structure that is used for character or byte
				862	arrays.</li>
				863
				864	<li>a <a href="#ds_bit">bit</a> container provides an efficient way to store and
				865	perform set operations on sets of numeric id's, while automatically
				866	eliminating duplicates. Bit containers require a maximum of 1 bit for each
				867	identifier you want to store.
				868	</li>
				869	</ul>
				870
				871	<p>
				872	Once the proper category of container is determined, you can fine tune the
				873	memory use, constant factors, and cache behaviors of access by intelligently
				874	picking a member of the category. Note that constant factors and cache behavior
				875	can be a big deal. If you have a vector that usually only contains a few
				876	elements (but could contain many), for example, it's much better to use
				877	<a href="#dss_smallvector">SmallVector</a> than <a href="#dss_vector">vector</a>
				878	. Doing so avoids (relatively) expensive malloc/free calls, which dwarf the
				879	cost of adding the elements to the container. </p>
				880
				881	</div>
				882
				883	<!-- ======================================================================= -->
				884	<div class="doc_subsection">
				885	<a name="ds_sequential">Sequential Containers (std::vector, std::list, etc)</a>
				886	</div>
				887
				888	<div class="doc_text">
				889	There are a variety of sequential containers available for you, based on your
				890	needs. Pick the first in this section that will do what you want.
				891	</div>
				892
				893	<!-- _______________________________________________________________________ -->
				894	<div class="doc_subsubsection">
				895	<a name="dss_fixedarrays">Fixed Size Arrays</a>
				896	</div>
				897
				898	<div class="doc_text">
				899	<p>Fixed size arrays are very simple and very fast. They are good if you know
				900	exactly how many elements you have, or you have a (low) upper bound on how many
				901	you have.</p>
				902	</div>
				903
				904	<!-- _______________________________________________________________________ -->
				905	<div class="doc_subsubsection">
				906	<a name="dss_heaparrays">Heap Allocated Arrays</a>
				907	</div>
				908
				909	<div class="doc_text">
				910	<p>Heap allocated arrays (new[] + delete[]) are also simple. They are good if
				911	the number of elements is variable, if you know how many elements you will need
				912	before the array is allocated, and if the array is usually large (if not,
				913	consider a <a href="#dss_smallvector">SmallVector</a>). The cost of a heap
				914	allocated array is the cost of the new/delete (aka malloc/free). Also note that
				915	if you are allocating an array of a type with a constructor, the constructor and
				916	destructors will be run for every element in the array (re-sizable vectors only
				917	construct those elements actually used).</p>
				918	</div>
				919
				920	<!-- _______________________________________________________________________ -->
				921	<div class="doc_subsubsection">
				922	<a name="dss_smallvector">"llvm/ADT/SmallVector.h"</a>
				923	</div>
				924
				925	<div class="doc_text">
				926	<p><tt>SmallVector<Type, N></tt> is a simple class that looks and smells
				927	just like <tt>vector<Type></tt>:
				928	it supports efficient iteration, lays out elements in memory order (so you can
				929	do pointer arithmetic between elements), supports efficient push_back/pop_back
				930	operations, supports efficient random access to its elements, etc.</p>
				931
				932	<p>The advantage of SmallVector is that it allocates space for
				933	some number of elements (N) <b>in the object itself</b>. Because of this, if
				934	the SmallVector is dynamically smaller than N, no malloc is performed. This can
				935	be a big win in cases where the malloc/free call is far more expensive than the
				936	code that fiddles around with the elements.</p>
				937
				938	<p>This is good for vectors that are "usually small" (e.g. the number of
				939	predecessors/successors of a block is usually less than 8). On the other hand,
				940	this makes the size of the SmallVector itself large, so you don't want to
				941	allocate lots of them (doing so will waste a lot of space). As such,
				942	SmallVectors are most useful when on the stack.</p>
				943
				944	<p>SmallVector also provides a nice portable and efficient replacement for
				945	<tt>alloca</tt>.</p>
				946
				947	</div>
				948
				949	<!-- _______________________________________________________________________ -->
				950	<div class="doc_subsubsection">
				951	<a name="dss_vector"><vector></a>
				952	</div>
				953
				954	<div class="doc_text">
				955	<p>
				956	std::vector is well loved and respected. It is useful when SmallVector isn't:
				957	when the size of the vector is often large (thus the small optimization will
				958	rarely be a benefit) or if you will be allocating many instances of the vector
				959	itself (which would waste space for elements that aren't in the container).
				960	vector is also useful when interfacing with code that expects vectors :).
				961	</p>
				962
				963	<p>One worthwhile note about std::vector: avoid code like this:</p>
				964
				965	<div class="doc_code">
				966	<pre>
				967	for ( ... ) {
				968	std::vector<foo> V;
				969	use V;
				970	}
				971	</pre>
				972	</div>
				973
				974	<p>Instead, write this as:</p>
				975
				976	<div class="doc_code">
				977	<pre>
				978	std::vector<foo> V;
				979	for ( ... ) {
				980	use V;
				981	V.clear();
				982	}
				983	</pre>
				984	</div>
				985
				986	<p>Doing so will save (at least) one heap allocation and free per iteration of
				987	the loop.</p>
				988
				989	</div>
				990
				991	<!-- _______________________________________________________________________ -->
				992	<div class="doc_subsubsection">
				993	<a name="dss_deque"><deque></a>
				994	</div>
				995
				996	<div class="doc_text">
				997	<p>std::deque is, in some senses, a generalized version of std::vector. Like
				998	std::vector, it provides constant time random access and other similar
				999	properties, but it also provides efficient access to the front of the list. It
				1000	does not guarantee continuity of elements within memory.</p>
				1001
				1002	<p>In exchange for this extra flexibility, std::deque has significantly higher
				1003	constant factor costs than std::vector. If possible, use std::vector or
				1004	something cheaper.</p>
				1005	</div>
				1006
				1007	<!-- _______________________________________________________________________ -->
				1008	<div class="doc_subsubsection">
				1009	<a name="dss_list"><list></a>
				1010	</div>
				1011
				1012	<div class="doc_text">
				1013	<p>std::list is an extremely inefficient class that is rarely useful.
				1014	It performs a heap allocation for every element inserted into it, thus having an
				1015	extremely high constant factor, particularly for small data types. std::list
				1016	also only supports bidirectional iteration, not random access iteration.</p>
				1017
				1018	<p>In exchange for this high cost, std::list supports efficient access to both
				1019	ends of the list (like std::deque, but unlike std::vector or SmallVector). In
				1020	addition, the iterator invalidation characteristics of std::list are stronger
				1021	than that of a vector class: inserting or removing an element into the list does
				1022	not invalidate iterator or pointers to other elements in the list.</p>
				1023	</div>
				1024
				1025	<!-- _______________________________________________________________________ -->
				1026	<div class="doc_subsubsection">
				1027	<a name="dss_ilist">llvm/ADT/ilist.h</a>
				1028	</div>
				1029
				1030	<div class="doc_text">
				1031	<p><tt>ilist<T></tt> implements an 'intrusive' doubly-linked list. It is
				1032	intrusive, because it requires the element to store and provide access to the
				1033	prev/next pointers for the list.</p>
				1034
				1035	<p><tt>ilist</tt> has the same drawbacks as <tt>std::list</tt>, and additionally
				1036	requires an <tt>ilist_traits</tt> implementation for the element type, but it
				1037	provides some novel characteristics. In particular, it can efficiently store
				1038	polymorphic objects, the traits class is informed when an element is inserted or
				1039	removed from the list, and <tt>ilist</tt>s are guaranteed to support a
				1040	constant-time splice operation.</p>
				1041
				1042	<p>These properties are exactly what we want for things like
				1043	<tt>Instruction</tt>s and basic blocks, which is why these are implemented with
				1044	<tt>ilist</tt>s.</p>
				1045
				1046	Related classes of interest are explained in the following subsections:
				1047	<ul>
				1048	<li><a href="#dss_ilist_traits">ilist_traits</a></li>
				1049	<li><a href="#dss_iplist">iplist</a></li>
				1050	<li><a href="#dss_ilist_node">llvm/ADT/ilist_node.h</a></li>
				1051	<li><a href="#dss_ilist_sentinel">Sentinels</a></li>
				1052	</ul>
				1053	</div>
				1054
				1055	<!-- _______________________________________________________________________ -->
				1056	<div class="doc_subsubsection">
				1057	<a name="dss_ilist_traits">ilist_traits</a>
				1058	</div>
				1059
				1060	<div class="doc_text">
				1061	<p><tt>ilist_traits<T></tt> is <tt>ilist<T></tt>'s customization
				1062	mechanism. <tt>iplist<T></tt> (and consequently <tt>ilist<T></tt>)
				1063	publicly derive from this traits class.</p>
				1064	</div>
				1065
				1066	<!-- _______________________________________________________________________ -->
				1067	<div class="doc_subsubsection">
				1068	<a name="dss_iplist">iplist</a>
				1069	</div>
				1070
				1071	<div class="doc_text">
				1072	<p><tt>iplist<T></tt> is <tt>ilist<T></tt>'s base and as such
				1073	supports a slightly narrower interface. Notably, inserters from
				1074	<tt>T&</tt> are absent.</p>
				1075
				1076	<p><tt>ilist_traits<T></tt> is a public base of this class and can be
				1077	used for a wide variety of customizations.</p>
				1078	</div>
				1079
				1080	<!-- _______________________________________________________________________ -->
				1081	<div class="doc_subsubsection">
				1082	<a name="dss_ilist_node">llvm/ADT/ilist_node.h</a>
				1083	</div>
				1084
				1085	<div class="doc_text">
				1086	<p><tt>ilist_node<T></tt> implements a the forward and backward links
				1087	that are expected by the <tt>ilist<T></tt> (and analogous containers)
				1088	in the default manner.</p>
				1089
				1090	<p><tt>ilist_node<T></tt>s are meant to be embedded in the node type
				1091	<tt>T</tt>, usually <tt>T</tt> publicly derives from
				1092	<tt>ilist_node<T></tt>.</p>
				1093	</div>
				1094
				1095	<!-- _______________________________________________________________________ -->
				1096	<div class="doc_subsubsection">
				1097	<a name="dss_ilist_sentinel">Sentinels</a>
				1098	</div>
				1099
				1100	<div class="doc_text">
				1101	<p><tt>ilist</tt>s have another speciality that must be considered. To be a good
				1102	citizen in the C++ ecosystem, it needs to support the standard container
				1103	operations, such as <tt>begin</tt> and <tt>end</tt> iterators, etc. Also, the
				1104	<tt>operator--</tt> must work correctly on the <tt>end</tt> iterator in the
				1105	case of non-empty <tt>ilist</tt>s.</p>
				1106
				1107	<p>The only sensible solution to this problem is to allocate a so-called
				1108	<i>sentinel</i> along with the intrusive list, which serves as the <tt>end</tt>
				1109	iterator, providing the back-link to the last element. However conforming to the
				1110	C++ convention it is illegal to <tt>operator++</tt> beyond the sentinel and it
				1111	also must not be dereferenced.</p>
				1112
				1113	<p>These constraints allow for some implementation freedom to the <tt>ilist</tt>
				1114	how to allocate and store the sentinel. The corresponding policy is dictated
				1115	by <tt>ilist_traits<T></tt>. By default a <tt>T</tt> gets heap-allocated
				1116	whenever the need for a sentinel arises.</p>
				1117
				1118	<p>While the default policy is sufficient in most cases, it may break down when
				1119	<tt>T</tt> does not provide a default constructor. Also, in the case of many
				1120	instances of <tt>ilist</tt>s, the memory overhead of the associated sentinels
				1121	is wasted. To alleviate the situation with numerous and voluminous
				1122	<tt>T</tt>-sentinels, sometimes a trick is employed, leading to <i>ghostly
				1123	sentinels</i>.</p>
				1124
				1125	<p>Ghostly sentinels are obtained by specially-crafted <tt>ilist_traits<T></tt>
				1126	which superpose the sentinel with the <tt>ilist</tt> instance in memory. Pointer
				1127	arithmetic is used to obtain the sentinel, which is relative to the
				1128	<tt>ilist</tt>'s <tt>this</tt> pointer. The <tt>ilist</tt> is augmented by an
				1129	extra pointer, which serves as the back-link of the sentinel. This is the only
				1130	field in the ghostly sentinel which can be legally accessed.</p>
				1131	</div>
				1132
				1133	<!-- _______________________________________________________________________ -->
				1134	<div class="doc_subsubsection">
				1135	<a name="dss_other">Other Sequential Container options</a>
				1136	</div>
				1137
				1138	<div class="doc_text">
				1139	<p>Other STL containers are available, such as std::string.</p>
				1140
				1141	<p>There are also various STL adapter classes such as std::queue,
				1142	std::priority_queue, std::stack, etc. These provide simplified access to an
				1143	underlying container but don't affect the cost of the container itself.</p>
				1144
				1145	</div>
				1146
				1147
				1148	<!-- ======================================================================= -->
				1149	<div class="doc_subsection">
				1150	<a name="ds_set">Set-Like Containers (std::set, SmallSet, SetVector, etc)</a>
				1151	</div>
				1152
				1153	<div class="doc_text">
				1154
				1155	<p>Set-like containers are useful when you need to canonicalize multiple values
				1156	into a single representation. There are several different choices for how to do
				1157	this, providing various trade-offs.</p>
				1158
				1159	</div>
				1160
				1161
				1162	<!-- _______________________________________________________________________ -->
				1163	<div class="doc_subsubsection">
				1164	<a name="dss_sortedvectorset">A sorted 'vector'</a>
				1165	</div>
				1166
				1167	<div class="doc_text">
				1168
				1169	<p>If you intend to insert a lot of elements, then do a lot of queries, a
				1170	great approach is to use a vector (or other sequential container) with
				1171	std::sort+std::unique to remove duplicates. This approach works really well if
				1172	your usage pattern has these two distinct phases (insert then query), and can be
				1173	coupled with a good choice of <a href="#ds_sequential">sequential container</a>.
				1174	</p>
				1175
				1176	<p>
				1177	This combination provides the several nice properties: the result data is
				1178	contiguous in memory (good for cache locality), has few allocations, is easy to
				1179	address (iterators in the final vector are just indices or pointers), and can be
				1180	efficiently queried with a standard binary or radix search.</p>
				1181
				1182	</div>
				1183
				1184	<!-- _______________________________________________________________________ -->
				1185	<div class="doc_subsubsection">
				1186	<a name="dss_smallset">"llvm/ADT/SmallSet.h"</a>
				1187	</div>
				1188
				1189	<div class="doc_text">
				1190
				1191	<p>If you have a set-like data structure that is usually small and whose elements
				1192	are reasonably small, a <tt>SmallSet<Type, N></tt> is a good choice. This set
				1193	has space for N elements in place (thus, if the set is dynamically smaller than
				1194	N, no malloc traffic is required) and accesses them with a simple linear search.
				1195	When the set grows beyond 'N' elements, it allocates a more expensive representation that
				1196	guarantees efficient access (for most types, it falls back to std::set, but for
				1197	pointers it uses something far better, <a
				1198	href="#dss_smallptrset">SmallPtrSet</a>).</p>
				1199
				1200	<p>The magic of this class is that it handles small sets extremely efficiently,
				1201	but gracefully handles extremely large sets without loss of efficiency. The
				1202	drawback is that the interface is quite small: it supports insertion, queries
				1203	and erasing, but does not support iteration.</p>
				1204
				1205	</div>
				1206
				1207	<!-- _______________________________________________________________________ -->
				1208	<div class="doc_subsubsection">
				1209	<a name="dss_smallptrset">"llvm/ADT/SmallPtrSet.h"</a>
				1210	</div>
				1211
				1212	<div class="doc_text">
				1213
				1214	<p>SmallPtrSet has all the advantages of SmallSet (and a SmallSet of pointers is
				1215	transparently implemented with a SmallPtrSet), but also supports iterators. If
				1216	more than 'N' insertions are performed, a single quadratically
				1217	probed hash table is allocated and grows as needed, providing extremely
				1218	efficient access (constant time insertion/deleting/queries with low constant
				1219	factors) and is very stingy with malloc traffic.</p>
				1220
				1221	<p>Note that, unlike std::set, the iterators of SmallPtrSet are invalidated
				1222	whenever an insertion occurs. Also, the values visited by the iterators are not
				1223	visited in sorted order.</p>
				1224
				1225	</div>
				1226
				1227	<!-- _______________________________________________________________________ -->
				1228	<div class="doc_subsubsection">
				1229	<a name="dss_denseset">"llvm/ADT/DenseSet.h"</a>
				1230	</div>
				1231
				1232	<div class="doc_text">
				1233
				1234	<p>
				1235	DenseSet is a simple quadratically probed hash table. It excels at supporting
				1236	small values: it uses a single allocation to hold all of the pairs that
				1237	are currently inserted in the set. DenseSet is a great way to unique small
				1238	values that are not simple pointers (use <a
				1239	href="#dss_smallptrset">SmallPtrSet</a> for pointers). Note that DenseSet has
				1240	the same requirements for the value type that <a
				1241	href="#dss_densemap">DenseMap</a> has.
				1242	</p>
				1243
				1244	</div>
				1245
				1246	<!-- _______________________________________________________________________ -->
				1247	<div class="doc_subsubsection">
				1248	<a name="dss_FoldingSet">"llvm/ADT/FoldingSet.h"</a>
				1249	</div>
				1250
				1251	<div class="doc_text">
				1252
				1253	<p>
				1254	FoldingSet is an aggregate class that is really good at uniquing
				1255	expensive-to-create or polymorphic objects. It is a combination of a chained
				1256	hash table with intrusive links (uniqued objects are required to inherit from
				1257	FoldingSetNode) that uses <a href="#dss_smallvector">SmallVector</a> as part of
				1258	its ID process.</p>
				1259
				1260	<p>Consider a case where you want to implement a "getOrCreateFoo" method for
				1261	a complex object (for example, a node in the code generator). The client has a
				1262	description of what it wants to generate (it knows the opcode and all the
				1263	operands), but we don't want to 'new' a node, then try inserting it into a set
				1264	only to find out it already exists, at which point we would have to delete it
				1265	and return the node that already exists.
				1266	</p>
				1267
				1268	<p>To support this style of client, FoldingSet perform a query with a
				1269	FoldingSetNodeID (which wraps SmallVector) that can be used to describe the
				1270	element that we want to query for. The query either returns the element
				1271	matching the ID or it returns an opaque ID that indicates where insertion should
				1272	take place. Construction of the ID usually does not require heap traffic.</p>
				1273
				1274	<p>Because FoldingSet uses intrusive links, it can support polymorphic objects
				1275	in the set (for example, you can have SDNode instances mixed with LoadSDNodes).
				1276	Because the elements are individually allocated, pointers to the elements are
				1277	stable: inserting or removing elements does not invalidate any pointers to other
				1278	elements.
				1279	</p>
				1280
				1281	</div>
				1282
				1283	<!-- _______________________________________________________________________ -->
				1284	<div class="doc_subsubsection">
				1285	<a name="dss_set"><set></a>
				1286	</div>
				1287
				1288	<div class="doc_text">
				1289
				1290	<p><tt>std::set</tt> is a reasonable all-around set class, which is decent at
				1291	many things but great at nothing. std::set allocates memory for each element
				1292	inserted (thus it is very malloc intensive) and typically stores three pointers
				1293	per element in the set (thus adding a large amount of per-element space
				1294	overhead). It offers guaranteed log(n) performance, which is not particularly
				1295	fast from a complexity standpoint (particularly if the elements of the set are
				1296	expensive to compare, like strings), and has extremely high constant factors for
				1297	lookup, insertion and removal.</p>
				1298
				1299	<p>The advantages of std::set are that its iterators are stable (deleting or
				1300	inserting an element from the set does not affect iterators or pointers to other
				1301	elements) and that iteration over the set is guaranteed to be in sorted order.
				1302	If the elements in the set are large, then the relative overhead of the pointers
				1303	and malloc traffic is not a big deal, but if the elements of the set are small,
				1304	std::set is almost never a good choice.</p>
				1305
				1306	</div>
				1307
				1308	<!-- _______________________________________________________________________ -->
				1309	<div class="doc_subsubsection">
				1310	<a name="dss_setvector">"llvm/ADT/SetVector.h"</a>
				1311	</div>
				1312
				1313	<div class="doc_text">
				1314	<p>LLVM's SetVector<Type> is an adapter class that combines your choice of
				1315	a set-like container along with a <a href="#ds_sequential">Sequential
				1316	Container</a>. The important property
				1317	that this provides is efficient insertion with uniquing (duplicate elements are
				1318	ignored) with iteration support. It implements this by inserting elements into
				1319	both a set-like container and the sequential container, using the set-like
				1320	container for uniquing and the sequential container for iteration.
				1321	</p>
				1322
				1323	<p>The difference between SetVector and other sets is that the order of
				1324	iteration is guaranteed to match the order of insertion into the SetVector.
				1325	This property is really important for things like sets of pointers. Because
				1326	pointer values are non-deterministic (e.g. vary across runs of the program on
				1327	different machines), iterating over the pointers in the set will
				1328	not be in a well-defined order.</p>
				1329
				1330	<p>
				1331	The drawback of SetVector is that it requires twice as much space as a normal
				1332	set and has the sum of constant factors from the set-like container and the
				1333	sequential container that it uses. Use it only if you need to iterate over
				1334	the elements in a deterministic order. SetVector is also expensive to delete
				1335	elements out of (linear time), unless you use it's "pop_back" method, which is
				1336	faster.
				1337	</p>
				1338
				1339	<p>SetVector is an adapter class that defaults to using std::vector and std::set
				1340	for the underlying containers, so it is quite expensive. However,
				1341	<tt>"llvm/ADT/SetVector.h"</tt> also provides a SmallSetVector class, which
				1342	defaults to using a SmallVector and SmallSet of a specified size. If you use
				1343	this, and if your sets are dynamically smaller than N, you will save a lot of
				1344	heap traffic.</p>
				1345
				1346	</div>
				1347
				1348	<!-- _______________________________________________________________________ -->
				1349	<div class="doc_subsubsection">
				1350	<a name="dss_uniquevector">"llvm/ADT/UniqueVector.h"</a>
				1351	</div>
				1352
				1353	<div class="doc_text">
				1354
				1355	<p>
				1356	UniqueVector is similar to <a href="#dss_setvector">SetVector</a>, but it
				1357	retains a unique ID for each element inserted into the set. It internally
				1358	contains a map and a vector, and it assigns a unique ID for each value inserted
				1359	into the set.</p>
				1360
				1361	<p>UniqueVector is very expensive: its cost is the sum of the cost of
				1362	maintaining both the map and vector, it has high complexity, high constant
				1363	factors, and produces a lot of malloc traffic. It should be avoided.</p>
				1364
				1365	</div>
				1366
				1367
				1368	<!-- _______________________________________________________________________ -->
				1369	<div class="doc_subsubsection">
				1370	<a name="dss_otherset">Other Set-Like Container Options</a>
				1371	</div>
				1372
				1373	<div class="doc_text">
				1374
				1375	<p>
				1376	The STL provides several other options, such as std::multiset and the various
				1377	"hash_set" like containers (whether from C++ TR1 or from the SGI library). We
				1378	never use hash_set and unordered_set because they are generally very expensive
				1379	(each insertion requires a malloc) and very non-portable.
				1380	</p>
				1381
				1382	<p>std::multiset is useful if you're not interested in elimination of
				1383	duplicates, but has all the drawbacks of std::set. A sorted vector (where you
				1384	don't delete duplicate entries) or some other approach is almost always
				1385	better.</p>
				1386
				1387	</div>
				1388
				1389	<!-- ======================================================================= -->
				1390	<div class="doc_subsection">
				1391	<a name="ds_map">Map-Like Containers (std::map, DenseMap, etc)</a>
				1392	</div>
				1393
				1394	<div class="doc_text">
				1395	Map-like containers are useful when you want to associate data to a key. As
				1396	usual, there are a lot of different ways to do this. :)
				1397	</div>
				1398
				1399	<!-- _______________________________________________________________________ -->
				1400	<div class="doc_subsubsection">
				1401	<a name="dss_sortedvectormap">A sorted 'vector'</a>
				1402	</div>
				1403
				1404	<div class="doc_text">
				1405
				1406	<p>
				1407	If your usage pattern follows a strict insert-then-query approach, you can
				1408	trivially use the same approach as <a href="#dss_sortedvectorset">sorted vectors
				1409	for set-like containers</a>. The only difference is that your query function
				1410	(which uses std::lower_bound to get efficient log(n) lookup) should only compare
				1411	the key, not both the key and value. This yields the same advantages as sorted
				1412	vectors for sets.
				1413	</p>
				1414	</div>
				1415
				1416	<!-- _______________________________________________________________________ -->
				1417	<div class="doc_subsubsection">
				1418	<a name="dss_stringmap">"llvm/ADT/StringMap.h"</a>
				1419	</div>
				1420
				1421	<div class="doc_text">
				1422
				1423	<p>
				1424	Strings are commonly used as keys in maps, and they are difficult to support
				1425	efficiently: they are variable length, inefficient to hash and compare when
				1426	long, expensive to copy, etc. StringMap is a specialized container designed to
				1427	cope with these issues. It supports mapping an arbitrary range of bytes to an
				1428	arbitrary other object.</p>
				1429
				1430	<p>The StringMap implementation uses a quadratically-probed hash table, where
				1431	the buckets store a pointer to the heap allocated entries (and some other
				1432	stuff). The entries in the map must be heap allocated because the strings are
				1433	variable length. The string data (key) and the element object (value) are
				1434	stored in the same allocation with the string data immediately after the element
				1435	object. This container guarantees the "<tt>(char*)(&Value+1)</tt>" points
				1436	to the key string for a value.</p>
				1437
				1438	<p>The StringMap is very fast for several reasons: quadratic probing is very
				1439	cache efficient for lookups, the hash value of strings in buckets is not
				1440	recomputed when lookup up an element, StringMap rarely has to touch the
				1441	memory for unrelated objects when looking up a value (even when hash collisions
				1442	happen), hash table growth does not recompute the hash values for strings
				1443	already in the table, and each pair in the map is store in a single allocation
				1444	(the string data is stored in the same allocation as the Value of a pair).</p>
				1445
				1446	<p>StringMap also provides query methods that take byte ranges, so it only ever
				1447	copies a string if a value is inserted into the table.</p>
				1448	</div>
				1449
				1450	<!-- _______________________________________________________________________ -->
				1451	<div class="doc_subsubsection">
				1452	<a name="dss_indexedmap">"llvm/ADT/IndexedMap.h"</a>
				1453	</div>
				1454
				1455	<div class="doc_text">
				1456	<p>
				1457	IndexedMap is a specialized container for mapping small dense integers (or
				1458	values that can be mapped to small dense integers) to some other type. It is
				1459	internally implemented as a vector with a mapping function that maps the keys to
				1460	the dense integer range.
				1461	</p>
				1462
				1463	<p>
				1464	This is useful for cases like virtual registers in the LLVM code generator: they
				1465	have a dense mapping that is offset by a compile-time constant (the first
				1466	virtual register ID).</p>
				1467
				1468	</div>
				1469
				1470	<!-- _______________________________________________________________________ -->
				1471	<div class="doc_subsubsection">
				1472	<a name="dss_densemap">"llvm/ADT/DenseMap.h"</a>
				1473	</div>
				1474
				1475	<div class="doc_text">
				1476
				1477	<p>
				1478	DenseMap is a simple quadratically probed hash table. It excels at supporting
				1479	small keys and values: it uses a single allocation to hold all of the pairs that
				1480	are currently inserted in the map. DenseMap is a great way to map pointers to
				1481	pointers, or map other small types to each other.
				1482	</p>
				1483
				1484	<p>
				1485	There are several aspects of DenseMap that you should be aware of, however. The
				1486	iterators in a densemap are invalidated whenever an insertion occurs, unlike
				1487	map. Also, because DenseMap allocates space for a large number of key/value
				1488	pairs (it starts with 64 by default), it will waste a lot of space if your keys
				1489	or values are large. Finally, you must implement a partial specialization of
				1490	DenseMapInfo for the key that you want, if it isn't already supported. This
				1491	is required to tell DenseMap about two special marker values (which can never be
				1492	inserted into the map) that it needs internally.</p>
				1493
				1494	</div>
				1495
				1496	<!-- _______________________________________________________________________ -->
				1497	<div class="doc_subsubsection">
				1498	<a name="dss_valuemap">"llvm/ADT/ValueMap.h"</a>
				1499	</div>
				1500
				1501	<div class="doc_text">
				1502
				1503	<p>
				1504	ValueMap is a wrapper around a <a href="#dss_densemap">DenseMap</a> mapping
				1505	Value*s (or subclasses) to another type. When a Value is deleted or RAUW'ed,
				1506	ValueMap will update itself so the new version of the key is mapped to the same
				1507	value, just as if the key were a WeakVH. You can configure exactly how this
				1508	happens, and what else happens on these two events, by passing
				1509	a <code>Config</code> parameter to the ValueMap template.</p>
				1510
				1511	</div>
				1512
				1513	<!-- _______________________________________________________________________ -->
				1514	<div class="doc_subsubsection">
				1515	<a name="dss_map"><map></a>
				1516	</div>
				1517
				1518	<div class="doc_text">
				1519
				1520	<p>
				1521	std::map has similar characteristics to <a href="#dss_set">std::set</a>: it uses
				1522	a single allocation per pair inserted into the map, it offers log(n) lookup with
				1523	an extremely large constant factor, imposes a space penalty of 3 pointers per
				1524	pair in the map, etc.</p>
				1525
				1526	<p>std::map is most useful when your keys or values are very large, if you need
				1527	to iterate over the collection in sorted order, or if you need stable iterators
				1528	into the map (i.e. they don't get invalidated if an insertion or deletion of
				1529	another element takes place).</p>
				1530
				1531	</div>
				1532
				1533	<!-- _______________________________________________________________________ -->
				1534	<div class="doc_subsubsection">
				1535	<a name="dss_othermap">Other Map-Like Container Options</a>
				1536	</div>
				1537
				1538	<div class="doc_text">
				1539
				1540	<p>
				1541	The STL provides several other options, such as std::multimap and the various
				1542	"hash_map" like containers (whether from C++ TR1 or from the SGI library). We
				1543	never use hash_set and unordered_set because they are generally very expensive
				1544	(each insertion requires a malloc) and very non-portable.</p>
				1545
				1546	<p>std::multimap is useful if you want to map a key to multiple values, but has
				1547	all the drawbacks of std::map. A sorted vector or some other approach is almost
				1548	always better.</p>
				1549
				1550	</div>
				1551
				1552	<!-- ======================================================================= -->
				1553	<div class="doc_subsection">
				1554	<a name="ds_string">String-like containers</a>
				1555	</div>
				1556
				1557	<div class="doc_text">
				1558
				1559	<p>
				1560	TODO: const char* vs stringref vs smallstring vs std::string. Describe twine,
				1561	xref to #string_apis.
				1562	</p>
				1563
				1564	</div>
				1565
				1566	<!-- ======================================================================= -->
				1567	<div class="doc_subsection">
				1568	<a name="ds_bit">Bit storage containers (BitVector, SparseBitVector)</a>
				1569	</div>
				1570
				1571	<div class="doc_text">
				1572	<p>Unlike the other containers, there are only two bit storage containers, and
				1573	choosing when to use each is relatively straightforward.</p>
				1574
				1575	<p>One additional option is
				1576	<tt>std::vector<bool></tt>: we discourage its use for two reasons 1) the
				1577	implementation in many common compilers (e.g. commonly available versions of
				1578	GCC) is extremely inefficient and 2) the C++ standards committee is likely to
				1579	deprecate this container and/or change it significantly somehow. In any case,
				1580	please don't use it.</p>
				1581	</div>
				1582
				1583	<!-- _______________________________________________________________________ -->
				1584	<div class="doc_subsubsection">
				1585	<a name="dss_bitvector">BitVector</a>
				1586	</div>
				1587
				1588	<div class="doc_text">
				1589	<p> The BitVector container provides a dynamic size set of bits for manipulation.
				1590	It supports individual bit setting/testing, as well as set operations. The set
				1591	operations take time O(size of bitvector), but operations are performed one word
				1592	at a time, instead of one bit at a time. This makes the BitVector very fast for
				1593	set operations compared to other containers. Use the BitVector when you expect
				1594	the number of set bits to be high (IE a dense set).
				1595	</p>
				1596	</div>
				1597
				1598	<!-- _______________________________________________________________________ -->
				1599	<div class="doc_subsubsection">
				1600	<a name="dss_smallbitvector">SmallBitVector</a>
				1601	</div>
				1602
				1603	<div class="doc_text">
				1604	<p> The SmallBitVector container provides the same interface as BitVector, but
				1605	it is optimized for the case where only a small number of bits, less than
				1606	25 or so, are needed. It also transparently supports larger bit counts, but
				1607	slightly less efficiently than a plain BitVector, so SmallBitVector should
				1608	only be used when larger counts are rare.
				1609	</p>
				1610
				1611	<p>
				1612	At this time, SmallBitVector does not support set operations (and, or, xor),
				1613	and its operator[] does not provide an assignable lvalue.
				1614	</p>
				1615	</div>
				1616
				1617	<!-- _______________________________________________________________________ -->
				1618	<div class="doc_subsubsection">
				1619	<a name="dss_sparsebitvector">SparseBitVector</a>
				1620	</div>
				1621
				1622	<div class="doc_text">
				1623	<p> The SparseBitVector container is much like BitVector, with one major
				1624	difference: Only the bits that are set, are stored. This makes the
				1625	SparseBitVector much more space efficient than BitVector when the set is sparse,
				1626	as well as making set operations O(number of set bits) instead of O(size of
				1627	universe). The downside to the SparseBitVector is that setting and testing of random bits is O(N), and on large SparseBitVectors, this can be slower than BitVector. In our implementation, setting or testing bits in sorted order
				1628	(either forwards or reverse) is O(1) worst case. Testing and setting bits within 128 bits (depends on size) of the current bit is also O(1). As a general statement, testing/setting bits in a SparseBitVector is O(distance away from last set bit).
				1629	</p>
				1630	</div>
				1631
				1632	<!-- *********************************************************************** -->
				1633	<div class="doc_section">
				1634	<a name="common">Helpful Hints for Common Operations</a>
				1635	</div>
				1636	<!-- *********************************************************************** -->
				1637
				1638	<div class="doc_text">
				1639
				1640	<p>This section describes how to perform some very simple transformations of
				1641	LLVM code. This is meant to give examples of common idioms used, showing the
				1642	practical side of LLVM transformations. <p> Because this is a "how-to" section,
				1643	you should also read about the main classes that you will be working with. The
				1644	<a href="#coreclasses">Core LLVM Class Hierarchy Reference</a> contains details
				1645	and descriptions of the main classes that you should know about.</p>
				1646
				1647	</div>
				1648
				1649	<!-- NOTE: this section should be heavy on example code -->
				1650	<!-- ======================================================================= -->
				1651	<div class="doc_subsection">
				1652	<a name="inspection">Basic Inspection and Traversal Routines</a>
				1653	</div>
				1654
				1655	<div class="doc_text">
				1656
				1657	<p>The LLVM compiler infrastructure have many different data structures that may
				1658	be traversed. Following the example of the C++ standard template library, the
				1659	techniques used to traverse these various data structures are all basically the
				1660	same. For a enumerable sequence of values, the <tt>XXXbegin()</tt> function (or
				1661	method) returns an iterator to the start of the sequence, the <tt>XXXend()</tt>
				1662	function returns an iterator pointing to one past the last valid element of the
				1663	sequence, and there is some <tt>XXXiterator</tt> data type that is common
				1664	between the two operations.</p>
				1665
				1666	<p>Because the pattern for iteration is common across many different aspects of
				1667	the program representation, the standard template library algorithms may be used
				1668	on them, and it is easier to remember how to iterate. First we show a few common
				1669	examples of the data structures that need to be traversed. Other data
				1670	structures are traversed in very similar ways.</p>
				1671
				1672	</div>
				1673
				1674	<!-- _______________________________________________________________________ -->
				1675	<div class="doc_subsubsection">
				1676	<a name="iterate_function">Iterating over the </a><a
				1677	href="#BasicBlock"><tt>BasicBlock</tt></a>s in a <a
				1678	href="#Function"><tt>Function</tt></a>
				1679	</div>
				1680
				1681	<div class="doc_text">
				1682
				1683	<p>It's quite common to have a <tt>Function</tt> instance that you'd like to
				1684	transform in some way; in particular, you'd like to manipulate its
				1685	<tt>BasicBlock</tt>s. To facilitate this, you'll need to iterate over all of
				1686	the <tt>BasicBlock</tt>s that constitute the <tt>Function</tt>. The following is
				1687	an example that prints the name of a <tt>BasicBlock</tt> and the number of
				1688	<tt>Instruction</tt>s it contains:</p>
				1689
				1690	<div class="doc_code">
				1691	<pre>
				1692	// <i>func is a pointer to a Function instance</i>
				1693	for (Function::iterator i = func->begin(), e = func->end(); i != e; ++i)
				1694	// <i>Print out the name of the basic block if it has one, and then the</i>
				1695	// <i>number of instructions that it contains</i>
				1696	errs() << "Basic block (name=" << i->getName() << ") has "
				1697	<< i->size() << " instructions.\n";
				1698	</pre>
				1699	</div>
				1700
				1701	<p>Note that i can be used as if it were a pointer for the purposes of
				1702	invoking member functions of the <tt>Instruction</tt> class. This is
				1703	because the indirection operator is overloaded for the iterator
				1704	classes. In the above code, the expression <tt>i->size()</tt> is
				1705	exactly equivalent to <tt>(*i).size()</tt> just like you'd expect.</p>
				1706
				1707	</div>
				1708
				1709	<!-- _______________________________________________________________________ -->
				1710	<div class="doc_subsubsection">
				1711	<a name="iterate_basicblock">Iterating over the </a><a
				1712	href="#Instruction"><tt>Instruction</tt></a>s in a <a
				1713	href="#BasicBlock"><tt>BasicBlock</tt></a>
				1714	</div>
				1715
				1716	<div class="doc_text">
				1717
				1718	<p>Just like when dealing with <tt>BasicBlock</tt>s in <tt>Function</tt>s, it's
				1719	easy to iterate over the individual instructions that make up
				1720	<tt>BasicBlock</tt>s. Here's a code snippet that prints out each instruction in
				1721	a <tt>BasicBlock</tt>:</p>
				1722
				1723	<div class="doc_code">
				1724	<pre>
				1725	// <i>blk is a pointer to a BasicBlock instance</i>
				1726	for (BasicBlock::iterator i = blk->begin(), e = blk->end(); i != e; ++i)
				1727	// <i>The next statement works since operator<<(ostream&,...)</i>
				1728	// <i>is overloaded for Instruction&</i>
				1729	errs() << *i << "\n";
				1730	</pre>
				1731	</div>
				1732
				1733	<p>However, this isn't really the best way to print out the contents of a
				1734	<tt>BasicBlock</tt>! Since the ostream operators are overloaded for virtually
				1735	anything you'll care about, you could have just invoked the print routine on the
				1736	basic block itself: <tt>errs() << *blk << "\n";</tt>.</p>
				1737
				1738	</div>
				1739
				1740	<!-- _______________________________________________________________________ -->
				1741	<div class="doc_subsubsection">
				1742	<a name="iterate_institer">Iterating over the </a><a
				1743	href="#Instruction"><tt>Instruction</tt></a>s in a <a
				1744	href="#Function"><tt>Function</tt></a>
				1745	</div>
				1746
				1747	<div class="doc_text">
				1748
				1749	<p>If you're finding that you commonly iterate over a <tt>Function</tt>'s
				1750	<tt>BasicBlock</tt>s and then that <tt>BasicBlock</tt>'s <tt>Instruction</tt>s,
				1751	<tt>InstIterator</tt> should be used instead. You'll need to include <a
				1752	href="/doxygen/InstIterator_8h-source.html"><tt>llvm/Support/InstIterator.h</tt></a>,
				1753	and then instantiate <tt>InstIterator</tt>s explicitly in your code. Here's a
				1754	small example that shows how to dump all instructions in a function to the standard error stream:<p>
				1755
				1756	<div class="doc_code">
				1757	<pre>
				1758	#include "<a href="/doxygen/InstIterator_8h-source.html">llvm/Support/InstIterator.h</a>"
				1759
				1760	// <i>F is a pointer to a Function instance</i>
				1761	for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
				1762	errs() << *I << "\n";
				1763	</pre>
				1764	</div>
				1765
				1766	<p>Easy, isn't it? You can also use <tt>InstIterator</tt>s to fill a
				1767	work list with its initial contents. For example, if you wanted to
				1768	initialize a work list to contain all instructions in a <tt>Function</tt>
				1769	F, all you would need to do is something like:</p>
				1770
				1771	<div class="doc_code">
				1772	<pre>
				1773	std::set<Instruction*> worklist;
				1774	// or better yet, SmallPtrSet<Instruction*, 64> worklist;
				1775
				1776	for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
				1777	worklist.insert(&*I);
				1778	</pre>
				1779	</div>
				1780
				1781	<p>The STL set <tt>worklist</tt> would now contain all instructions in the
				1782	<tt>Function</tt> pointed to by F.</p>
				1783
				1784	</div>
				1785
				1786	<!-- _______________________________________________________________________ -->
				1787	<div class="doc_subsubsection">
				1788	<a name="iterate_convert">Turning an iterator into a class pointer (and
				1789	vice-versa)</a>
				1790	</div>
				1791
				1792	<div class="doc_text">
				1793
				1794	<p>Sometimes, it'll be useful to grab a reference (or pointer) to a class
				1795	instance when all you've got at hand is an iterator. Well, extracting
				1796	a reference or a pointer from an iterator is very straight-forward.
				1797	Assuming that <tt>i</tt> is a <tt>BasicBlock::iterator</tt> and <tt>j</tt>
				1798	is a <tt>BasicBlock::const_iterator</tt>:</p>
				1799
				1800	<div class="doc_code">
				1801	<pre>
				1802	Instruction& inst = *i; // <i>Grab reference to instruction reference</i>
				1803	Instruction* pinst = &*i; // <i>Grab pointer to instruction reference</i>
				1804	const Instruction& inst = *j;
				1805	</pre>
				1806	</div>
				1807
				1808	<p>However, the iterators you'll be working with in the LLVM framework are
				1809	special: they will automatically convert to a ptr-to-instance type whenever they
				1810	need to. Instead of dereferencing the iterator and then taking the address of
				1811	the result, you can simply assign the iterator to the proper pointer type and
				1812	you get the dereference and address-of operation as a result of the assignment
				1813	(behind the scenes, this is a result of overloading casting mechanisms). Thus
				1814	the last line of the last example,</p>
				1815
				1816	<div class="doc_code">
				1817	<pre>
				1818	Instruction pinst = &i;
				1819	</pre>
				1820	</div>
				1821
				1822	<p>is semantically equivalent to</p>
				1823
				1824	<div class="doc_code">
				1825	<pre>
				1826	Instruction *pinst = i;
				1827	</pre>
				1828	</div>
				1829
				1830	<p>It's also possible to turn a class pointer into the corresponding iterator,
				1831	and this is a constant time operation (very efficient). The following code
				1832	snippet illustrates use of the conversion constructors provided by LLVM
				1833	iterators. By using these, you can explicitly grab the iterator of something
				1834	without actually obtaining it via iteration over some structure:</p>
				1835
				1836	<div class="doc_code">
				1837	<pre>
				1838	void printNextInstruction(Instruction* inst) {
				1839	BasicBlock::iterator it(inst);
				1840	++it; // <i>After this line, it refers to the instruction after *inst</i>
				1841	if (it != inst->getParent()->end()) errs() << *it << "\n";
				1842	}
				1843	</pre>
				1844	</div>
				1845
				1846	</div>
				1847
				1848	<!--_______________________________________________________________________-->
				1849	<div class="doc_subsubsection">
				1850	<a name="iterate_complex">Finding call sites: a slightly more complex
				1851	example</a>
				1852	</div>
				1853
				1854	<div class="doc_text">
				1855
				1856	<p>Say that you're writing a FunctionPass and would like to count all the
				1857	locations in the entire module (that is, across every <tt>Function</tt>) where a
				1858	certain function (i.e., some <tt>Function</tt>*) is already in scope. As you'll
				1859	learn later, you may want to use an <tt>InstVisitor</tt> to accomplish this in a
				1860	much more straight-forward manner, but this example will allow us to explore how
				1861	you'd do it if you didn't have <tt>InstVisitor</tt> around. In pseudo-code, this
				1862	is what we want to do:</p>
				1863
				1864	<div class="doc_code">
				1865	<pre>
				1866	initialize callCounter to zero
				1867	for each Function f in the Module
				1868	for each BasicBlock b in f
				1869	for each Instruction i in b
				1870	if (i is a CallInst and calls the given function)
				1871	increment callCounter
				1872	</pre>
				1873	</div>
				1874
				1875	<p>And the actual code is (remember, because we're writing a
				1876	<tt>FunctionPass</tt>, our <tt>FunctionPass</tt>-derived class simply has to
				1877	override the <tt>runOnFunction</tt> method):</p>
				1878
				1879	<div class="doc_code">
				1880	<pre>
				1881	Function* targetFunc = ...;
				1882
				1883	class OurFunctionPass : public FunctionPass {
				1884	public:
				1885	OurFunctionPass(): callCounter(0) { }
				1886
				1887	virtual runOnFunction(Function& F) {
				1888	for (Function::iterator b = F.begin(), be = F.end(); b != be; ++b) {
				1889	for (BasicBlock::iterator i = b->begin(), ie = b->end(); i != ie; ++i) {
				1890	if (<a href="#CallInst">CallInst</a>* callInst = <a href="#isa">dyn_cast</a><<a
				1891	href="#CallInst">CallInst</a>>(&*i)) {
				1892	// <i>We know we've encountered a call instruction, so we</i>
				1893	// <i>need to determine if it's a call to the</i>
				1894	// <i>function pointed to by m_func or not.</i>
				1895	if (callInst->getCalledFunction() == targetFunc)
				1896	++callCounter;
				1897	}
				1898	}
				1899	}
				1900	}
				1901
				1902	private:
				1903	unsigned callCounter;
				1904	};
				1905	</pre>
				1906	</div>
				1907
				1908	</div>
				1909
				1910	<!--_______________________________________________________________________-->
				1911	<div class="doc_subsubsection">
				1912	<a name="calls_and_invokes">Treating calls and invokes the same way</a>
				1913	</div>
				1914
				1915	<div class="doc_text">
				1916
				1917	<p>You may have noticed that the previous example was a bit oversimplified in
				1918	that it did not deal with call sites generated by 'invoke' instructions. In
				1919	this, and in other situations, you may find that you want to treat
				1920	<tt>CallInst</tt>s and <tt>InvokeInst</tt>s the same way, even though their
				1921	most-specific common base class is <tt>Instruction</tt>, which includes lots of
				1922	less closely-related things. For these cases, LLVM provides a handy wrapper
				1923	class called <a
				1924	href="http://llvm.org/doxygen/classllvm_1_1CallSite.html"><tt>CallSite</tt></a>.
				1925	It is essentially a wrapper around an <tt>Instruction</tt> pointer, with some
				1926	methods that provide functionality common to <tt>CallInst</tt>s and
				1927	<tt>InvokeInst</tt>s.</p>
				1928
				1929	<p>This class has "value semantics": it should be passed by value, not by
				1930	reference and it should not be dynamically allocated or deallocated using
				1931	<tt>operator new</tt> or <tt>operator delete</tt>. It is efficiently copyable,
				1932	assignable and constructable, with costs equivalents to that of a bare pointer.
				1933	If you look at its definition, it has only a single pointer member.</p>
				1934
				1935	</div>
				1936
				1937	<!--_______________________________________________________________________-->
				1938	<div class="doc_subsubsection">
				1939	<a name="iterate_chains">Iterating over def-use & use-def chains</a>
				1940	</div>
				1941
				1942	<div class="doc_text">
				1943
				1944	<p>Frequently, we might have an instance of the <a
				1945	href="/doxygen/classllvm_1_1Value.html">Value Class</a> and we want to
				1946	determine which <tt>User</tt>s use the <tt>Value</tt>. The list of all
				1947	<tt>User</tt>s of a particular <tt>Value</tt> is called a <i>def-use</i> chain.
				1948	For example, let's say we have a <tt>Function*</tt> named <tt>F</tt> to a
				1949	particular function <tt>foo</tt>. Finding all of the instructions that
				1950	<i>use</i> <tt>foo</tt> is as simple as iterating over the <i>def-use</i> chain
				1951	of <tt>F</tt>:</p>
				1952
				1953	<div class="doc_code">
				1954	<pre>
				1955	Function *F = ...;
				1956
				1957	for (Value::use_iterator i = F->use_begin(), e = F->use_end(); i != e; ++i)
				1958	if (Instruction Inst = dyn_cast<Instruction>(i)) {
				1959	errs() << "F is used in instruction:\n";
				1960	errs() << *Inst << "\n";
				1961	}
				1962	</pre>
				1963	</div>
				1964
				1965	<p>Alternately, it's common to have an instance of the <a
				1966	href="/doxygen/classllvm_1_1User.html">User Class</a> and need to know what
				1967	<tt>Value</tt>s are used by it. The list of all <tt>Value</tt>s used by a
				1968	<tt>User</tt> is known as a <i>use-def</i> chain. Instances of class
				1969	<tt>Instruction</tt> are common <tt>User</tt>s, so we might want to iterate over
				1970	all of the values that a particular instruction uses (that is, the operands of
				1971	the particular <tt>Instruction</tt>):</p>
				1972
				1973	<div class="doc_code">
				1974	<pre>
				1975	Instruction *pi = ...;
				1976
				1977	for (User::op_iterator i = pi->op_begin(), e = pi->op_end(); i != e; ++i) {
				1978	Value v = i;
				1979	// <i>...</i>
				1980	}
				1981	</pre>
				1982	</div>
				1983
				1984	<!--
				1985	def-use chains ("finding all users of"): Value::use_begin/use_end
				1986	use-def chains ("finding all values used"): User::op_begin/op_end [op=operand]
				1987	-->
				1988
				1989	</div>
				1990
				1991	<!--_______________________________________________________________________-->
				1992	<div class="doc_subsubsection">
				1993	<a name="iterate_preds">Iterating over predecessors &
				1994	successors of blocks</a>
				1995	</div>
				1996
				1997	<div class="doc_text">
				1998
				1999	<p>Iterating over the predecessors and successors of a block is quite easy
				2000	with the routines defined in <tt>"llvm/Support/CFG.h"</tt>. Just use code like
				2001	this to iterate over all predecessors of BB:</p>
				2002
				2003	<div class="doc_code">
				2004	<pre>
				2005	#include "llvm/Support/CFG.h"
				2006	BasicBlock *BB = ...;
				2007
				2008	for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
				2009	BasicBlock Pred = PI;
				2010	// <i>...</i>
				2011	}
				2012	</pre>
				2013	</div>
				2014
				2015	<p>Similarly, to iterate over successors use
				2016	succ_iterator/succ_begin/succ_end.</p>
				2017
				2018	</div>
				2019
				2020
				2021	<!-- ======================================================================= -->
				2022	<div class="doc_subsection">
				2023	<a name="simplechanges">Making simple changes</a>
				2024	</div>
				2025
				2026	<div class="doc_text">
				2027
				2028	<p>There are some primitive transformation operations present in the LLVM
				2029	infrastructure that are worth knowing about. When performing
				2030	transformations, it's fairly common to manipulate the contents of basic
				2031	blocks. This section describes some of the common methods for doing so
				2032	and gives example code.</p>
				2033
				2034	</div>
				2035
				2036	<!--_______________________________________________________________________-->
				2037	<div class="doc_subsubsection">
				2038	<a name="schanges_creating">Creating and inserting new
				2039	<tt>Instruction</tt>s</a>
				2040	</div>
				2041
				2042	<div class="doc_text">
				2043
				2044	<p><i>Instantiating Instructions</i></p>
				2045
				2046	<p>Creation of <tt>Instruction</tt>s is straight-forward: simply call the
				2047	constructor for the kind of instruction to instantiate and provide the necessary
				2048	parameters. For example, an <tt>AllocaInst</tt> only <i>requires</i> a
				2049	(const-ptr-to) <tt>Type</tt>. Thus:</p>
				2050
				2051	<div class="doc_code">
				2052	<pre>
				2053	AllocaInst* ai = new AllocaInst(Type::Int32Ty);
				2054	</pre>
				2055	</div>
				2056
				2057	<p>will create an <tt>AllocaInst</tt> instance that represents the allocation of
				2058	one integer in the current stack frame, at run time. Each <tt>Instruction</tt>
				2059	subclass is likely to have varying default parameters which change the semantics
				2060	of the instruction, so refer to the <a
				2061	href="/doxygen/classllvm_1_1Instruction.html">doxygen documentation for the subclass of
				2062	Instruction</a> that you're interested in instantiating.</p>
				2063
				2064	<p><i>Naming values</i></p>
				2065
				2066	<p>It is very useful to name the values of instructions when you're able to, as
				2067	this facilitates the debugging of your transformations. If you end up looking
				2068	at generated LLVM machine code, you definitely want to have logical names
				2069	associated with the results of instructions! By supplying a value for the
				2070	<tt>Name</tt> (default) parameter of the <tt>Instruction</tt> constructor, you
				2071	associate a logical name with the result of the instruction's execution at
				2072	run time. For example, say that I'm writing a transformation that dynamically
				2073	allocates space for an integer on the stack, and that integer is going to be
				2074	used as some kind of index by some other code. To accomplish this, I place an
				2075	<tt>AllocaInst</tt> at the first point in the first <tt>BasicBlock</tt> of some
				2076	<tt>Function</tt>, and I'm intending to use it within the same
				2077	<tt>Function</tt>. I might do:</p>
				2078
				2079	<div class="doc_code">
				2080	<pre>
				2081	AllocaInst* pa = new AllocaInst(Type::Int32Ty, 0, "indexLoc");
				2082	</pre>
				2083	</div>
				2084
				2085	<p>where <tt>indexLoc</tt> is now the logical name of the instruction's
				2086	execution value, which is a pointer to an integer on the run time stack.</p>
				2087
				2088	<p><i>Inserting instructions</i></p>
				2089
				2090	<p>There are essentially two ways to insert an <tt>Instruction</tt>
				2091	into an existing sequence of instructions that form a <tt>BasicBlock</tt>:</p>
				2092
				2093	<ul>
				2094	<li>Insertion into an explicit instruction list
				2095
				2096	<p>Given a <tt>BasicBlock* pb</tt>, an <tt>Instruction* pi</tt> within that
				2097	<tt>BasicBlock</tt>, and a newly-created instruction we wish to insert
				2098	before <tt>*pi</tt>, we do the following: </p>
				2099
				2100	<div class="doc_code">
				2101	<pre>
				2102	BasicBlock *pb = ...;
				2103	Instruction *pi = ...;
				2104	Instruction *newInst = new Instruction(...);
				2105
				2106	pb->getInstList().insert(pi, newInst); // <i>Inserts newInst before pi in pb</i>
				2107	</pre>
				2108	</div>
				2109
				2110	<p>Appending to the end of a <tt>BasicBlock</tt> is so common that
				2111	the <tt>Instruction</tt> class and <tt>Instruction</tt>-derived
				2112	classes provide constructors which take a pointer to a
				2113	<tt>BasicBlock</tt> to be appended to. For example code that
				2114	looked like: </p>
				2115
				2116	<div class="doc_code">
				2117	<pre>
				2118	BasicBlock *pb = ...;
				2119	Instruction *newInst = new Instruction(...);
				2120
				2121	pb->getInstList().push_back(newInst); // <i>Appends newInst to pb</i>
				2122	</pre>
				2123	</div>
				2124
				2125	<p>becomes: </p>
				2126
				2127	<div class="doc_code">
				2128	<pre>
				2129	BasicBlock *pb = ...;
				2130	Instruction *newInst = new Instruction(..., pb);
				2131	</pre>
				2132	</div>
				2133
				2134	<p>which is much cleaner, especially if you are creating
				2135	long instruction streams.</p></li>
				2136
				2137	<li>Insertion into an implicit instruction list
				2138
				2139	<p><tt>Instruction</tt> instances that are already in <tt>BasicBlock</tt>s
				2140	are implicitly associated with an existing instruction list: the instruction
				2141	list of the enclosing basic block. Thus, we could have accomplished the same
				2142	thing as the above code without being given a <tt>BasicBlock</tt> by doing:
				2143	</p>
				2144
				2145	<div class="doc_code">
				2146	<pre>
				2147	Instruction *pi = ...;
				2148	Instruction *newInst = new Instruction(...);
				2149
				2150	pi->getParent()->getInstList().insert(pi, newInst);
				2151	</pre>
				2152	</div>
				2153
				2154	<p>In fact, this sequence of steps occurs so frequently that the
				2155	<tt>Instruction</tt> class and <tt>Instruction</tt>-derived classes provide
				2156	constructors which take (as a default parameter) a pointer to an
				2157	<tt>Instruction</tt> which the newly-created <tt>Instruction</tt> should
				2158	precede. That is, <tt>Instruction</tt> constructors are capable of
				2159	inserting the newly-created instance into the <tt>BasicBlock</tt> of a
				2160	provided instruction, immediately before that instruction. Using an
				2161	<tt>Instruction</tt> constructor with a <tt>insertBefore</tt> (default)
				2162	parameter, the above code becomes:</p>
				2163
				2164	<div class="doc_code">
				2165	<pre>
				2166	Instruction* pi = ...;
				2167	Instruction* newInst = new Instruction(..., pi);
				2168	</pre>
				2169	</div>
				2170
				2171	<p>which is much cleaner, especially if you're creating a lot of
				2172	instructions and adding them to <tt>BasicBlock</tt>s.</p></li>
				2173	</ul>
				2174
				2175	</div>
				2176
				2177	<!--_______________________________________________________________________-->
				2178	<div class="doc_subsubsection">
				2179	<a name="schanges_deleting">Deleting <tt>Instruction</tt>s</a>
				2180	</div>
				2181
				2182	<div class="doc_text">
				2183
				2184	<p>Deleting an instruction from an existing sequence of instructions that form a
				2185	<a href="#BasicBlock"><tt>BasicBlock</tt></a> is very straight-forward. First,
				2186	you must have a pointer to the instruction that you wish to delete. Second, you
				2187	need to obtain the pointer to that instruction's basic block. You use the
				2188	pointer to the basic block to get its list of instructions and then use the
				2189	erase function to remove your instruction. For example:</p>
				2190
				2191	<div class="doc_code">
				2192	<pre>
				2193	<a href="#Instruction">Instruction</a> *I = .. ;
				2194	I->eraseFromParent();
				2195	</pre>
				2196	</div>
				2197
				2198	</div>
				2199
				2200	<!--_______________________________________________________________________-->
				2201	<div class="doc_subsubsection">
				2202	<a name="schanges_replacing">Replacing an <tt>Instruction</tt> with another
				2203	<tt>Value</tt></a>
				2204	</div>
				2205
				2206	<div class="doc_text">
				2207
				2208	<p><i>Replacing individual instructions</i></p>
				2209
				2210	<p>Including "<a href="/doxygen/BasicBlockUtils_8h-source.html">llvm/Transforms/Utils/BasicBlockUtils.h</a>"
				2211	permits use of two very useful replace functions: <tt>ReplaceInstWithValue</tt>
				2212	and <tt>ReplaceInstWithInst</tt>.</p>
				2213
				2214	<h4><a name="schanges_deleting">Deleting <tt>Instruction</tt>s</a></h4>
				2215
				2216	<ul>
				2217	<li><tt>ReplaceInstWithValue</tt>
				2218
				2219	<p>This function replaces all uses of a given instruction with a value,
				2220	and then removes the original instruction. The following example
				2221	illustrates the replacement of the result of a particular
				2222	<tt>AllocaInst</tt> that allocates memory for a single integer with a null
				2223	pointer to an integer.</p>
				2224
				2225	<div class="doc_code">
				2226	<pre>
				2227	AllocaInst* instToReplace = ...;
				2228	BasicBlock::iterator ii(instToReplace);
				2229
				2230	ReplaceInstWithValue(instToReplace->getParent()->getInstList(), ii,
				2231	Constant::getNullValue(PointerType::getUnqual(Type::Int32Ty)));
				2232	</pre></div></li>
				2233
				2234	<li><tt>ReplaceInstWithInst</tt>
				2235
				2236	<p>This function replaces a particular instruction with another
				2237	instruction, inserting the new instruction into the basic block at the
				2238	location where the old instruction was, and replacing any uses of the old
				2239	instruction with the new instruction. The following example illustrates
				2240	the replacement of one <tt>AllocaInst</tt> with another.</p>
				2241
				2242	<div class="doc_code">
				2243	<pre>
				2244	AllocaInst* instToReplace = ...;
				2245	BasicBlock::iterator ii(instToReplace);
				2246
				2247	ReplaceInstWithInst(instToReplace->getParent()->getInstList(), ii,
				2248	new AllocaInst(Type::Int32Ty, 0, "ptrToReplacedInt"));
				2249	</pre></div></li>
				2250	</ul>
				2251
				2252	<p><i>Replacing multiple uses of <tt>User</tt>s and <tt>Value</tt>s</i></p>
				2253
				2254	<p>You can use <tt>Value::replaceAllUsesWith</tt> and
				2255	<tt>User::replaceUsesOfWith</tt> to change more than one use at a time. See the
				2256	doxygen documentation for the <a href="/doxygen/classllvm_1_1Value.html">Value Class</a>
				2257	and <a href="/doxygen/classllvm_1_1User.html">User Class</a>, respectively, for more
				2258	information.</p>
				2259
				2260	<!-- Value::replaceAllUsesWith User::replaceUsesOfWith Point out:
				2261	include/llvm/Transforms/Utils/ especially BasicBlockUtils.h with:
				2262	ReplaceInstWithValue, ReplaceInstWithInst -->
				2263
				2264	</div>
				2265
				2266	<!--_______________________________________________________________________-->
				2267	<div class="doc_subsubsection">
				2268	<a name="schanges_deletingGV">Deleting <tt>GlobalVariable</tt>s</a>
				2269	</div>
				2270
				2271	<div class="doc_text">
				2272
				2273	<p>Deleting a global variable from a module is just as easy as deleting an
				2274	Instruction. First, you must have a pointer to the global variable that you wish
				2275	to delete. You use this pointer to erase it from its parent, the module.
				2276	For example:</p>
				2277
				2278	<div class="doc_code">
				2279	<pre>
				2280	<a href="#GlobalVariable">GlobalVariable</a> *GV = .. ;
				2281
				2282	GV->eraseFromParent();
				2283	</pre>
				2284	</div>
				2285
				2286	</div>
				2287
				2288	<!-- ======================================================================= -->
				2289	<div class="doc_subsection">
				2290	<a name="create_types">How to Create Types</a>
				2291	</div>
				2292
				2293	<div class="doc_text">
				2294
				2295	<p>In generating IR, you may need some complex types. If you know these types
				2296	statically, you can use <tt>TypeBuilder<...>::get()</tt>, defined
				2297	in <tt>llvm/Support/TypeBuilder.h</tt>, to retrieve them. <tt>TypeBuilder</tt>
				2298	has two forms depending on whether you're building types for cross-compilation
				2299	or native library use. <tt>TypeBuilder<T, true></tt> requires
				2300	that <tt>T</tt> be independent of the host environment, meaning that it's built
				2301	out of types from
				2302	the <a href="/doxygen/namespacellvm_1_1types.html"><tt>llvm::types</tt></a>
				2303	namespace and pointers, functions, arrays, etc. built of
				2304	those. <tt>TypeBuilder<T, false></tt> additionally allows native C types
				2305	whose size may depend on the host compiler. For example,</p>
				2306
				2307	<div class="doc_code">
				2308	<pre>
				2309	FunctionType ft = TypeBuilder<types::i<8>(types::i<32>), true>::get();
				2310	</pre>
				2311	</div>
				2312
				2313	<p>is easier to read and write than the equivalent</p>
				2314
				2315	<div class="doc_code">
				2316	<pre>
				2317	std::vector<const Type*> params;
				2318	params.push_back(PointerType::getUnqual(Type::Int32Ty));
				2319	FunctionType *ft = FunctionType::get(Type::Int8Ty, params, false);
				2320	</pre>
				2321	</div>
				2322
				2323	<p>See the <a href="/doxygen/TypeBuilder_8h-source.html#l00001">class
				2324	comment</a> for more details.</p>
				2325
				2326	</div>
				2327
				2328	<!-- *********************************************************************** -->
				2329	<div class="doc_section">
				2330	<a name="threading">Threads and LLVM</a>
				2331	</div>
				2332	<!-- *********************************************************************** -->
				2333
				2334	<div class="doc_text">
				2335	<p>
				2336	This section describes the interaction of the LLVM APIs with multithreading,
				2337	both on the part of client applications, and in the JIT, in the hosted
				2338	application.
				2339	</p>
				2340
				2341	<p>
				2342	Note that LLVM's support for multithreading is still relatively young. Up
				2343	through version 2.5, the execution of threaded hosted applications was
				2344	supported, but not threaded client access to the APIs. While this use case is
				2345	now supported, clients <em>must</em> adhere to the guidelines specified below to
				2346	ensure proper operation in multithreaded mode.
				2347	</p>
				2348
				2349	<p>
				2350	Note that, on Unix-like platforms, LLVM requires the presence of GCC's atomic
				2351	intrinsics in order to support threaded operation. If you need a
				2352	multhreading-capable LLVM on a platform without a suitably modern system
				2353	compiler, consider compiling LLVM and LLVM-GCC in single-threaded mode, and
				2354	using the resultant compiler to build a copy of LLVM with multithreading
				2355	support.
				2356	</p>
				2357	</div>
				2358
				2359	<!-- ======================================================================= -->
				2360	<div class="doc_subsection">
				2361	<a name="startmultithreaded">Entering and Exiting Multithreaded Mode</a>
				2362	</div>
				2363
				2364	<div class="doc_text">
				2365
				2366	<p>
				2367	In order to properly protect its internal data structures while avoiding
				2368	excessive locking overhead in the single-threaded case, the LLVM must intialize
				2369	certain data structures necessary to provide guards around its internals. To do
				2370	so, the client program must invoke <tt>llvm_start_multithreaded()</tt> before
				2371	making any concurrent LLVM API calls. To subsequently tear down these
				2372	structures, use the <tt>llvm_stop_multithreaded()</tt> call. You can also use
				2373	the <tt>llvm_is_multithreaded()</tt> call to check the status of multithreaded
				2374	mode.
				2375	</p>
				2376
				2377	<p>
				2378	Note that both of these calls must be made <em>in isolation</em>. That is to
				2379	say that no other LLVM API calls may be executing at any time during the
				2380	execution of <tt>llvm_start_multithreaded()</tt> or <tt>llvm_stop_multithreaded
				2381	</tt>. It's is the client's responsibility to enforce this isolation.
				2382	</p>
				2383
				2384	<p>
				2385	The return value of <tt>llvm_start_multithreaded()</tt> indicates the success or
				2386	failure of the initialization. Failure typically indicates that your copy of
				2387	LLVM was built without multithreading support, typically because GCC atomic
				2388	intrinsics were not found in your system compiler. In this case, the LLVM API
				2389	will not be safe for concurrent calls. However, it <em>will</em> be safe for
				2390	hosting threaded applications in the JIT, though <a href="#jitthreading">care
				2391	must be taken</a> to ensure that side exits and the like do not accidentally
				2392	result in concurrent LLVM API calls.
				2393	</p>
				2394	</div>
				2395
				2396	<!-- ======================================================================= -->
				2397	<div class="doc_subsection">
				2398	<a name="shutdown">Ending Execution with <tt>llvm_shutdown()</tt></a>
				2399	</div>
				2400
				2401	<div class="doc_text">
				2402	<p>
				2403	When you are done using the LLVM APIs, you should call <tt>llvm_shutdown()</tt>
				2404	to deallocate memory used for internal structures. This will also invoke
				2405	<tt>llvm_stop_multithreaded()</tt> if LLVM is operating in multithreaded mode.
				2406	As such, <tt>llvm_shutdown()</tt> requires the same isolation guarantees as
				2407	<tt>llvm_stop_multithreaded()</tt>.
				2408	</p>
				2409
				2410	<p>
				2411	Note that, if you use scope-based shutdown, you can use the
				2412	<tt>llvm_shutdown_obj</tt> class, which calls <tt>llvm_shutdown()</tt> in its
				2413	destructor.
				2414	</div>
				2415
				2416	<!-- ======================================================================= -->
				2417	<div class="doc_subsection">
				2418	<a name="managedstatic">Lazy Initialization with <tt>ManagedStatic</tt></a>
				2419	</div>
				2420
				2421	<div class="doc_text">
				2422	<p>
				2423	<tt>ManagedStatic</tt> is a utility class in LLVM used to implement static
				2424	initialization of static resources, such as the global type tables. Before the
				2425	invocation of <tt>llvm_shutdown()</tt>, it implements a simple lazy
				2426	initialization scheme. Once <tt>llvm_start_multithreaded()</tt> returns,
				2427	however, it uses double-checked locking to implement thread-safe lazy
				2428	initialization.
				2429	</p>
				2430
				2431	<p>
				2432	Note that, because no other threads are allowed to issue LLVM API calls before
				2433	<tt>llvm_start_multithreaded()</tt> returns, it is possible to have
				2434	<tt>ManagedStatic</tt>s of <tt>llvm::sys::Mutex</tt>s.
				2435	</p>
				2436
				2437	<p>
				2438	The <tt>llvm_acquire_global_lock()</tt> and <tt>llvm_release_global_lock</tt>
				2439	APIs provide access to the global lock used to implement the double-checked
				2440	locking for lazy initialization. These should only be used internally to LLVM,
				2441	and only if you know what you're doing!
				2442	</p>
				2443	</div>
				2444
				2445	<!-- ======================================================================= -->
				2446	<div class="doc_subsection">
				2447	<a name="llvmcontext">Achieving Isolation with <tt>LLVMContext</tt></a>
				2448	</div>
				2449
				2450	<div class="doc_text">
				2451	<p>
				2452	<tt>LLVMContext</tt> is an opaque class in the LLVM API which clients can use
				2453	to operate multiple, isolated instances of LLVM concurrently within the same
				2454	address space. For instance, in a hypothetical compile-server, the compilation
				2455	of an individual translation unit is conceptually independent from all the
				2456	others, and it would be desirable to be able to compile incoming translation
				2457	units concurrently on independent server threads. Fortunately,
				2458	<tt>LLVMContext</tt> exists to enable just this kind of scenario!
				2459	</p>
				2460
				2461	<p>
				2462	Conceptually, <tt>LLVMContext</tt> provides isolation. Every LLVM entity
				2463	(<tt>Module</tt>s, <tt>Value</tt>s, <tt>Type</tt>s, <tt>Constant</tt>s, etc.)
				2464	in LLVM's in-memory IR belongs to an <tt>LLVMContext</tt>. Entities in
				2465	different contexts <em>cannot</em> interact with each other: <tt>Module</tt>s in
				2466	different contexts cannot be linked together, <tt>Function</tt>s cannot be added
				2467	to <tt>Module</tt>s in different contexts, etc. What this means is that is is
				2468	safe to compile on multiple threads simultaneously, as long as no two threads
				2469	operate on entities within the same context.
				2470	</p>
				2471
				2472	<p>
				2473	In practice, very few places in the API require the explicit specification of a
				2474	<tt>LLVMContext</tt>, other than the <tt>Type</tt> creation/lookup APIs.
				2475	Because every <tt>Type</tt> carries a reference to its owning context, most
				2476	other entities can determine what context they belong to by looking at their
				2477	own <tt>Type</tt>. If you are adding new entities to LLVM IR, please try to
				2478	maintain this interface design.
				2479	</p>
				2480
				2481	<p>
				2482	For clients that do <em>not</em> require the benefits of isolation, LLVM
				2483	provides a convenience API <tt>getGlobalContext()</tt>. This returns a global,
				2484	lazily initialized <tt>LLVMContext</tt> that may be used in situations where
				2485	isolation is not a concern.
				2486	</p>
				2487	</div>
				2488
				2489	<!-- ======================================================================= -->
				2490	<div class="doc_subsection">
				2491	<a name="jitthreading">Threads and the JIT</a>
				2492	</div>
				2493
				2494	<div class="doc_text">
				2495	<p>
				2496	LLVM's "eager" JIT compiler is safe to use in threaded programs. Multiple
				2497	threads can call <tt>ExecutionEngine::getPointerToFunction()</tt> or
				2498	<tt>ExecutionEngine::runFunction()</tt> concurrently, and multiple threads can
				2499	run code output by the JIT concurrently. The user must still ensure that only
				2500	one thread accesses IR in a given <tt>LLVMContext</tt> while another thread
				2501	might be modifying it. One way to do that is to always hold the JIT lock while
				2502	accessing IR outside the JIT (the JIT <em>modifies</em> the IR by adding
				2503	<tt>CallbackVH</tt>s). Another way is to only
				2504	call <tt>getPointerToFunction()</tt> from the <tt>LLVMContext</tt>'s thread.
				2505	</p>
				2506
				2507	<p>When the JIT is configured to compile lazily (using
				2508	<tt>ExecutionEngine::DisableLazyCompilation(false)</tt>), there is currently a
				2509	<a href="http://llvm.org/bugs/show_bug.cgi?id=5184">race condition</a> in
				2510	updating call sites after a function is lazily-jitted. It's still possible to
				2511	use the lazy JIT in a threaded program if you ensure that only one thread at a
				2512	time can call any particular lazy stub and that the JIT lock guards any IR
				2513	access, but we suggest using only the eager JIT in threaded programs.
				2514	</p>
				2515	</div>
				2516
				2517	<!-- *********************************************************************** -->
				2518	<div class="doc_section">
				2519	<a name="advanced">Advanced Topics</a>
				2520	</div>
				2521	<!-- *********************************************************************** -->
				2522
				2523	<div class="doc_text">
				2524	<p>
				2525	This section describes some of the advanced or obscure API's that most clients
				2526	do not need to be aware of. These API's tend manage the inner workings of the
				2527	LLVM system, and only need to be accessed in unusual circumstances.
				2528	</p>
				2529	</div>
				2530
				2531	<!-- ======================================================================= -->
				2532	<div class="doc_subsection">
				2533	<a name="TypeResolve">LLVM Type Resolution</a>
				2534	</div>
				2535
				2536	<div class="doc_text">
				2537
				2538	<p>
				2539	The LLVM type system has a very simple goal: allow clients to compare types for
				2540	structural equality with a simple pointer comparison (aka a shallow compare).
				2541	This goal makes clients much simpler and faster, and is used throughout the LLVM
				2542	system.
				2543	</p>
				2544
				2545	<p>
				2546	Unfortunately achieving this goal is not a simple matter. In particular,
				2547	recursive types and late resolution of opaque types makes the situation very
				2548	difficult to handle. Fortunately, for the most part, our implementation makes
				2549	most clients able to be completely unaware of the nasty internal details. The
				2550	primary case where clients are exposed to the inner workings of it are when
				2551	building a recursive type. In addition to this case, the LLVM bitcode reader,
				2552	assembly parser, and linker also have to be aware of the inner workings of this
				2553	system.
				2554	</p>
				2555
				2556	<p>
				2557	For our purposes below, we need three concepts. First, an "Opaque Type" is
				2558	exactly as defined in the <a href="LangRef.html#t_opaque">language
				2559	reference</a>. Second an "Abstract Type" is any type which includes an
				2560	opaque type as part of its type graph (for example "<tt>{ opaque, i32 }</tt>").
				2561	Third, a concrete type is a type that is not an abstract type (e.g. "<tt>{ i32,
				2562	float }</tt>").
				2563	</p>
				2564
				2565	</div>
				2566
				2567	<!-- ______________________________________________________________________ -->
				2568	<div class="doc_subsubsection">
				2569	<a name="BuildRecType">Basic Recursive Type Construction</a>
				2570	</div>
				2571
				2572	<div class="doc_text">
				2573
				2574	<p>
				2575	Because the most common question is "how do I build a recursive type with LLVM",
				2576	we answer it now and explain it as we go. Here we include enough to cause this
				2577	to be emitted to an output .ll file:
				2578	</p>
				2579
				2580	<div class="doc_code">
				2581	<pre>
				2582	%mylist = type { %mylist*, i32 }
				2583	</pre>
				2584	</div>
				2585
				2586	<p>
				2587	To build this, use the following LLVM APIs:
				2588	</p>
				2589
				2590	<div class="doc_code">
				2591	<pre>
				2592	// <i>Create the initial outer struct</i>
				2593	<a href="#PATypeHolder">PATypeHolder</a> StructTy = OpaqueType::get();
				2594	std::vector<const Type*> Elts;
				2595	Elts.push_back(PointerType::getUnqual(StructTy));
				2596	Elts.push_back(Type::Int32Ty);
				2597	StructType *NewSTy = StructType::get(Elts);
				2598
				2599	// <i>At this point, NewSTy = "{ opaque*, i32 }". Tell VMCore that</i>
				2600	// <i>the struct and the opaque type are actually the same.</i>
				2601	cast<OpaqueType>(StructTy.get())-><a href="#refineAbstractTypeTo">refineAbstractTypeTo</a>(NewSTy);
				2602
				2603	// <i>NewSTy is potentially invalidated, but StructTy (a <a href="#PATypeHolder">PATypeHolder</a>) is</i>
				2604	// <i>kept up-to-date</i>
				2605	NewSTy = cast<StructType>(StructTy.get());
				2606
				2607	// <i>Add a name for the type to the module symbol table (optional)</i>
				2608	MyModule->addTypeName("mylist", NewSTy);
				2609	</pre>
				2610	</div>
				2611
				2612	<p>
				2613	This code shows the basic approach used to build recursive types: build a
				2614	non-recursive type using 'opaque', then use type unification to close the cycle.
				2615	The type unification step is performed by the <tt><a
				2616	href="#refineAbstractTypeTo">refineAbstractTypeTo</a></tt> method, which is
				2617	described next. After that, we describe the <a
				2618	href="#PATypeHolder">PATypeHolder class</a>.
				2619	</p>
				2620
				2621	</div>
				2622
				2623	<!-- ______________________________________________________________________ -->
				2624	<div class="doc_subsubsection">
				2625	<a name="refineAbstractTypeTo">The <tt>refineAbstractTypeTo</tt> method</a>
				2626	</div>
				2627
				2628	<div class="doc_text">
				2629	<p>
				2630	The <tt>refineAbstractTypeTo</tt> method starts the type unification process.
				2631	While this method is actually a member of the DerivedType class, it is most
				2632	often used on OpaqueType instances. Type unification is actually a recursive
				2633	process. After unification, types can become structurally isomorphic to
				2634	existing types, and all duplicates are deleted (to preserve pointer equality).
				2635	</p>
				2636
				2637	<p>
				2638	In the example above, the OpaqueType object is definitely deleted.
				2639	Additionally, if there is an "{ \2*, i32}" type already created in the system,
				2640	the pointer and struct type created are <b>also</b> deleted. Obviously whenever
				2641	a type is deleted, any "Type*" pointers in the program are invalidated. As
				2642	such, it is safest to avoid having <i>any</i> "Type*" pointers to abstract types
				2643	live across a call to <tt>refineAbstractTypeTo</tt> (note that non-abstract
				2644	types can never move or be deleted). To deal with this, the <a
				2645	href="#PATypeHolder">PATypeHolder</a> class is used to maintain a stable
				2646	reference to a possibly refined type, and the <a
				2647	href="#AbstractTypeUser">AbstractTypeUser</a> class is used to update more
				2648	complex datastructures.
				2649	</p>
				2650
				2651	</div>
				2652
				2653	<!-- ______________________________________________________________________ -->
				2654	<div class="doc_subsubsection">
				2655	<a name="PATypeHolder">The PATypeHolder Class</a>
				2656	</div>
				2657
				2658	<div class="doc_text">
				2659	<p>
				2660	PATypeHolder is a form of a "smart pointer" for Type objects. When VMCore
				2661	happily goes about nuking types that become isomorphic to existing types, it
				2662	automatically updates all PATypeHolder objects to point to the new type. In the
				2663	example above, this allows the code to maintain a pointer to the resultant
				2664	resolved recursive type, even though the Type*'s are potentially invalidated.
				2665	</p>
				2666
				2667	<p>
				2668	PATypeHolder is an extremely light-weight object that uses a lazy union-find
				2669	implementation to update pointers. For example the pointer from a Value to its
				2670	Type is maintained by PATypeHolder objects.
				2671	</p>
				2672
				2673	</div>
				2674
				2675	<!-- ______________________________________________________________________ -->
				2676	<div class="doc_subsubsection">
				2677	<a name="AbstractTypeUser">The AbstractTypeUser Class</a>
				2678	</div>
				2679
				2680	<div class="doc_text">
				2681
				2682	<p>
				2683	Some data structures need more to perform more complex updates when types get
				2684	resolved. To support this, a class can derive from the AbstractTypeUser class.
				2685	This class
				2686	allows it to get callbacks when certain types are resolved. To register to get
				2687	callbacks for a particular type, the DerivedType::{add/remove}AbstractTypeUser
				2688	methods can be called on a type. Note that these methods only work for <i>
				2689	abstract</i> types. Concrete types (those that do not include any opaque
				2690	objects) can never be refined.
				2691	</p>
				2692	</div>
				2693
				2694
				2695	<!-- ======================================================================= -->
				2696	<div class="doc_subsection">
				2697	<a name="SymbolTable">The <tt>ValueSymbolTable</tt> and
				2698	<tt>TypeSymbolTable</tt> classes</a>
				2699	</div>
				2700
				2701	<div class="doc_text">
				2702	<p>The <tt><a href="http://llvm.org/doxygen/classllvm_1_1ValueSymbolTable.html">
				2703	ValueSymbolTable</a></tt> class provides a symbol table that the <a
				2704	href="#Function"><tt>Function</tt></a> and <a href="#Module">
				2705	<tt>Module</tt></a> classes use for naming value definitions. The symbol table
				2706	can provide a name for any <a href="#Value"><tt>Value</tt></a>.
				2707	The <tt><a href="http://llvm.org/doxygen/classllvm_1_1TypeSymbolTable.html">
				2708	TypeSymbolTable</a></tt> class is used by the <tt>Module</tt> class to store
				2709	names for types.</p>
				2710
				2711	<p>Note that the <tt>SymbolTable</tt> class should not be directly accessed
				2712	by most clients. It should only be used when iteration over the symbol table
				2713	names themselves are required, which is very special purpose. Note that not
				2714	all LLVM
				2715	<tt><a href="#Value">Value</a></tt>s have names, and those without names (i.e. they have
				2716	an empty name) do not exist in the symbol table.
				2717	</p>
				2718
				2719	<p>These symbol tables support iteration over the values/types in the symbol
				2720	table with <tt>begin/end/iterator</tt> and supports querying to see if a
				2721	specific name is in the symbol table (with <tt>lookup</tt>). The
				2722	<tt>ValueSymbolTable</tt> class exposes no public mutator methods, instead,
				2723	simply call <tt>setName</tt> on a value, which will autoinsert it into the
				2724	appropriate symbol table. For types, use the Module::addTypeName method to
				2725	insert entries into the symbol table.</p>
				2726
				2727	</div>
				2728
				2729
				2730
				2731	<!-- ======================================================================= -->
				2732	<div class="doc_subsection">
				2733	<a name="UserLayout">The <tt>User</tt> and owned <tt>Use</tt> classes' memory layout</a>
				2734	</div>
				2735
				2736	<div class="doc_text">
				2737	<p>The <tt><a href="http://llvm.org/doxygen/classllvm_1_1User.html">
				2738	User</a></tt> class provides a basis for expressing the ownership of <tt>User</tt>
				2739	towards other <tt><a href="http://llvm.org/doxygen/classllvm_1_1Value.html">
				2740	Value</a></tt>s. The <tt><a href="http://llvm.org/doxygen/classllvm_1_1Use.html">
				2741	Use</a></tt> helper class is employed to do the bookkeeping and to facilitate <i>O(1)</i>
				2742	addition and removal.</p>
				2743
				2744	<!-- ______________________________________________________________________ -->
				2745	<div class="doc_subsubsection">
				2746	<a name="Use2User">Interaction and relationship between <tt>User</tt> and <tt>Use</tt> objects</a>
				2747	</div>
				2748
				2749	<div class="doc_text">
				2750	<p>
				2751	A subclass of <tt>User</tt> can choose between incorporating its <tt>Use</tt> objects
				2752	or refer to them out-of-line by means of a pointer. A mixed variant
				2753	(some <tt>Use</tt>s inline others hung off) is impractical and breaks the invariant
				2754	that the <tt>Use</tt> objects belonging to the same <tt>User</tt> form a contiguous array.
				2755	</p>
				2756	</div>
				2757
				2758	<p>
				2759	We have 2 different layouts in the <tt>User</tt> (sub)classes:
				2760	<ul>
				2761	<li><p>Layout a)
				2762	The <tt>Use</tt> object(s) are inside (resp. at fixed offset) of the <tt>User</tt>
				2763	object and there are a fixed number of them.</p>
				2764
				2765	<li><p>Layout b)
				2766	The <tt>Use</tt> object(s) are referenced by a pointer to an
				2767	array from the <tt>User</tt> object and there may be a variable
				2768	number of them.</p>
				2769	</ul>
				2770	<p>
				2771	As of v2.4 each layout still possesses a direct pointer to the
				2772	start of the array of <tt>Use</tt>s. Though not mandatory for layout a),
				2773	we stick to this redundancy for the sake of simplicity.
				2774	The <tt>User</tt> object also stores the number of <tt>Use</tt> objects it
				2775	has. (Theoretically this information can also be calculated
				2776	given the scheme presented below.)</p>
				2777	<p>
				2778	Special forms of allocation operators (<tt>operator new</tt>)
				2779	enforce the following memory layouts:</p>
				2780
				2781	<ul>
				2782	<li><p>Layout a) is modelled by prepending the <tt>User</tt> object by the <tt>Use[]</tt> array.</p>
				2783
				2784	<pre>
				2785	...---.---.---.---.-------...
				2786	\| P \| P \| P \| P \| User
				2787	'''---'---'---'---'-------'''
				2788	</pre>
				2789
				2790	<li><p>Layout b) is modelled by pointing at the <tt>Use[]</tt> array.</p>
				2791	<pre>
				2792	.-------...
				2793	\| User
				2794	'-------'''
				2795	\|
				2796	v
				2797	.---.---.---.---...
				2798	\| P \| P \| P \| P \|
				2799	'---'---'---'---'''
				2800	</pre>
				2801	</ul>
				2802	<i>(In the above figures '<tt>P</tt>' stands for the <tt>Use**</tt> that
				2803	is stored in each <tt>Use</tt> object in the member <tt>Use::Prev</tt>)</i>
				2804
				2805	<!-- ______________________________________________________________________ -->
				2806	<div class="doc_subsubsection">
				2807	<a name="Waymarking">The waymarking algorithm</a>
				2808	</div>
				2809
				2810	<div class="doc_text">
				2811	<p>
				2812	Since the <tt>Use</tt> objects are deprived of the direct (back)pointer to
				2813	their <tt>User</tt> objects, there must be a fast and exact method to
				2814	recover it. This is accomplished by the following scheme:</p>
				2815	</div>
				2816
				2817	A bit-encoding in the 2 LSBits (least significant bits) of the <tt>Use::Prev</tt> allows to find the
				2818	start of the <tt>User</tt> object:
				2819	<ul>
				2820	<li><tt>00</tt> —> binary digit 0</li>
				2821	<li><tt>01</tt> —> binary digit 1</li>
				2822	<li><tt>10</tt> —> stop and calculate (<tt>s</tt>)</li>
				2823	<li><tt>11</tt> —> full stop (<tt>S</tt>)</li>
				2824	</ul>
				2825	<p>
				2826	Given a <tt>Use*</tt>, all we have to do is to walk till we get
				2827	a stop and we either have a <tt>User</tt> immediately behind or
				2828	we have to walk to the next stop picking up digits
				2829	and calculating the offset:</p>
				2830	<pre>
				2831	.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.---.----------------
				2832	\| 1 \| s \| 1 \| 0 \| 1 \| 0 \| s \| 1 \| 1 \| 0 \| s \| 1 \| 1 \| s \| 1 \| S \| User (or User*)
				2833	'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'---'----------------
				2834	\|+15 \|+10 \|+6 \|+3 \|+1
				2835	\| \| \| \| \|__>
				2836	\| \| \| \|__________>
				2837	\| \| \|______________________>
				2838	\| \|______________________________________>
				2839	\|__________________________________________________________>
				2840	</pre>
				2841	<p>
				2842	Only the significant number of bits need to be stored between the
				2843	stops, so that the <i>worst case is 20 memory accesses</i> when there are
				2844	1000 <tt>Use</tt> objects associated with a <tt>User</tt>.</p>
				2845
				2846	<!-- ______________________________________________________________________ -->
				2847	<div class="doc_subsubsection">
				2848	<a name="ReferenceImpl">Reference implementation</a>
				2849	</div>
				2850
				2851	<div class="doc_text">
				2852	<p>
				2853	The following literate Haskell fragment demonstrates the concept:</p>
				2854	</div>
				2855
				2856	<div class="doc_code">
				2857	<pre>
				2858	> import Test.QuickCheck
				2859	>
				2860	> digits :: Int -> [Char] -> [Char]
				2861	> digits 0 acc = '0' : acc
				2862	> digits 1 acc = '1' : acc
				2863	> digits n acc = digits (n `div` 2) $ digits (n `mod` 2) acc
				2864	>
				2865	> dist :: Int -> [Char] -> [Char]
				2866	> dist 0 [] = ['S']
				2867	> dist 0 acc = acc
				2868	> dist 1 acc = let r = dist 0 acc in 's' : digits (length r) r
				2869	> dist n acc = dist (n - 1) $ dist 1 acc
				2870	>
				2871	> takeLast n ss = reverse $ take n $ reverse ss
				2872	>
				2873	> test = takeLast 40 $ dist 20 []
				2874	>
				2875	</pre>
				2876	</div>
				2877	<p>
				2878	Printing <test> gives: <tt>"1s100000s11010s10100s1111s1010s110s11s1S"</tt></p>
				2879	<p>
				2880	The reverse algorithm computes the length of the string just by examining
				2881	a certain prefix:</p>
				2882
				2883	<div class="doc_code">
				2884	<pre>
				2885	> pref :: [Char] -> Int
				2886	> pref "S" = 1
				2887	> pref ('s':'1':rest) = decode 2 1 rest
				2888	> pref (_:rest) = 1 + pref rest
				2889	>
				2890	> decode walk acc ('0':rest) = decode (walk + 1) (acc * 2) rest
				2891	> decode walk acc ('1':rest) = decode (walk + 1) (acc * 2 + 1) rest
				2892	> decode walk acc _ = walk + acc
				2893	>
				2894	</pre>
				2895	</div>
				2896	<p>
				2897	Now, as expected, printing <pref test> gives <tt>40</tt>.</p>
				2898	<p>
				2899	We can <i>quickCheck</i> this with following property:</p>
				2900
				2901	<div class="doc_code">
				2902	<pre>
				2903	> testcase = dist 2000 []
				2904	> testcaseLength = length testcase
				2905	>
				2906	> identityProp n = n > 0 && n <= testcaseLength ==> length arr == pref arr
				2907	> where arr = takeLast n testcase
				2908	>
				2909	</pre>
				2910	</div>
				2911	<p>
				2912	As expected <quickCheck identityProp> gives:</p>
				2913
				2914	<pre>
				2915	*Main> quickCheck identityProp
				2916	OK, passed 100 tests.
				2917	</pre>
				2918	<p>
				2919	Let's be a bit more exhaustive:</p>
				2920
				2921	<div class="doc_code">
				2922	<pre>
				2923	>
				2924	> deepCheck p = check (defaultConfig { configMaxTest = 500 }) p
				2925	>
				2926	</pre>
				2927	</div>
				2928	<p>
				2929	And here is the result of <deepCheck identityProp>:</p>
				2930
				2931	<pre>
				2932	*Main> deepCheck identityProp
				2933	OK, passed 500 tests.
				2934	</pre>
				2935
				2936	<!-- ______________________________________________________________________ -->
				2937	<div class="doc_subsubsection">
				2938	<a name="Tagging">Tagging considerations</a>
				2939	</div>
				2940
				2941	<p>
				2942	To maintain the invariant that the 2 LSBits of each <tt>Use**</tt> in <tt>Use</tt>
				2943	never change after being set up, setters of <tt>Use::Prev</tt> must re-tag the
				2944	new <tt>Use**</tt> on every modification. Accordingly getters must strip the
				2945	tag bits.</p>
				2946	<p>
				2947	For layout b) instead of the <tt>User</tt> we find a pointer (<tt>User*</tt> with LSBit set).
				2948	Following this pointer brings us to the <tt>User</tt>. A portable trick ensures
				2949	that the first bytes of <tt>User</tt> (if interpreted as a pointer) never has
				2950	the LSBit set. (Portability is relying on the fact that all known compilers place the
				2951	<tt>vptr</tt> in the first word of the instances.)</p>
				2952
				2953	</div>
				2954
				2955	<!-- *********************************************************************** -->
				2956	<div class="doc_section">
				2957	<a name="coreclasses">The Core LLVM Class Hierarchy Reference </a>
				2958	</div>
				2959	<!-- *********************************************************************** -->
				2960
				2961	<div class="doc_text">
				2962	<p><tt>#include "<a href="/doxygen/Type_8h-source.html">llvm/Type.h</a>"</tt>
				2963	<br>doxygen info: <a href="/doxygen/classllvm_1_1Type.html">Type Class</a></p>
				2964
				2965	<p>The Core LLVM classes are the primary means of representing the program
				2966	being inspected or transformed. The core LLVM classes are defined in
				2967	header files in the <tt>include/llvm/</tt> directory, and implemented in
				2968	the <tt>lib/VMCore</tt> directory.</p>
				2969
				2970	</div>
				2971
				2972	<!-- ======================================================================= -->
				2973	<div class="doc_subsection">
				2974	<a name="Type">The <tt>Type</tt> class and Derived Types</a>
				2975	</div>
				2976
				2977	<div class="doc_text">
				2978
				2979	<p><tt>Type</tt> is a superclass of all type classes. Every <tt>Value</tt> has
				2980	a <tt>Type</tt>. <tt>Type</tt> cannot be instantiated directly but only
				2981	through its subclasses. Certain primitive types (<tt>VoidType</tt>,
				2982	<tt>LabelType</tt>, <tt>FloatType</tt> and <tt>DoubleType</tt>) have hidden
				2983	subclasses. They are hidden because they offer no useful functionality beyond
				2984	what the <tt>Type</tt> class offers except to distinguish themselves from
				2985	other subclasses of <tt>Type</tt>.</p>
				2986	<p>All other types are subclasses of <tt>DerivedType</tt>. Types can be
				2987	named, but this is not a requirement. There exists exactly
				2988	one instance of a given shape at any one time. This allows type equality to
				2989	be performed with address equality of the Type Instance. That is, given two
				2990	<tt>Type*</tt> values, the types are identical if the pointers are identical.
				2991	</p>
				2992	</div>
				2993
				2994	<!-- _______________________________________________________________________ -->
				2995	<div class="doc_subsubsection">
				2996	<a name="m_Type">Important Public Methods</a>
				2997	</div>
				2998
				2999	<div class="doc_text">
				3000
				3001	<ul>
				3002	<li><tt>bool isInteger() const</tt>: Returns true for any integer type.</li>
				3003
				3004	<li><tt>bool isFloatingPoint()</tt>: Return true if this is one of the two
				3005	floating point types.</li>
				3006
				3007	<li><tt>bool isAbstract()</tt>: Return true if the type is abstract (contains
				3008	an OpaqueType anywhere in its definition).</li>
				3009
				3010	<li><tt>bool isSized()</tt>: Return true if the type has known size. Things
				3011	that don't have a size are abstract types, labels and void.</li>
				3012
				3013	</ul>
				3014	</div>
				3015
				3016	<!-- _______________________________________________________________________ -->
				3017	<div class="doc_subsubsection">
				3018	<a name="derivedtypes">Important Derived Types</a>
				3019	</div>
				3020	<div class="doc_text">
				3021	<dl>
				3022	<dt><tt>IntegerType</tt></dt>
				3023	<dd>Subclass of DerivedType that represents integer types of any bit width.
				3024	Any bit width between <tt>IntegerType::MIN_INT_BITS</tt> (1) and
				3025	<tt>IntegerType::MAX_INT_BITS</tt> (~8 million) can be represented.
				3026	<ul>
				3027	<li><tt>static const IntegerType* get(unsigned NumBits)</tt>: get an integer
				3028	type of a specific bit width.</li>
				3029	<li><tt>unsigned getBitWidth() const</tt>: Get the bit width of an integer
				3030	type.</li>
				3031	</ul>
				3032	</dd>
				3033	<dt><tt>SequentialType</tt></dt>
				3034	<dd>This is subclassed by ArrayType and PointerType
				3035	<ul>
				3036	<li><tt>const Type * getElementType() const</tt>: Returns the type of each
				3037	of the elements in the sequential type. </li>
				3038	</ul>
				3039	</dd>
				3040	<dt><tt>ArrayType</tt></dt>
				3041	<dd>This is a subclass of SequentialType and defines the interface for array
				3042	types.
				3043	<ul>
				3044	<li><tt>unsigned getNumElements() const</tt>: Returns the number of
				3045	elements in the array. </li>
				3046	</ul>
				3047	</dd>
				3048	<dt><tt>PointerType</tt></dt>
				3049	<dd>Subclass of SequentialType for pointer types.</dd>
				3050	<dt><tt>VectorType</tt></dt>
				3051	<dd>Subclass of SequentialType for vector types. A
				3052	vector type is similar to an ArrayType but is distinguished because it is
				3053	a first class type whereas ArrayType is not. Vector types are used for
				3054	vector operations and are usually small vectors of of an integer or floating
				3055	point type.</dd>
				3056	<dt><tt>StructType</tt></dt>
				3057	<dd>Subclass of DerivedTypes for struct types.</dd>
				3058	<dt><tt><a name="FunctionType">FunctionType</a></tt></dt>
				3059	<dd>Subclass of DerivedTypes for function types.
				3060	<ul>
				3061	<li><tt>bool isVarArg() const</tt>: Returns true if its a vararg
				3062	function</li>
				3063	<li><tt> const Type * getReturnType() const</tt>: Returns the
				3064	return type of the function.</li>
				3065	<li><tt>const Type * getParamType (unsigned i)</tt>: Returns
				3066	the type of the ith parameter.</li>
				3067	<li><tt> const unsigned getNumParams() const</tt>: Returns the
				3068	number of formal parameters.</li>
				3069	</ul>
				3070	</dd>
				3071	<dt><tt>OpaqueType</tt></dt>
				3072	<dd>Sublcass of DerivedType for abstract types. This class
				3073	defines no content and is used as a placeholder for some other type. Note
				3074	that OpaqueType is used (temporarily) during type resolution for forward
				3075	references of types. Once the referenced type is resolved, the OpaqueType
				3076	is replaced with the actual type. OpaqueType can also be used for data
				3077	abstraction. At link time opaque types can be resolved to actual types
				3078	of the same name.</dd>
				3079	</dl>
				3080	</div>
				3081
				3082
				3083
				3084	<!-- ======================================================================= -->
				3085	<div class="doc_subsection">
				3086	<a name="Module">The <tt>Module</tt> class</a>
				3087	</div>
				3088
				3089	<div class="doc_text">
				3090
				3091	<p><tt>#include "<a
				3092	href="/doxygen/Module_8h-source.html">llvm/Module.h</a>"</tt><br> doxygen info:
				3093	<a href="/doxygen/classllvm_1_1Module.html">Module Class</a></p>
				3094
				3095	<p>The <tt>Module</tt> class represents the top level structure present in LLVM
				3096	programs. An LLVM module is effectively either a translation unit of the
				3097	original program or a combination of several translation units merged by the
				3098	linker. The <tt>Module</tt> class keeps track of a list of <a
				3099	href="#Function"><tt>Function</tt></a>s, a list of <a
				3100	href="#GlobalVariable"><tt>GlobalVariable</tt></a>s, and a <a
				3101	href="#SymbolTable"><tt>SymbolTable</tt></a>. Additionally, it contains a few
				3102	helpful member functions that try to make common operations easy.</p>
				3103
				3104	</div>
				3105
				3106	<!-- _______________________________________________________________________ -->
				3107	<div class="doc_subsubsection">
				3108	<a name="m_Module">Important Public Members of the <tt>Module</tt> class</a>
				3109	</div>
				3110
				3111	<div class="doc_text">
				3112
				3113	<ul>
				3114	<li><tt>Module::Module(std::string name = "")</tt></li>
				3115	</ul>
				3116
				3117	<p>Constructing a <a href="#Module">Module</a> is easy. You can optionally
				3118	provide a name for it (probably based on the name of the translation unit).</p>
				3119
				3120	<ul>
				3121	<li><tt>Module::iterator</tt> - Typedef for function list iterator<br>
				3122	<tt>Module::const_iterator</tt> - Typedef for const_iterator.<br>
				3123
				3124	<tt>begin()</tt>, <tt>end()</tt>
				3125	<tt>size()</tt>, <tt>empty()</tt>
				3126
				3127	<p>These are forwarding methods that make it easy to access the contents of
				3128	a <tt>Module</tt> object's <a href="#Function"><tt>Function</tt></a>
				3129	list.</p></li>
				3130
				3131	<li><tt>Module::FunctionListType &getFunctionList()</tt>
				3132
				3133	<p> Returns the list of <a href="#Function"><tt>Function</tt></a>s. This is
				3134	necessary to use when you need to update the list or perform a complex
				3135	action that doesn't have a forwarding method.</p>
				3136
				3137	<p><!-- Global Variable --></p></li>
				3138	</ul>
				3139
				3140	<hr>
				3141
				3142	<ul>
				3143	<li><tt>Module::global_iterator</tt> - Typedef for global variable list iterator<br>
				3144
				3145	<tt>Module::const_global_iterator</tt> - Typedef for const_iterator.<br>
				3146
				3147	<tt>global_begin()</tt>, <tt>global_end()</tt>
				3148	<tt>global_size()</tt>, <tt>global_empty()</tt>
				3149
				3150	<p> These are forwarding methods that make it easy to access the contents of
				3151	a <tt>Module</tt> object's <a
				3152	href="#GlobalVariable"><tt>GlobalVariable</tt></a> list.</p></li>
				3153
				3154	<li><tt>Module::GlobalListType &getGlobalList()</tt>
				3155
				3156	<p>Returns the list of <a
				3157	href="#GlobalVariable"><tt>GlobalVariable</tt></a>s. This is necessary to
				3158	use when you need to update the list or perform a complex action that
				3159	doesn't have a forwarding method.</p>
				3160
				3161	<p><!-- Symbol table stuff --> </p></li>
				3162	</ul>
				3163
				3164	<hr>
				3165
				3166	<ul>
				3167	<li><tt><a href="#SymbolTable">SymbolTable</a> *getSymbolTable()</tt>
				3168
				3169	<p>Return a reference to the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
				3170	for this <tt>Module</tt>.</p>
				3171
				3172	<p><!-- Convenience methods --></p></li>
				3173	</ul>
				3174
				3175	<hr>
				3176
				3177	<ul>
				3178	<li><tt><a href="#Function">Function</a> *getFunction(const std::string
				3179	&Name, const <a href="#FunctionType">FunctionType</a> *Ty)</tt>
				3180
				3181	<p>Look up the specified function in the <tt>Module</tt> <a
				3182	href="#SymbolTable"><tt>SymbolTable</tt></a>. If it does not exist, return
				3183	<tt>null</tt>.</p></li>
				3184
				3185	<li><tt><a href="#Function">Function</a> *getOrInsertFunction(const
				3186	std::string &Name, const <a href="#FunctionType">FunctionType</a> *T)</tt>
				3187
				3188	<p>Look up the specified function in the <tt>Module</tt> <a
				3189	href="#SymbolTable"><tt>SymbolTable</tt></a>. If it does not exist, add an
				3190	external declaration for the function and return it.</p></li>
				3191
				3192	<li><tt>std::string getTypeName(const <a href="#Type">Type</a> *Ty)</tt>
				3193
				3194	<p>If there is at least one entry in the <a
				3195	href="#SymbolTable"><tt>SymbolTable</tt></a> for the specified <a
				3196	href="#Type"><tt>Type</tt></a>, return it. Otherwise return the empty
				3197	string.</p></li>
				3198
				3199	<li><tt>bool addTypeName(const std::string &Name, const <a
				3200	href="#Type">Type</a> *Ty)</tt>
				3201
				3202	<p>Insert an entry in the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
				3203	mapping <tt>Name</tt> to <tt>Ty</tt>. If there is already an entry for this
				3204	name, true is returned and the <a
				3205	href="#SymbolTable"><tt>SymbolTable</tt></a> is not modified.</p></li>
				3206	</ul>
				3207
				3208	</div>
				3209
				3210
				3211	<!-- ======================================================================= -->
				3212	<div class="doc_subsection">
				3213	<a name="Value">The <tt>Value</tt> class</a>
				3214	</div>
				3215
				3216	<div class="doc_text">
				3217
				3218	<p><tt>#include "<a href="/doxygen/Value_8h-source.html">llvm/Value.h</a>"</tt>
				3219	<br>
				3220	doxygen info: <a href="/doxygen/classllvm_1_1Value.html">Value Class</a></p>
				3221
				3222	<p>The <tt>Value</tt> class is the most important class in the LLVM Source
				3223	base. It represents a typed value that may be used (among other things) as an
				3224	operand to an instruction. There are many different types of <tt>Value</tt>s,
				3225	such as <a href="#Constant"><tt>Constant</tt></a>s,<a
				3226	href="#Argument"><tt>Argument</tt></a>s. Even <a
				3227	href="#Instruction"><tt>Instruction</tt></a>s and <a
				3228	href="#Function"><tt>Function</tt></a>s are <tt>Value</tt>s.</p>
				3229
				3230	<p>A particular <tt>Value</tt> may be used many times in the LLVM representation
				3231	for a program. For example, an incoming argument to a function (represented
				3232	with an instance of the <a href="#Argument">Argument</a> class) is "used" by
				3233	every instruction in the function that references the argument. To keep track
				3234	of this relationship, the <tt>Value</tt> class keeps a list of all of the <a
				3235	href="#User"><tt>User</tt></a>s that is using it (the <a
				3236	href="#User"><tt>User</tt></a> class is a base class for all nodes in the LLVM
				3237	graph that can refer to <tt>Value</tt>s). This use list is how LLVM represents
				3238	def-use information in the program, and is accessible through the <tt>use_</tt>*
				3239	methods, shown below.</p>
				3240
				3241	<p>Because LLVM is a typed representation, every LLVM <tt>Value</tt> is typed,
				3242	and this <a href="#Type">Type</a> is available through the <tt>getType()</tt>
				3243	method. In addition, all LLVM values can be named. The "name" of the
				3244	<tt>Value</tt> is a symbolic string printed in the LLVM code:</p>
				3245
				3246	<div class="doc_code">
				3247	<pre>
				3248	%<b>foo</b> = add i32 1, 2
				3249	</pre>
				3250	</div>
				3251
				3252	<p><a name="nameWarning">The name of this instruction is "foo".</a> <b>NOTE</b>
				3253	that the name of any value may be missing (an empty string), so names should
				3254	<b>ONLY</b> be used for debugging (making the source code easier to read,
				3255	debugging printouts), they should not be used to keep track of values or map
				3256	between them. For this purpose, use a <tt>std::map</tt> of pointers to the
				3257	<tt>Value</tt> itself instead.</p>
				3258
				3259	<p>One important aspect of LLVM is that there is no distinction between an SSA
				3260	variable and the operation that produces it. Because of this, any reference to
				3261	the value produced by an instruction (or the value available as an incoming
				3262	argument, for example) is represented as a direct pointer to the instance of
				3263	the class that
				3264	represents this value. Although this may take some getting used to, it
				3265	simplifies the representation and makes it easier to manipulate.</p>
				3266
				3267	</div>
				3268
				3269	<!-- _______________________________________________________________________ -->
				3270	<div class="doc_subsubsection">
				3271	<a name="m_Value">Important Public Members of the <tt>Value</tt> class</a>
				3272	</div>
				3273
				3274	<div class="doc_text">
				3275
				3276	<ul>
				3277	<li><tt>Value::use_iterator</tt> - Typedef for iterator over the
				3278	use-list<br>
				3279	<tt>Value::use_const_iterator</tt> - Typedef for const_iterator over
				3280	the use-list<br>
				3281	<tt>unsigned use_size()</tt> - Returns the number of users of the
				3282	value.<br>
				3283	<tt>bool use_empty()</tt> - Returns true if there are no users.<br>
				3284	<tt>use_iterator use_begin()</tt> - Get an iterator to the start of
				3285	the use-list.<br>
				3286	<tt>use_iterator use_end()</tt> - Get an iterator to the end of the
				3287	use-list.<br>
				3288	<tt><a href="#User">User</a> *use_back()</tt> - Returns the last
				3289	element in the list.
				3290	<p> These methods are the interface to access the def-use
				3291	information in LLVM. As with all other iterators in LLVM, the naming
				3292	conventions follow the conventions defined by the <a href="#stl">STL</a>.</p>
				3293	</li>
				3294	<li><tt><a href="#Type">Type</a> *getType() const</tt>
				3295	<p>This method returns the Type of the Value.</p>
				3296	</li>
				3297	<li><tt>bool hasName() const</tt><br>
				3298	<tt>std::string getName() const</tt><br>
				3299	<tt>void setName(const std::string &Name)</tt>
				3300	<p> This family of methods is used to access and assign a name to a <tt>Value</tt>,
				3301	be aware of the <a href="#nameWarning">precaution above</a>.</p>
				3302	</li>
				3303	<li><tt>void replaceAllUsesWith(Value *V)</tt>
				3304
				3305	<p>This method traverses the use list of a <tt>Value</tt> changing all <a
				3306	href="#User"><tt>User</tt>s</a> of the current value to refer to
				3307	"<tt>V</tt>" instead. For example, if you detect that an instruction always
				3308	produces a constant value (for example through constant folding), you can
				3309	replace all uses of the instruction with the constant like this:</p>
				3310
				3311	<div class="doc_code">
				3312	<pre>
				3313	Inst->replaceAllUsesWith(ConstVal);
				3314	</pre>
				3315	</div>
				3316
				3317	</ul>
				3318
				3319	</div>
				3320
				3321	<!-- ======================================================================= -->
				3322	<div class="doc_subsection">
				3323	<a name="User">The <tt>User</tt> class</a>
				3324	</div>
				3325
				3326	<div class="doc_text">
				3327
				3328	<p>
				3329	<tt>#include "<a href="/doxygen/User_8h-source.html">llvm/User.h</a>"</tt><br>
				3330	doxygen info: <a href="/doxygen/classllvm_1_1User.html">User Class</a><br>
				3331	Superclass: <a href="#Value"><tt>Value</tt></a></p>
				3332
				3333	<p>The <tt>User</tt> class is the common base class of all LLVM nodes that may
				3334	refer to <a href="#Value"><tt>Value</tt></a>s. It exposes a list of "Operands"
				3335	that are all of the <a href="#Value"><tt>Value</tt></a>s that the User is
				3336	referring to. The <tt>User</tt> class itself is a subclass of
				3337	<tt>Value</tt>.</p>
				3338
				3339	<p>The operands of a <tt>User</tt> point directly to the LLVM <a
				3340	href="#Value"><tt>Value</tt></a> that it refers to. Because LLVM uses Static
				3341	Single Assignment (SSA) form, there can only be one definition referred to,
				3342	allowing this direct connection. This connection provides the use-def
				3343	information in LLVM.</p>
				3344
				3345	</div>
				3346
				3347	<!-- _______________________________________________________________________ -->
				3348	<div class="doc_subsubsection">
				3349	<a name="m_User">Important Public Members of the <tt>User</tt> class</a>
				3350	</div>
				3351
				3352	<div class="doc_text">
				3353
				3354	<p>The <tt>User</tt> class exposes the operand list in two ways: through
				3355	an index access interface and through an iterator based interface.</p>
				3356
				3357	<ul>
				3358	<li><tt>Value *getOperand(unsigned i)</tt><br>
				3359	<tt>unsigned getNumOperands()</tt>
				3360	<p> These two methods expose the operands of the <tt>User</tt> in a
				3361	convenient form for direct access.</p></li>
				3362
				3363	<li><tt>User::op_iterator</tt> - Typedef for iterator over the operand
				3364	list<br>
				3365	<tt>op_iterator op_begin()</tt> - Get an iterator to the start of
				3366	the operand list.<br>
				3367	<tt>op_iterator op_end()</tt> - Get an iterator to the end of the
				3368	operand list.
				3369	<p> Together, these methods make up the iterator based interface to
				3370	the operands of a <tt>User</tt>.</p></li>
				3371	</ul>
				3372
				3373	</div>
				3374
				3375	<!-- ======================================================================= -->
				3376	<div class="doc_subsection">
				3377	<a name="Instruction">The <tt>Instruction</tt> class</a>
				3378	</div>
				3379
				3380	<div class="doc_text">
				3381
				3382	<p><tt>#include "</tt><tt><a
				3383	href="/doxygen/Instruction_8h-source.html">llvm/Instruction.h</a>"</tt><br>
				3384	doxygen info: <a href="/doxygen/classllvm_1_1Instruction.html">Instruction Class</a><br>
				3385	Superclasses: <a href="#User"><tt>User</tt></a>, <a
				3386	href="#Value"><tt>Value</tt></a></p>
				3387
				3388	<p>The <tt>Instruction</tt> class is the common base class for all LLVM
				3389	instructions. It provides only a few methods, but is a very commonly used
				3390	class. The primary data tracked by the <tt>Instruction</tt> class itself is the
				3391	opcode (instruction type) and the parent <a
				3392	href="#BasicBlock"><tt>BasicBlock</tt></a> the <tt>Instruction</tt> is embedded
				3393	into. To represent a specific type of instruction, one of many subclasses of
				3394	<tt>Instruction</tt> are used.</p>
				3395
				3396	<p> Because the <tt>Instruction</tt> class subclasses the <a
				3397	href="#User"><tt>User</tt></a> class, its operands can be accessed in the same
				3398	way as for other <a href="#User"><tt>User</tt></a>s (with the
				3399	<tt>getOperand()</tt>/<tt>getNumOperands()</tt> and
				3400	<tt>op_begin()</tt>/<tt>op_end()</tt> methods).</p> <p> An important file for
				3401	the <tt>Instruction</tt> class is the <tt>llvm/Instruction.def</tt> file. This
				3402	file contains some meta-data about the various different types of instructions
				3403	in LLVM. It describes the enum values that are used as opcodes (for example
				3404	<tt>Instruction::Add</tt> and <tt>Instruction::ICmp</tt>), as well as the
				3405	concrete sub-classes of <tt>Instruction</tt> that implement the instruction (for
				3406	example <tt><a href="#BinaryOperator">BinaryOperator</a></tt> and <tt><a
				3407	href="#CmpInst">CmpInst</a></tt>). Unfortunately, the use of macros in
				3408	this file confuses doxygen, so these enum values don't show up correctly in the
				3409	<a href="/doxygen/classllvm_1_1Instruction.html">doxygen output</a>.</p>
				3410
				3411	</div>
				3412
				3413	<!-- _______________________________________________________________________ -->
				3414	<div class="doc_subsubsection">
				3415	<a name="s_Instruction">Important Subclasses of the <tt>Instruction</tt>
				3416	class</a>
				3417	</div>
				3418	<div class="doc_text">
				3419	<ul>
				3420	<li><tt><a name="BinaryOperator">BinaryOperator</a></tt>
				3421	<p>This subclasses represents all two operand instructions whose operands
				3422	must be the same type, except for the comparison instructions.</p></li>
				3423	<li><tt><a name="CastInst">CastInst</a></tt>
				3424	<p>This subclass is the parent of the 12 casting instructions. It provides
				3425	common operations on cast instructions.</p>
				3426	<li><tt><a name="CmpInst">CmpInst</a></tt>
				3427	<p>This subclass respresents the two comparison instructions,
				3428	<a href="LangRef.html#i_icmp">ICmpInst</a> (integer opreands), and
				3429	<a href="LangRef.html#i_fcmp">FCmpInst</a> (floating point operands).</p>
				3430	<li><tt><a name="TerminatorInst">TerminatorInst</a></tt>
				3431	<p>This subclass is the parent of all terminator instructions (those which
				3432	can terminate a block).</p>
				3433	</ul>
				3434	</div>
				3435
				3436	<!-- _______________________________________________________________________ -->
				3437	<div class="doc_subsubsection">
				3438	<a name="m_Instruction">Important Public Members of the <tt>Instruction</tt>
				3439	class</a>
				3440	</div>
				3441
				3442	<div class="doc_text">
				3443
				3444	<ul>
				3445	<li><tt><a href="#BasicBlock">BasicBlock</a> *getParent()</tt>
				3446	<p>Returns the <a href="#BasicBlock"><tt>BasicBlock</tt></a> that
				3447	this <tt>Instruction</tt> is embedded into.</p></li>
				3448	<li><tt>bool mayWriteToMemory()</tt>
				3449	<p>Returns true if the instruction writes to memory, i.e. it is a
				3450	<tt>call</tt>,<tt>free</tt>,<tt>invoke</tt>, or <tt>store</tt>.</p></li>
				3451	<li><tt>unsigned getOpcode()</tt>
				3452	<p>Returns the opcode for the <tt>Instruction</tt>.</p></li>
				3453	<li><tt><a href="#Instruction">Instruction</a> *clone() const</tt>
				3454	<p>Returns another instance of the specified instruction, identical
				3455	in all ways to the original except that the instruction has no parent
				3456	(ie it's not embedded into a <a href="#BasicBlock"><tt>BasicBlock</tt></a>),
				3457	and it has no name</p></li>
				3458	</ul>
				3459
				3460	</div>
				3461
				3462	<!-- ======================================================================= -->
				3463	<div class="doc_subsection">
				3464	<a name="Constant">The <tt>Constant</tt> class and subclasses</a>
				3465	</div>
				3466
				3467	<div class="doc_text">
				3468
				3469	<p>Constant represents a base class for different types of constants. It
				3470	is subclassed by ConstantInt, ConstantArray, etc. for representing
				3471	the various types of Constants. <a href="#GlobalValue">GlobalValue</a> is also
				3472	a subclass, which represents the address of a global variable or function.
				3473	</p>
				3474
				3475	</div>
				3476
				3477	<!-- _______________________________________________________________________ -->
				3478	<div class="doc_subsubsection">Important Subclasses of Constant </div>
				3479	<div class="doc_text">
				3480	<ul>
				3481	<li>ConstantInt : This subclass of Constant represents an integer constant of
				3482	any width.
				3483	<ul>
				3484	<li><tt>const APInt& getValue() const</tt>: Returns the underlying
				3485	value of this constant, an APInt value.</li>
				3486	<li><tt>int64_t getSExtValue() const</tt>: Converts the underlying APInt
				3487	value to an int64_t via sign extension. If the value (not the bit width)
				3488	of the APInt is too large to fit in an int64_t, an assertion will result.
				3489	For this reason, use of this method is discouraged.</li>
				3490	<li><tt>uint64_t getZExtValue() const</tt>: Converts the underlying APInt
				3491	value to a uint64_t via zero extension. IF the value (not the bit width)
				3492	of the APInt is too large to fit in a uint64_t, an assertion will result.
				3493	For this reason, use of this method is discouraged.</li>
				3494	<li><tt>static ConstantInt* get(const APInt& Val)</tt>: Returns the
				3495	ConstantInt object that represents the value provided by <tt>Val</tt>.
				3496	The type is implied as the IntegerType that corresponds to the bit width
				3497	of <tt>Val</tt>.</li>
				3498	<li><tt>static ConstantInt* get(const Type *Ty, uint64_t Val)</tt>:
				3499	Returns the ConstantInt object that represents the value provided by
				3500	<tt>Val</tt> for integer type <tt>Ty</tt>.</li>
				3501	</ul>
				3502	</li>
				3503	<li>ConstantFP : This class represents a floating point constant.
				3504	<ul>
				3505	<li><tt>double getValue() const</tt>: Returns the underlying value of
				3506	this constant. </li>
				3507	</ul>
				3508	</li>
				3509	<li>ConstantArray : This represents a constant array.
				3510	<ul>
				3511	<li><tt>const std::vector<Use> &getValues() const</tt>: Returns
				3512	a vector of component constants that makeup this array. </li>
				3513	</ul>
				3514	</li>
				3515	<li>ConstantStruct : This represents a constant struct.
				3516	<ul>
				3517	<li><tt>const std::vector<Use> &getValues() const</tt>: Returns
				3518	a vector of component constants that makeup this array. </li>
				3519	</ul>
				3520	</li>
				3521	<li>GlobalValue : This represents either a global variable or a function. In
				3522	either case, the value is a constant fixed address (after linking).
				3523	</li>
				3524	</ul>
				3525	</div>
				3526
				3527
				3528	<!-- ======================================================================= -->
				3529	<div class="doc_subsection">
				3530	<a name="GlobalValue">The <tt>GlobalValue</tt> class</a>
				3531	</div>
				3532
				3533	<div class="doc_text">
				3534
				3535	<p><tt>#include "<a
				3536	href="/doxygen/GlobalValue_8h-source.html">llvm/GlobalValue.h</a>"</tt><br>
				3537	doxygen info: <a href="/doxygen/classllvm_1_1GlobalValue.html">GlobalValue
				3538	Class</a><br>
				3539	Superclasses: <a href="#Constant"><tt>Constant</tt></a>,
				3540	<a href="#User"><tt>User</tt></a>, <a href="#Value"><tt>Value</tt></a></p>
				3541
				3542	<p>Global values (<a href="#GlobalVariable"><tt>GlobalVariable</tt></a>s or <a
				3543	href="#Function"><tt>Function</tt></a>s) are the only LLVM values that are
				3544	visible in the bodies of all <a href="#Function"><tt>Function</tt></a>s.
				3545	Because they are visible at global scope, they are also subject to linking with
				3546	other globals defined in different translation units. To control the linking
				3547	process, <tt>GlobalValue</tt>s know their linkage rules. Specifically,
				3548	<tt>GlobalValue</tt>s know whether they have internal or external linkage, as
				3549	defined by the <tt>LinkageTypes</tt> enumeration.</p>
				3550
				3551	<p>If a <tt>GlobalValue</tt> has internal linkage (equivalent to being
				3552	<tt>static</tt> in C), it is not visible to code outside the current translation
				3553	unit, and does not participate in linking. If it has external linkage, it is
				3554	visible to external code, and does participate in linking. In addition to
				3555	linkage information, <tt>GlobalValue</tt>s keep track of which <a
				3556	href="#Module"><tt>Module</tt></a> they are currently part of.</p>
				3557
				3558	<p>Because <tt>GlobalValue</tt>s are memory objects, they are always referred to
				3559	by their <b>address</b>. As such, the <a href="#Type"><tt>Type</tt></a> of a
				3560	global is always a pointer to its contents. It is important to remember this
				3561	when using the <tt>GetElementPtrInst</tt> instruction because this pointer must
				3562	be dereferenced first. For example, if you have a <tt>GlobalVariable</tt> (a
				3563	subclass of <tt>GlobalValue)</tt> that is an array of 24 ints, type <tt>[24 x
				3564	i32]</tt>, then the <tt>GlobalVariable</tt> is a pointer to that array. Although
				3565	the address of the first element of this array and the value of the
				3566	<tt>GlobalVariable</tt> are the same, they have different types. The
				3567	<tt>GlobalVariable</tt>'s type is <tt>[24 x i32]</tt>. The first element's type
				3568	is <tt>i32.</tt> Because of this, accessing a global value requires you to
				3569	dereference the pointer with <tt>GetElementPtrInst</tt> first, then its elements
				3570	can be accessed. This is explained in the <a href="LangRef.html#globalvars">LLVM
				3571	Language Reference Manual</a>.</p>
				3572
				3573	</div>
				3574
				3575	<!-- _______________________________________________________________________ -->
				3576	<div class="doc_subsubsection">
				3577	<a name="m_GlobalValue">Important Public Members of the <tt>GlobalValue</tt>
				3578	class</a>
				3579	</div>
				3580
				3581	<div class="doc_text">
				3582
				3583	<ul>
				3584	<li><tt>bool hasInternalLinkage() const</tt><br>
				3585	<tt>bool hasExternalLinkage() const</tt><br>
				3586	<tt>void setInternalLinkage(bool HasInternalLinkage)</tt>
				3587	<p> These methods manipulate the linkage characteristics of the <tt>GlobalValue</tt>.</p>
				3588	<p> </p>
				3589	</li>
				3590	<li><tt><a href="#Module">Module</a> *getParent()</tt>
				3591	<p> This returns the <a href="#Module"><tt>Module</tt></a> that the
				3592	GlobalValue is currently embedded into.</p></li>
				3593	</ul>
				3594
				3595	</div>
				3596
				3597	<!-- ======================================================================= -->
				3598	<div class="doc_subsection">
				3599	<a name="Function">The <tt>Function</tt> class</a>
				3600	</div>
				3601
				3602	<div class="doc_text">
				3603
				3604	<p><tt>#include "<a
				3605	href="/doxygen/Function_8h-source.html">llvm/Function.h</a>"</tt><br> doxygen
				3606	info: <a href="/doxygen/classllvm_1_1Function.html">Function Class</a><br>
				3607	Superclasses: <a href="#GlobalValue"><tt>GlobalValue</tt></a>,
				3608	<a href="#Constant"><tt>Constant</tt></a>,
				3609	<a href="#User"><tt>User</tt></a>,
				3610	<a href="#Value"><tt>Value</tt></a></p>
				3611
				3612	<p>The <tt>Function</tt> class represents a single procedure in LLVM. It is
				3613	actually one of the more complex classes in the LLVM hierarchy because it must
				3614	keep track of a large amount of data. The <tt>Function</tt> class keeps track
				3615	of a list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s, a list of formal
				3616	<a href="#Argument"><tt>Argument</tt></a>s, and a
				3617	<a href="#SymbolTable"><tt>SymbolTable</tt></a>.</p>
				3618
				3619	<p>The list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s is the most
				3620	commonly used part of <tt>Function</tt> objects. The list imposes an implicit
				3621	ordering of the blocks in the function, which indicate how the code will be
				3622	laid out by the backend. Additionally, the first <a
				3623	href="#BasicBlock"><tt>BasicBlock</tt></a> is the implicit entry node for the
				3624	<tt>Function</tt>. It is not legal in LLVM to explicitly branch to this initial
				3625	block. There are no implicit exit nodes, and in fact there may be multiple exit
				3626	nodes from a single <tt>Function</tt>. If the <a
				3627	href="#BasicBlock"><tt>BasicBlock</tt></a> list is empty, this indicates that
				3628	the <tt>Function</tt> is actually a function declaration: the actual body of the
				3629	function hasn't been linked in yet.</p>
				3630
				3631	<p>In addition to a list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s, the
				3632	<tt>Function</tt> class also keeps track of the list of formal <a
				3633	href="#Argument"><tt>Argument</tt></a>s that the function receives. This
				3634	container manages the lifetime of the <a href="#Argument"><tt>Argument</tt></a>
				3635	nodes, just like the <a href="#BasicBlock"><tt>BasicBlock</tt></a> list does for
				3636	the <a href="#BasicBlock"><tt>BasicBlock</tt></a>s.</p>
				3637
				3638	<p>The <a href="#SymbolTable"><tt>SymbolTable</tt></a> is a very rarely used
				3639	LLVM feature that is only used when you have to look up a value by name. Aside
				3640	from that, the <a href="#SymbolTable"><tt>SymbolTable</tt></a> is used
				3641	internally to make sure that there are not conflicts between the names of <a
				3642	href="#Instruction"><tt>Instruction</tt></a>s, <a
				3643	href="#BasicBlock"><tt>BasicBlock</tt></a>s, or <a
				3644	href="#Argument"><tt>Argument</tt></a>s in the function body.</p>
				3645
				3646	<p>Note that <tt>Function</tt> is a <a href="#GlobalValue">GlobalValue</a>
				3647	and therefore also a <a href="#Constant">Constant</a>. The value of the function
				3648	is its address (after linking) which is guaranteed to be constant.</p>
				3649	</div>
				3650
				3651	<!-- _______________________________________________________________________ -->
				3652	<div class="doc_subsubsection">
				3653	<a name="m_Function">Important Public Members of the <tt>Function</tt>
				3654	class</a>
				3655	</div>
				3656
				3657	<div class="doc_text">
				3658
				3659	<ul>
				3660	<li><tt>Function(const </tt><tt><a href="#FunctionType">FunctionType</a>
				3661	Ty, LinkageTypes Linkage, const std::string &N = "", Module Parent = 0)</tt>
				3662
				3663	<p>Constructor used when you need to create new <tt>Function</tt>s to add
				3664	the the program. The constructor must specify the type of the function to
				3665	create and what type of linkage the function should have. The <a
				3666	href="#FunctionType"><tt>FunctionType</tt></a> argument
				3667	specifies the formal arguments and return value for the function. The same
				3668	<a href="#FunctionType"><tt>FunctionType</tt></a> value can be used to
				3669	create multiple functions. The <tt>Parent</tt> argument specifies the Module
				3670	in which the function is defined. If this argument is provided, the function
				3671	will automatically be inserted into that module's list of
				3672	functions.</p></li>
				3673
				3674	<li><tt>bool isDeclaration()</tt>
				3675
				3676	<p>Return whether or not the <tt>Function</tt> has a body defined. If the
				3677	function is "external", it does not have a body, and thus must be resolved
				3678	by linking with a function defined in a different translation unit.</p></li>
				3679
				3680	<li><tt>Function::iterator</tt> - Typedef for basic block list iterator<br>
				3681	<tt>Function::const_iterator</tt> - Typedef for const_iterator.<br>
				3682
				3683	<tt>begin()</tt>, <tt>end()</tt>
				3684	<tt>size()</tt>, <tt>empty()</tt>
				3685
				3686	<p>These are forwarding methods that make it easy to access the contents of
				3687	a <tt>Function</tt> object's <a href="#BasicBlock"><tt>BasicBlock</tt></a>
				3688	list.</p></li>
				3689
				3690	<li><tt>Function::BasicBlockListType &getBasicBlockList()</tt>
				3691
				3692	<p>Returns the list of <a href="#BasicBlock"><tt>BasicBlock</tt></a>s. This
				3693	is necessary to use when you need to update the list or perform a complex
				3694	action that doesn't have a forwarding method.</p></li>
				3695
				3696	<li><tt>Function::arg_iterator</tt> - Typedef for the argument list
				3697	iterator<br>
				3698	<tt>Function::const_arg_iterator</tt> - Typedef for const_iterator.<br>
				3699
				3700	<tt>arg_begin()</tt>, <tt>arg_end()</tt>
				3701	<tt>arg_size()</tt>, <tt>arg_empty()</tt>
				3702
				3703	<p>These are forwarding methods that make it easy to access the contents of
				3704	a <tt>Function</tt> object's <a href="#Argument"><tt>Argument</tt></a>
				3705	list.</p></li>
				3706
				3707	<li><tt>Function::ArgumentListType &getArgumentList()</tt>
				3708
				3709	<p>Returns the list of <a href="#Argument"><tt>Argument</tt></a>s. This is
				3710	necessary to use when you need to update the list or perform a complex
				3711	action that doesn't have a forwarding method.</p></li>
				3712
				3713	<li><tt><a href="#BasicBlock">BasicBlock</a> &getEntryBlock()</tt>
				3714
				3715	<p>Returns the entry <a href="#BasicBlock"><tt>BasicBlock</tt></a> for the
				3716	function. Because the entry block for the function is always the first
				3717	block, this returns the first block of the <tt>Function</tt>.</p></li>
				3718
				3719	<li><tt><a href="#Type">Type</a> *getReturnType()</tt><br>
				3720	<tt><a href="#FunctionType">FunctionType</a> *getFunctionType()</tt>
				3721
				3722	<p>This traverses the <a href="#Type"><tt>Type</tt></a> of the
				3723	<tt>Function</tt> and returns the return type of the function, or the <a
				3724	href="#FunctionType"><tt>FunctionType</tt></a> of the actual
				3725	function.</p></li>
				3726
				3727	<li><tt><a href="#SymbolTable">SymbolTable</a> *getSymbolTable()</tt>
				3728
				3729	<p> Return a pointer to the <a href="#SymbolTable"><tt>SymbolTable</tt></a>
				3730	for this <tt>Function</tt>.</p></li>
				3731	</ul>
				3732
				3733	</div>
				3734
				3735	<!-- ======================================================================= -->
				3736	<div class="doc_subsection">
				3737	<a name="GlobalVariable">The <tt>GlobalVariable</tt> class</a>
				3738	</div>
				3739
				3740	<div class="doc_text">
				3741
				3742	<p><tt>#include "<a
				3743	href="/doxygen/GlobalVariable_8h-source.html">llvm/GlobalVariable.h</a>"</tt>
				3744	<br>
				3745	doxygen info: <a href="/doxygen/classllvm_1_1GlobalVariable.html">GlobalVariable
				3746	Class</a><br>
				3747	Superclasses: <a href="#GlobalValue"><tt>GlobalValue</tt></a>,
				3748	<a href="#Constant"><tt>Constant</tt></a>,
				3749	<a href="#User"><tt>User</tt></a>,
				3750	<a href="#Value"><tt>Value</tt></a></p>
				3751
				3752	<p>Global variables are represented with the (surprise surprise)
				3753	<tt>GlobalVariable</tt> class. Like functions, <tt>GlobalVariable</tt>s are also
				3754	subclasses of <a href="#GlobalValue"><tt>GlobalValue</tt></a>, and as such are
				3755	always referenced by their address (global values must live in memory, so their
				3756	"name" refers to their constant address). See
				3757	<a href="#GlobalValue"><tt>GlobalValue</tt></a> for more on this. Global
				3758	variables may have an initial value (which must be a
				3759	<a href="#Constant"><tt>Constant</tt></a>), and if they have an initializer,
				3760	they may be marked as "constant" themselves (indicating that their contents
				3761	never change at runtime).</p>
				3762	</div>
				3763
				3764	<!-- _______________________________________________________________________ -->
				3765	<div class="doc_subsubsection">
				3766	<a name="m_GlobalVariable">Important Public Members of the
				3767	<tt>GlobalVariable</tt> class</a>
				3768	</div>
				3769
				3770	<div class="doc_text">
				3771
				3772	<ul>
				3773	<li><tt>GlobalVariable(const </tt><tt><a href="#Type">Type</a> *Ty, bool
				3774	isConstant, LinkageTypes& Linkage, <a href="#Constant">Constant</a>
				3775	Initializer = 0, const std::string &Name = "", Module Parent = 0)</tt>
				3776
				3777	<p>Create a new global variable of the specified type. If
				3778	<tt>isConstant</tt> is true then the global variable will be marked as
				3779	unchanging for the program. The Linkage parameter specifies the type of
				3780	linkage (internal, external, weak, linkonce, appending) for the variable.
				3781	If the linkage is InternalLinkage, WeakAnyLinkage, WeakODRLinkage,
				3782	LinkOnceAnyLinkage or LinkOnceODRLinkage,  then the resultant
				3783	global variable will have internal linkage. AppendingLinkage concatenates
				3784	together all instances (in different translation units) of the variable
				3785	into a single variable but is only applicable to arrays.  See
				3786	the <a href="LangRef.html#modulestructure">LLVM Language Reference</a> for
				3787	further details on linkage types. Optionally an initializer, a name, and the
				3788	module to put the variable into may be specified for the global variable as
				3789	well.</p></li>
				3790
				3791	<li><tt>bool isConstant() const</tt>
				3792
				3793	<p>Returns true if this is a global variable that is known not to
				3794	be modified at runtime.</p></li>
				3795
				3796	<li><tt>bool hasInitializer()</tt>
				3797
				3798	<p>Returns true if this <tt>GlobalVariable</tt> has an intializer.</p></li>
				3799
				3800	<li><tt><a href="#Constant">Constant</a> *getInitializer()</tt>
				3801
				3802	<p>Returns the initial value for a <tt>GlobalVariable</tt>. It is not legal
				3803	to call this method if there is no initializer.</p></li>
				3804	</ul>
				3805
				3806	</div>
				3807
				3808
				3809	<!-- ======================================================================= -->
				3810	<div class="doc_subsection">
				3811	<a name="BasicBlock">The <tt>BasicBlock</tt> class</a>
				3812	</div>
				3813
				3814	<div class="doc_text">
				3815
				3816	<p><tt>#include "<a
				3817	href="/doxygen/BasicBlock_8h-source.html">llvm/BasicBlock.h</a>"</tt><br>
				3818	doxygen info: <a href="/doxygen/classllvm_1_1BasicBlock.html">BasicBlock
				3819	Class</a><br>
				3820	Superclass: <a href="#Value"><tt>Value</tt></a></p>
				3821
				3822	<p>This class represents a single entry multiple exit section of the code,
				3823	commonly known as a basic block by the compiler community. The
				3824	<tt>BasicBlock</tt> class maintains a list of <a
				3825	href="#Instruction"><tt>Instruction</tt></a>s, which form the body of the block.
				3826	Matching the language definition, the last element of this list of instructions
				3827	is always a terminator instruction (a subclass of the <a
				3828	href="#TerminatorInst"><tt>TerminatorInst</tt></a> class).</p>
				3829
				3830	<p>In addition to tracking the list of instructions that make up the block, the
				3831	<tt>BasicBlock</tt> class also keeps track of the <a
				3832	href="#Function"><tt>Function</tt></a> that it is embedded into.</p>
				3833
				3834	<p>Note that <tt>BasicBlock</tt>s themselves are <a
				3835	href="#Value"><tt>Value</tt></a>s, because they are referenced by instructions
				3836	like branches and can go in the switch tables. <tt>BasicBlock</tt>s have type
				3837	<tt>label</tt>.</p>
				3838
				3839	</div>
				3840
				3841	<!-- _______________________________________________________________________ -->
				3842	<div class="doc_subsubsection">
				3843	<a name="m_BasicBlock">Important Public Members of the <tt>BasicBlock</tt>
				3844	class</a>
				3845	</div>
				3846
				3847	<div class="doc_text">
				3848	<ul>
				3849
				3850	<li><tt>BasicBlock(const std::string &Name = "", </tt><tt><a
				3851	href="#Function">Function</a> *Parent = 0)</tt>
				3852
				3853	<p>The <tt>BasicBlock</tt> constructor is used to create new basic blocks for
				3854	insertion into a function. The constructor optionally takes a name for the new
				3855	block, and a <a href="#Function"><tt>Function</tt></a> to insert it into. If
				3856	the <tt>Parent</tt> parameter is specified, the new <tt>BasicBlock</tt> is
				3857	automatically inserted at the end of the specified <a
				3858	href="#Function"><tt>Function</tt></a>, if not specified, the BasicBlock must be
				3859	manually inserted into the <a href="#Function"><tt>Function</tt></a>.</p></li>
				3860
				3861	<li><tt>BasicBlock::iterator</tt> - Typedef for instruction list iterator<br>
				3862	<tt>BasicBlock::const_iterator</tt> - Typedef for const_iterator.<br>
				3863	<tt>begin()</tt>, <tt>end()</tt>, <tt>front()</tt>, <tt>back()</tt>,
				3864	<tt>size()</tt>, <tt>empty()</tt>
				3865	STL-style functions for accessing the instruction list.
				3866
				3867	<p>These methods and typedefs are forwarding functions that have the same
				3868	semantics as the standard library methods of the same names. These methods
				3869	expose the underlying instruction list of a basic block in a way that is easy to
				3870	manipulate. To get the full complement of container operations (including
				3871	operations to update the list), you must use the <tt>getInstList()</tt>
				3872	method.</p></li>
				3873
				3874	<li><tt>BasicBlock::InstListType &getInstList()</tt>
				3875
				3876	<p>This method is used to get access to the underlying container that actually
				3877	holds the Instructions. This method must be used when there isn't a forwarding
				3878	function in the <tt>BasicBlock</tt> class for the operation that you would like
				3879	to perform. Because there are no forwarding functions for "updating"
				3880	operations, you need to use this if you want to update the contents of a
				3881	<tt>BasicBlock</tt>.</p></li>
				3882
				3883	<li><tt><a href="#Function">Function</a> *getParent()</tt>
				3884
				3885	<p> Returns a pointer to <a href="#Function"><tt>Function</tt></a> the block is
				3886	embedded into, or a null pointer if it is homeless.</p></li>
				3887
				3888	<li><tt><a href="#TerminatorInst">TerminatorInst</a> *getTerminator()</tt>
				3889
				3890	<p> Returns a pointer to the terminator instruction that appears at the end of
				3891	the <tt>BasicBlock</tt>. If there is no terminator instruction, or if the last
				3892	instruction in the block is not a terminator, then a null pointer is
				3893	returned.</p></li>
				3894
				3895	</ul>
				3896
				3897	</div>
				3898
				3899
				3900	<!-- ======================================================================= -->
				3901	<div class="doc_subsection">
				3902	<a name="Argument">The <tt>Argument</tt> class</a>
				3903	</div>
				3904
				3905	<div class="doc_text">
				3906
				3907	<p>This subclass of Value defines the interface for incoming formal
				3908	arguments to a function. A Function maintains a list of its formal
				3909	arguments. An argument has a pointer to the parent Function.</p>
				3910
				3911	</div>
				3912
				3913	<!-- *********************************************************************** -->
				3914	<hr>
				3915	<address>
				3916	<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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				3920
				3921	<a href="mailto:dhurjati@cs.uiuc.edu">Dinakar Dhurjati</a> and
				3922	<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
				3923	<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
				3924	Last modified: $Date: 2010-01-29 11:10:38 -0800 (Fri, 29 Jan 2010) $
				3925	</address>
				3926
				3927	</body>
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