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				4	<head>
NAKAMURA Takumi	5c6e4df	2011-10-31 11:21:59 +0000	[diff] [blame^]	5	<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
Misha Brukman	a97e6cb	2004-07-01 15:33:24 +0000	[diff] [blame]	6	<title>LLVM Alias Analysis Infrastructure</title>
Misha Brukman	6e4afd0	2004-01-15 19:04:12 +0000	[diff] [blame]	7	<link rel="stylesheet" href="llvm.css" type="text/css">
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	8	</head>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	9	<body>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	10
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	11	<h1>
Misha Brukman	a97e6cb	2004-07-01 15:33:24 +0000	[diff] [blame]	12	LLVM Alias Analysis Infrastructure
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	13	</h1>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	14
				15	<ol>
Misha Brukman	b215425	2003-10-23 02:29:42 +0000	[diff] [blame]	16	<li><a href="#introduction">Introduction</a></li>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	17
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	18	<li><a href="#overview"><tt>AliasAnalysis</tt> Class Overview</a>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	19	<ul>
Misha Brukman	b215425	2003-10-23 02:29:42 +0000	[diff] [blame]	20	<li><a href="#pointers">Representation of Pointers</a></li>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	21	<li><a href="#alias">The <tt>alias</tt> method</a></li>
Misha Brukman	b215425	2003-10-23 02:29:42 +0000	[diff] [blame]	22	<li><a href="#ModRefInfo">The <tt>getModRefInfo</tt> methods</a></li>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	23	<li><a href="#OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a></li>
				24	</ul>
				25	</li>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	26
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	27	<li><a href="#writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	28	<ul>
Misha Brukman	b215425	2003-10-23 02:29:42 +0000	[diff] [blame]	29	<li><a href="#passsubclasses">Different Pass styles</a></li>
				30	<li><a href="#requiredcalls">Required initialization calls</a></li>
				31	<li><a href="#interfaces">Interfaces which may be specified</a></li>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	32	<li><a href="#chaining"><tt>AliasAnalysis</tt> chaining behavior</a></li>
				33	<li><a href="#updating">Updating analysis results for transformations</a></li>
Misha Brukman	b215425	2003-10-23 02:29:42 +0000	[diff] [blame]	34	<li><a href="#implefficiency">Efficiency Issues</a></li>
Dan Gohman	01e5988	2010-12-15 23:09:41 +0000	[diff] [blame]	35	<li><a href="#limitations">Limitations</a></li>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	36	</ul>
				37	</li>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	38
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	39	<li><a href="#using">Using alias analysis results</a>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	40	<ul>
Chris Lattner	4f4365e	2009-04-25 21:11:37 +0000	[diff] [blame]	41	<li><a href="#memdep">Using the <tt>MemoryDependenceAnalysis</tt> Pass</a></li>
Misha Brukman	b215425	2003-10-23 02:29:42 +0000	[diff] [blame]	42	<li><a href="#ast">Using the <tt>AliasSetTracker</tt> class</a></li>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	43	<li><a href="#direct">Using the <tt>AliasAnalysis</tt> interface directly</a></li>
				44	</ul>
				45	</li>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	46
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	47	<li><a href="#exist">Existing alias analysis implementations and clients</a>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	48	<ul>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	49	<li><a href="#impls">Available <tt>AliasAnalysis</tt> implementations</a></li>
				50	<li><a href="#aliasanalysis-xforms">Alias analysis driven transformations</a></li>
Misha Brukman	a97e6cb	2004-07-01 15:33:24 +0000	[diff] [blame]	51	<li><a href="#aliasanalysis-debug">Clients for debugging and evaluation of
				52	implementations</a></li>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	53	</ul>
				54	</li>
Owen Anderson	d8b4728	2007-10-02 00:44:20 +0000	[diff] [blame]	55	<li><a href="#memdep">Memory Dependence Analysis</a></li>
Misha Brukman	b215425	2003-10-23 02:29:42 +0000	[diff] [blame]	56	</ol>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	57
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	58	<div class="doc_author">
				59	<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	60	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	61
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	62	<!-- *********************************************************************** -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	63	<h2>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	64	<a name="introduction">Introduction</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	65	</h2>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	66	<!-- *********************************************************************** -->
				67
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	68	<div>
Misha Brukman	1f254d5	2003-11-21 22:30:25 +0000	[diff] [blame]	69
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	70	<p>Alias Analysis (aka Pointer Analysis) is a class of techniques which attempt
				71	to determine whether or not two pointers ever can point to the same object in
				72	memory. There are many different algorithms for alias analysis and many
				73	different ways of classifying them: flow-sensitive vs flow-insensitive,
				74	context-sensitive vs context-insensitive, field-sensitive vs field-insensitive,
				75	unification-based vs subset-based, etc. Traditionally, alias analyses respond
Duncan Sands	8036ca4	2007-03-30 12:22:09 +0000	[diff] [blame]	76	to a query with a <a href="#MustMayNo">Must, May, or No</a> alias response,
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	77	indicating that two pointers always point to the same object, might point to the
				78	same object, or are known to never point to the same object.</p>
Misha Brukman	1f254d5	2003-11-21 22:30:25 +0000	[diff] [blame]	79
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	80	<p>The LLVM <a
Reid Spencer	05fe4b0	2006-03-14 05:39:39 +0000	[diff] [blame]	81	href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	82	class is the primary interface used by clients and implementations of alias
				83	analyses in the LLVM system. This class is the common interface between clients
				84	of alias analysis information and the implementations providing it, and is
				85	designed to support a wide range of implementations and clients (but currently
				86	all clients are assumed to be flow-insensitive). In addition to simple alias
				87	analysis information, this class exposes Mod/Ref information from those
				88	implementations which can provide it, allowing for powerful analyses and
				89	transformations to work well together.</p>
Misha Brukman	1f254d5	2003-11-21 22:30:25 +0000	[diff] [blame]	90
				91	<p>This document contains information necessary to successfully implement this
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	92	interface, use it, and to test both sides. It also explains some of the finer
				93	points about what exactly results mean. If you feel that something is unclear
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	94	or should be added, please <a href="mailto:sabre@nondot.org">let me
Misha Brukman	1f254d5	2003-11-21 22:30:25 +0000	[diff] [blame]	95	know</a>.</p>
				96
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	97	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	98
				99	<!-- *********************************************************************** -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	100	<h2>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	101	<a name="overview"><tt>AliasAnalysis</tt> Class Overview</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	102	</h2>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	103	<!-- *********************************************************************** -->
				104
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	105	<div>
Misha Brukman	1f254d5	2003-11-21 22:30:25 +0000	[diff] [blame]	106
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	107	<p>The <a
Reid Spencer	05fe4b0	2006-03-14 05:39:39 +0000	[diff] [blame]	108	href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	109	class defines the interface that the various alias analysis implementations
				110	should support. This class exports two important enums: <tt>AliasResult</tt>
				111	and <tt>ModRefResult</tt> which represent the result of an alias query or a
				112	mod/ref query, respectively.</p>
Misha Brukman	1f254d5	2003-11-21 22:30:25 +0000	[diff] [blame]	113
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	114	<p>The <tt>AliasAnalysis</tt> interface exposes information about memory,
				115	represented in several different ways. In particular, memory objects are
				116	represented as a starting address and size, and function calls are represented
				117	as the actual <tt>call</tt> or <tt>invoke</tt> instructions that performs the
				118	call. The <tt>AliasAnalysis</tt> interface also exposes some helper methods
				119	which allow you to get mod/ref information for arbitrary instructions.</p>
Misha Brukman	1f254d5	2003-11-21 22:30:25 +0000	[diff] [blame]	120
Dan Gohman	9e86f43	2010-07-07 14:27:09 +0000	[diff] [blame]	121	<p>All <tt>AliasAnalysis</tt> interfaces require that in queries involving
				122	multiple values, values which are not
				123	<a href="LangRef.html#constants">constants</a> are all defined within the
				124	same function.</p>
				125
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	126	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	127	<h3>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	128	<a name="pointers">Representation of Pointers</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	129	</h3>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	130
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	131	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	132
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	133	<p>Most importantly, the <tt>AliasAnalysis</tt> class provides several methods
				134	which are used to query whether or not two memory objects alias, whether
				135	function calls can modify or read a memory object, etc. For all of these
				136	queries, memory objects are represented as a pair of their starting address (a
				137	symbolic LLVM <tt>Value*</tt>) and a static size.</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	138
				139	<p>Representing memory objects as a starting address and a size is critically
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	140	important for correct Alias Analyses. For example, consider this (silly, but
				141	possible) C code:</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	142
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	143	<div class="doc_code">
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	144	<pre>
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	145	int i;
				146	char C[2];
				147	char A[10];
				148	/* ... */
				149	for (i = 0; i != 10; ++i) {
				150	C[0] = A[i]; /* One byte store */
				151	C[1] = A[9-i]; /* One byte store */
				152	}
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	153	</pre>
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	154	</div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	155
				156	<p>In this case, the <tt>basicaa</tt> pass will disambiguate the stores to
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	157	<tt>C[0]</tt> and <tt>C[1]</tt> because they are accesses to two distinct
				158	locations one byte apart, and the accesses are each one byte. In this case, the
				159	LICM pass can use store motion to remove the stores from the loop. In
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	160	constrast, the following code:</p>
				161
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	162	<div class="doc_code">
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	163	<pre>
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	164	int i;
				165	char C[2];
				166	char A[10];
				167	/* ... */
				168	for (i = 0; i != 10; ++i) {
				169	((short)C)[0] = A[i]; / Two byte store! */
				170	C[1] = A[9-i]; /* One byte store */
				171	}
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	172	</pre>
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	173	</div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	174
				175	<p>In this case, the two stores to C do alias each other, because the access to
				176	the <tt>&C[0]</tt> element is a two byte access. If size information wasn't
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	177	available in the query, even the first case would have to conservatively assume
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	178	that the accesses alias.</p>
				179
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	180	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	181
				182	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	183	<h3>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	184	<a name="alias">The <tt>alias</tt> method</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	185	</h3>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	186
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	187	<div>
Dan Gohman	9e86f43	2010-07-07 14:27:09 +0000	[diff] [blame]	188	<p>The <tt>alias</tt> method is the primary interface used to determine whether
				189	or not two memory objects alias each other. It takes two memory objects as
Dan Gohman	4a34cbd	2010-12-10 19:38:58 +0000	[diff] [blame]	190	input and returns MustAlias, PartialAlias, MayAlias, or NoAlias as
				191	appropriate.</p>
Dan Gohman	9e86f43	2010-07-07 14:27:09 +0000	[diff] [blame]	192
				193	<p>Like all <tt>AliasAnalysis</tt> interfaces, the <tt>alias</tt> method requires
				194	that either the two pointer values be defined within the same function, or at
				195	least one of the values is a <a href="LangRef.html#constants">constant</a>.</p>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	196
				197	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	198	<h4>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	199	<a name="MustMayNo">Must, May, and No Alias Responses</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	200	</h4>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	201
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	202	<div>
Dan Gohman	1e10962	2010-07-02 18:41:32 +0000	[diff] [blame]	203	<p>The NoAlias response may be used when there is never an immediate dependence
				204	between any memory reference <i>based</i> on one pointer and any memory
				205	reference <i>based</i> the other. The most obvious example is when the two
				206	pointers point to non-overlapping memory ranges. Another is when the two
				207	pointers are only ever used for reading memory. Another is when the memory is
				208	freed and reallocated between accesses through one pointer and accesses through
				209	the other -- in this case, there is a dependence, but it's mediated by the free
				210	and reallocation.</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	211
Dan Gohman	efca7f9	2010-07-02 23:46:54 +0000	[diff] [blame]	212	<p>As an exception to this is with the
Dan Gohman	9e86f43	2010-07-07 14:27:09 +0000	[diff] [blame]	213	<a href="LangRef.html#noalias"><tt>noalias</tt></a> keyword; the "irrelevant"
				214	dependencies are ignored.</p>
Dan Gohman	efca7f9	2010-07-02 23:46:54 +0000	[diff] [blame]	215
Nick Lewycky	01557ce	2008-12-14 21:08:48 +0000	[diff] [blame]	216	<p>The MayAlias response is used whenever the two pointers might refer to the
Dan Gohman	4a34cbd	2010-12-10 19:38:58 +0000	[diff] [blame]	217	same object.</p>
				218
				219	<p>The PartialAlias response is used when the two memory objects are known
				220	to be overlapping in some way, but do not start at the same address.</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	221
Nick Lewycky	01557ce	2008-12-14 21:08:48 +0000	[diff] [blame]	222	<p>The MustAlias response may only be returned if the two memory objects are
				223	guaranteed to always start at exactly the same location. A MustAlias response
				224	implies that the pointers compare equal.</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	225
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	226	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	227
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	228	</div>
				229
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	230	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	231	<h3>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	232	<a name="ModRefInfo">The <tt>getModRefInfo</tt> methods</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	233	</h3>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	234
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	235	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	236
				237	<p>The <tt>getModRefInfo</tt> methods return information about whether the
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	238	execution of an instruction can read or modify a memory location. Mod/Ref
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	239	information is always conservative: if an instruction <b>might</b> read or write
				240	a location, ModRef is returned.</p>
				241
				242	<p>The <tt>AliasAnalysis</tt> class also provides a <tt>getModRefInfo</tt>
				243	method for testing dependencies between function calls. This method takes two
Dan Gohman	39429e2	2010-08-30 23:47:24 +0000	[diff] [blame]	244	call sites (CS1 & CS2), returns NoModRef if neither call writes to memory
				245	read or written by the other, Ref if CS1 reads memory written by CS2, Mod if CS1
				246	writes to memory read or written by CS2, or ModRef if CS1 might read or write
				247	memory written to by CS2. Note that this relation is not commutative.</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	248
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	249	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	250
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	251
				252	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	253	<h3>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	254	<a name="OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	255	</h3>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	256
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	257	<div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	258
				259	<p>
				260	Several other tidbits of information are often collected by various alias
				261	analysis implementations and can be put to good use by various clients.
				262	</p>
				263
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	264	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	265	<h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	266	The <tt>pointsToConstantMemory</tt> method
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	267	</h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	268
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	269	<div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	270
				271	<p>The <tt>pointsToConstantMemory</tt> method returns true if and only if the
				272	analysis can prove that the pointer only points to unchanging memory locations
				273	(functions, constant global variables, and the null pointer). This information
				274	can be used to refine mod/ref information: it is impossible for an unchanging
				275	memory location to be modified.</p>
				276
				277	</div>
				278
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	279	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	280	<h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	281	<a name="simplemodref">The <tt>doesNotAccessMemory</tt> and
				282	<tt>onlyReadsMemory</tt> methods</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	283	</h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	284
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	285	<div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	286
				287	<p>These methods are used to provide very simple mod/ref information for
				288	function calls. The <tt>doesNotAccessMemory</tt> method returns true for a
				289	function if the analysis can prove that the function never reads or writes to
				290	memory, or if the function only reads from constant memory. Functions with this
				291	property are side-effect free and only depend on their input arguments, allowing
				292	them to be eliminated if they form common subexpressions or be hoisted out of
				293	loops. Many common functions behave this way (e.g., <tt>sin</tt> and
				294	<tt>cos</tt>) but many others do not (e.g., <tt>acos</tt>, which modifies the
				295	<tt>errno</tt> variable).</p>
				296
				297	<p>The <tt>onlyReadsMemory</tt> method returns true for a function if analysis
				298	can prove that (at most) the function only reads from non-volatile memory.
				299	Functions with this property are side-effect free, only depending on their input
				300	arguments and the state of memory when they are called. This property allows
				301	calls to these functions to be eliminated and moved around, as long as there is
				302	no store instruction that changes the contents of memory. Note that all
				303	functions that satisfy the <tt>doesNotAccessMemory</tt> method also satisfies
				304	<tt>onlyReadsMemory</tt>.</p>
				305
				306	</div>
				307
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	308	</div>
				309
				310	</div>
				311
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	312	<!-- *********************************************************************** -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	313	<h2>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	314	<a name="writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	315	</h2>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	316	<!-- *********************************************************************** -->
				317
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	318	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	319
				320	<p>Writing a new alias analysis implementation for LLVM is quite
				321	straight-forward. There are already several implementations that you can use
				322	for examples, and the following information should help fill in any details.
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	323	For a examples, take a look at the <a href="#impls">various alias analysis
				324	implementations</a> included with LLVM.</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	325
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	326	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	327	<h3>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	328	<a name="passsubclasses">Different Pass styles</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	329	</h3>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	330
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	331	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	332
				333	<p>The first step to determining what type of <a
				334	href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias
				335	Analysis. As is the case with most other analyses and transformations, the
				336	answer should be fairly obvious from what type of problem you are trying to
				337	solve:</p>
				338
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	339	<ol>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	340	<li>If you require interprocedural analysis, it should be a
				341	<tt>Pass</tt>.</li>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	342	<li>If you are a function-local analysis, subclass <tt>FunctionPass</tt>.</li>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	343	<li>If you don't need to look at the program at all, subclass
				344	<tt>ImmutablePass</tt>.</li>
				345	</ol>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	346
				347	<p>In addition to the pass that you subclass, you should also inherit from the
Misha Brukman	700fd49	2003-05-07 21:47:16 +0000	[diff] [blame]	348	<tt>AliasAnalysis</tt> interface, of course, and use the
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	349	<tt>RegisterAnalysisGroup</tt> template to register as an implementation of
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	350	<tt>AliasAnalysis</tt>.</p>
				351
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	352	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	353
				354	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	355	<h3>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	356	<a name="requiredcalls">Required initialization calls</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	357	</h3>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	358
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	359	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	360
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	361	<p>Your subclass of <tt>AliasAnalysis</tt> is required to invoke two methods on
				362	the <tt>AliasAnalysis</tt> base class: <tt>getAnalysisUsage</tt> and
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	363	<tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
				364	<tt>getAnalysisUsage</tt> should explicitly call into the
				365	<tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
				366	declaring any pass dependencies your pass has. Thus you should have something
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	367	like this:</p>
				368
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	369	<div class="doc_code">
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	370	<pre>
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	371	void getAnalysisUsage(AnalysisUsage &AU) const {
				372	AliasAnalysis::getAnalysisUsage(AU);
				373	<i>// declare your dependencies here.</i>
				374	}
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	375	</pre>
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	376	</div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	377
				378	<p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
				379	from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	380	<tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, or <tt>InitializePass</tt>
				381	for an <tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	382
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	383	<div class="doc_code">
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	384	<pre>
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	385	bool run(Module &M) {
				386	InitializeAliasAnalysis(this);
				387	<i>// Perform analysis here...</i>
				388	return false;
				389	}
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	390	</pre>
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	391	</div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	392
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	393	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	394
				395	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	396	<h3>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	397	<a name="interfaces">Interfaces which may be specified</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	398	</h3>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	399
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	400	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	401
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	402	<p>All of the <a
				403	href="/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
				404	virtual methods default to providing <a href="#chaining">chaining</a> to another
				405	alias analysis implementation, which ends up returning conservatively correct
				406	information (returning "May" Alias and "Mod/Ref" for alias and mod/ref queries
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	407	respectively). Depending on the capabilities of the analysis you are
				408	implementing, you just override the interfaces you can improve.</p>
				409
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	410	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	411
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	412
				413
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	414	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	415	<h3>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	416	<a name="chaining"><tt>AliasAnalysis</tt> chaining behavior</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	417	</h3>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	418
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	419	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	420
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	421	<p>With only two special exceptions (the <tt><a
				422	href="#basic-aa">basicaa</a></tt> and <a href="#no-aa"><tt>no-aa</tt></a>
				423	passes) every alias analysis pass chains to another alias analysis
				424	implementation (for example, the user can specify "<tt>-basicaa -ds-aa
Chris Lattner	a9cf196	2010-03-01 19:24:17 +0000	[diff] [blame]	425	-licm</tt>" to get the maximum benefit from both alias
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	426	analyses). The alias analysis class automatically takes care of most of this
				427	for methods that you don't override. For methods that you do override, in code
				428	paths that return a conservative MayAlias or Mod/Ref result, simply return
				429	whatever the superclass computes. For example:</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	430
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	431	<div class="doc_code">
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	432	<pre>
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	433	AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
				434	const Value *V2, unsigned V2Size) {
				435	if (...)
				436	return NoAlias;
				437	...
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	438
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	439	<i>// Couldn't determine a must or no-alias result.</i>
				440	return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
				441	}
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	442	</pre>
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	443	</div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	444
				445	<p>In addition to analysis queries, you must make sure to unconditionally pass
				446	LLVM <a href="#updating">update notification</a> methods to the superclass as
				447	well if you override them, which allows all alias analyses in a change to be
				448	updated.</p>
				449
				450	</div>
				451
				452
				453	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	454	<h3>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	455	<a name="updating">Updating analysis results for transformations</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	456	</h3>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	457
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	458	<div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	459	<p>
				460	Alias analysis information is initially computed for a static snapshot of the
				461	program, but clients will use this information to make transformations to the
				462	code. All but the most trivial forms of alias analysis will need to have their
				463	analysis results updated to reflect the changes made by these transformations.
				464	</p>
				465
				466	<p>
Owen Anderson	ab6acc6	2011-01-03 21:38:41 +0000	[diff] [blame]	467	The <tt>AliasAnalysis</tt> interface exposes four methods which are used to
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	468	communicate program changes from the clients to the analysis implementations.
				469	Various alias analysis implementations should use these methods to ensure that
				470	their internal data structures are kept up-to-date as the program changes (for
				471	example, when an instruction is deleted), and clients of alias analysis must be
				472	sure to call these interfaces appropriately.
				473	</p>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	474
				475	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	476	<h4>The <tt>deleteValue</tt> method</h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	477
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	478	<div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	479	The <tt>deleteValue</tt> method is called by transformations when they remove an
				480	instruction or any other value from the program (including values that do not
				481	use pointers). Typically alias analyses keep data structures that have entries
				482	for each value in the program. When this method is called, they should remove
				483	any entries for the specified value, if they exist.
				484	</div>
				485
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	486	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	487	<h4>The <tt>copyValue</tt> method</h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	488
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	489	<div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	490	The <tt>copyValue</tt> method is used when a new value is introduced into the
				491	program. There is no way to introduce a value into the program that did not
				492	exist before (this doesn't make sense for a safe compiler transformation), so
				493	this is the only way to introduce a new value. This method indicates that the
				494	new value has exactly the same properties as the value being copied.
				495	</div>
				496
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	497	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	498	<h4>The <tt>replaceWithNewValue</tt> method</h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	499
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	500	<div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	501	This method is a simple helper method that is provided to make clients easier to
				502	use. It is implemented by copying the old analysis information to the new
				503	value, then deleting the old value. This method cannot be overridden by alias
				504	analysis implementations.
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	505	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	506
Owen Anderson	ab6acc6	2011-01-03 21:38:41 +0000	[diff] [blame]	507	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	508	<h4>The <tt>addEscapingUse</tt> method</h4>
Owen Anderson	ab6acc6	2011-01-03 21:38:41 +0000	[diff] [blame]	509
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	510	<div>
Owen Anderson	ab6acc6	2011-01-03 21:38:41 +0000	[diff] [blame]	511	<p>The <tt>addEscapingUse</tt> method is used when the uses of a pointer
				512	value have changed in ways that may invalidate precomputed analysis information.
				513	Implementations may either use this callback to provide conservative responses
				514	for points whose uses have change since analysis time, or may recompute some
				515	or all of their internal state to continue providing accurate responses.</p>
				516
				517	<p>In general, any new use of a pointer value is considered an escaping use,
				518	and must be reported through this callback, <em>except</em> for the
				519	uses below:</p>
				520
				521	<ul>
				522	<li>A <tt>bitcast</tt> or <tt>getelementptr</tt> of the pointer</li>
				523	<li>A <tt>store</tt> through the pointer (but not a <tt>store</tt>
				524	<em>of</em> the pointer)</li>
				525	<li>A <tt>load</tt> through the pointer</li>
				526	</ul>
				527	</div>
				528
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	529	</div>
				530
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	531	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	532	<h3>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	533	<a name="implefficiency">Efficiency Issues</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	534	</h3>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	535
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	536	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	537
				538	<p>From the LLVM perspective, the only thing you need to do to provide an
				539	efficient alias analysis is to make sure that alias analysis <b>queries</b> are
				540	serviced quickly. The actual calculation of the alias analysis results (the
				541	"run" method) is only performed once, but many (perhaps duplicate) queries may
				542	be performed. Because of this, try to move as much computation to the run
				543	method as possible (within reason).</p>
				544
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	545	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	546
Dan Gohman	3f43dc3	2010-06-24 19:34:03 +0000	[diff] [blame]	547	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	548	<h3>
Dan Gohman	49e41c5	2010-12-15 18:45:20 +0000	[diff] [blame]	549	<a name="limitations">Limitations</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	550	</h3>
Dan Gohman	3f43dc3	2010-06-24 19:34:03 +0000	[diff] [blame]	551
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	552	<div>
Dan Gohman	3f43dc3	2010-06-24 19:34:03 +0000	[diff] [blame]	553
Dan Gohman	49e41c5	2010-12-15 18:45:20 +0000	[diff] [blame]	554	<p>The AliasAnalysis infrastructure has several limitations which make
				555	writing a new <tt>AliasAnalysis</tt> implementation difficult.</p>
Dan Gohman	3f43dc3	2010-06-24 19:34:03 +0000	[diff] [blame]	556
				557	<p>There is no way to override the default alias analysis. It would
				558	be very useful to be able to do something like "opt -my-aa -O2" and
				559	have it use -my-aa for all passes which need AliasAnalysis, but there
				560	is currently no support for that, short of changing the source code
				561	and recompiling. Similarly, there is also no way of setting a chain
				562	of analyses as the default.</p>
				563
				564	<p>There is no way for transform passes to declare that they preserve
				565	<tt>AliasAnalysis</tt> implementations. The <tt>AliasAnalysis</tt>
				566	interface includes <tt>deleteValue</tt> and <tt>copyValue</tt> methods
				567	which are intended to allow a pass to keep an AliasAnalysis consistent,
				568	however there's no way for a pass to declare in its
				569	<tt>getAnalysisUsage</tt> that it does so. Some passes attempt to use
				570	<tt>AU.addPreserved<AliasAnalysis></tt>, however this doesn't
NAKAMURA Takumi	4d6deb0	2011-04-09 09:51:57 +0000	[diff] [blame]	571	actually have any effect.</p>
Dan Gohman	3f43dc3	2010-06-24 19:34:03 +0000	[diff] [blame]	572
				573	<p><tt>AliasAnalysisCounter</tt> (<tt>-count-aa</tt>) and <tt>AliasDebugger</tt>
				574	(<tt>-debug-aa</tt>) are implemented as <tt>ModulePass</tt> classes, so if your
				575	alias analysis uses <tt>FunctionPass</tt>, it won't be able to use
				576	these utilities. If you try to use them, the pass manager will
				577	silently route alias analysis queries directly to
				578	<tt>BasicAliasAnalysis</tt> instead.</p>
				579
				580	<p>Similarly, the <tt>opt -p</tt> option introduces <tt>ModulePass</tt>
				581	passes between each pass, which prevents the use of <tt>FunctionPass</tt>
				582	alias analysis passes.</p>
				583
Dan Gohman	49e41c5	2010-12-15 18:45:20 +0000	[diff] [blame]	584	<p>The <tt>AliasAnalysis</tt> API does have functions for notifying
				585	implementations when values are deleted or copied, however these
				586	aren't sufficient. There are many other ways that LLVM IR can be
				587	modified which could be relevant to <tt>AliasAnalysis</tt>
				588	implementations which can not be expressed.</p>
				589
				590	<p>The <tt>AliasAnalysisDebugger</tt> utility seems to suggest that
				591	<tt>AliasAnalysis</tt> implementations can expect that they will be
				592	informed of any relevant <tt>Value</tt> before it appears in an
				593	alias query. However, popular clients such as <tt>GVN</tt> don't
				594	support this, and are known to trigger errors when run with the
				595	<tt>AliasAnalysisDebugger</tt>.</p>
				596
				597	<p>Due to several of the above limitations, the most obvious use for
				598	the <tt>AliasAnalysisCounter</tt> utility, collecting stats on all
				599	alias queries in a compilation, doesn't work, even if the
				600	<tt>AliasAnalysis</tt> implementations don't use <tt>FunctionPass</tt>.
				601	There's no way to set a default, much less a default sequence,
				602	and there's no way to preserve it.</p>
				603
				604	<p>The <tt>AliasSetTracker</tt> class (which is used by <tt>LICM</tt>
				605	makes a non-deterministic number of alias queries. This can cause stats
				606	collected by <tt>AliasAnalysisCounter</tt> to have fluctuations among
				607	identical runs, for example. Another consequence is that debugging
				608	techniques involving pausing execution after a predetermined number
				609	of queries can be unreliable.</p>
				610
				611	<p>Many alias queries can be reformulated in terms of other alias
				612	queries. When multiple <tt>AliasAnalysis</tt> queries are chained together,
				613	it would make sense to start those queries from the beginning of the chain,
				614	with care taken to avoid infinite looping, however currently an
				615	implementation which wants to do this can only start such queries
				616	from itself.</p>
				617
Dan Gohman	3f43dc3	2010-06-24 19:34:03 +0000	[diff] [blame]	618	</div>
				619
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	620	</div>
				621
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	622	<!-- *********************************************************************** -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	623	<h2>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	624	<a name="using">Using alias analysis results</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	625	</h2>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	626	<!-- *********************************************************************** -->
				627
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	628	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	629
				630	<p>There are several different ways to use alias analysis results. In order of
				631	preference, these are...</p>
				632
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	633	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	634	<h3>
Chris Lattner	4f4365e	2009-04-25 21:11:37 +0000	[diff] [blame]	635	<a name="memdep">Using the <tt>MemoryDependenceAnalysis</tt> Pass</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	636	</h3>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	637
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	638	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	639
Chris Lattner	4f4365e	2009-04-25 21:11:37 +0000	[diff] [blame]	640	<p>The <tt>memdep</tt> pass uses alias analysis to provide high-level dependence
				641	information about memory-using instructions. This will tell you which store
				642	feeds into a load, for example. It uses caching and other techniques to be
				643	efficient, and is used by Dead Store Elimination, GVN, and memcpy optimizations.
				644	</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	645
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	646	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	647
				648	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	649	<h3>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	650	<a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	651	</h3>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	652
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	653	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	654
				655	<p>Many transformations need information about alias <b>sets</b> that are active
				656	in some scope, rather than information about pairwise aliasing. The <tt><a
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	657	href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class
				658	is used to efficiently build these Alias Sets from the pairwise alias analysis
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	659	information provided by the <tt>AliasAnalysis</tt> interface.</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	660
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	661	<p>First you initialize the AliasSetTracker by using the "<tt>add</tt>" methods
				662	to add information about various potentially aliasing instructions in the scope
				663	you are interested in. Once all of the alias sets are completed, your pass
				664	should simply iterate through the constructed alias sets, using the
				665	<tt>AliasSetTracker</tt> <tt>begin()</tt>/<tt>end()</tt> methods.</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	666
				667	<p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed
Chris Lattner	539ca70	2003-12-19 08:43:07 +0000	[diff] [blame]	668	to be disjoint, calculate mod/ref information and volatility for the set, and
				669	keep track of whether or not all of the pointers in the set are Must aliases.
				670	The AliasSetTracker also makes sure that sets are properly folded due to call
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	671	instructions, and can provide a list of pointers in each set.</p>
				672
				673	<p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	674	Invariant Code Motion</a> pass uses <tt>AliasSetTracker</tt>s to calculate alias
				675	sets for each loop nest. If an <tt>AliasSet</tt> in a loop is not modified,
				676	then all load instructions from that set may be hoisted out of the loop. If any
				677	alias sets are stored to <b>and</b> are must alias sets, then the stores may be
				678	sunk to outside of the loop, promoting the memory location to a register for the
				679	duration of the loop nest. Both of these transformations only apply if the
				680	pointer argument is loop-invariant.</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	681
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	682	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	683	<h4>
Chris Lattner	539ca70	2003-12-19 08:43:07 +0000	[diff] [blame]	684	The AliasSetTracker implementation
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	685	</h4>
Chris Lattner	539ca70	2003-12-19 08:43:07 +0000	[diff] [blame]	686
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	687	<div>
Chris Lattner	539ca70	2003-12-19 08:43:07 +0000	[diff] [blame]	688
				689	<p>The AliasSetTracker class is implemented to be as efficient as possible. It
				690	uses the union-find algorithm to efficiently merge AliasSets when a pointer is
				691	inserted into the AliasSetTracker that aliases multiple sets. The primary data
				692	structure is a hash table mapping pointers to the AliasSet they are in.</p>
				693
				694	<p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s
				695	that are in each AliasSet. Since the hash table already has entries for each
				696	LLVM Value* of interest, the AliasesSets thread the linked list through these
				697	hash-table nodes to avoid having to allocate memory unnecessarily, and to make
				698	merging alias sets extremely efficient (the linked list merge is constant time).
				699	</p>
				700
				701	<p>You shouldn't need to understand these details if you are just a client of
				702	the AliasSetTracker, but if you look at the code, hopefully this brief
				703	description will help make sense of why things are designed the way they
				704	are.</p>
				705
				706	</div>
				707
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	708	</div>
				709
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	710	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	711	<h3>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	712	<a name="direct">Using the <tt>AliasAnalysis</tt> interface directly</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	713	</h3>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	714
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	715	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	716
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	717	<p>If neither of these utility class are what your pass needs, you should use
				718	the interfaces exposed by the <tt>AliasAnalysis</tt> class directly. Try to use
				719	the higher-level methods when possible (e.g., use mod/ref information instead of
				720	the <a href="#alias"><tt>alias</tt></a> method directly if possible) to get the
				721	best precision and efficiency.</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	722
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	723	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	724
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	725	</div>
				726
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	727	<!-- *********************************************************************** -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	728	<h2>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	729	<a name="exist">Existing alias analysis implementations and clients</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	730	</h2>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	731	<!-- *********************************************************************** -->
				732
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	733	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	734
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	735	<p>If you're going to be working with the LLVM alias analysis infrastructure,
				736	you should know what clients and implementations of alias analysis are
				737	available. In particular, if you are implementing an alias analysis, you should
				738	be aware of the <a href="#aliasanalysis-debug">the clients</a> that are useful
				739	for monitoring and evaluating different implementations.</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	740
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	741	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	742	<h3>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	743	<a name="impls">Available <tt>AliasAnalysis</tt> implementations</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	744	</h3>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	745
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	746	<div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	747
				748	<p>This section lists the various implementations of the <tt>AliasAnalysis</tt>
Dan Gohman	b0b0da7	2011-05-24 22:01:32 +0000	[diff] [blame]	749	interface. With the exception of the <a href="#no-aa"><tt>-no-aa</tt></a>
				750	implementation, all of these <a href="#chaining">chain</a> to other alias
				751	analysis implementations.</p>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	752
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	753	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	754	<h4>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	755	<a name="no-aa">The <tt>-no-aa</tt> pass</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	756	</h4>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	757
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	758	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	759
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	760	<p>The <tt>-no-aa</tt> pass is just like what it sounds: an alias analysis that
				761	never returns any useful information. This pass can be useful if you think that
				762	alias analysis is doing something wrong and are trying to narrow down a
				763	problem.</p>
				764
				765	</div>
				766
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	767	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	768	<h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	769	<a name="basic-aa">The <tt>-basicaa</tt> pass</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	770	</h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	771
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	772	<div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	773
Dan Gohman	c66712f	2010-11-15 18:07:16 +0000	[diff] [blame]	774	<p>The <tt>-basicaa</tt> pass is an aggressive local analysis that "knows"
				775	many important facts:</p>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	776
				777	<ul>
				778	<li>Distinct globals, stack allocations, and heap allocations can never
				779	alias.</li>
				780	<li>Globals, stack allocations, and heap allocations never alias the null
				781	pointer.</li>
				782	<li>Different fields of a structure do not alias.</li>
				783	<li>Indexes into arrays with statically differing subscripts cannot alias.</li>
				784	<li>Many common standard C library functions <a
				785	href="#simplemodref">never access memory or only read memory</a>.</li>
				786	<li>Pointers that obviously point to constant globals
				787	"<tt>pointToConstantMemory</tt>".</li>
				788	<li>Function calls can not modify or references stack allocations if they never
				789	escape from the function that allocates them (a common case for automatic
				790	arrays).</li>
				791	</ul>
				792
				793	</div>
				794
Chris Lattner	100a4f8	2004-06-28 19:19:47 +0000	[diff] [blame]	795	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	796	<h4>
Chris Lattner	100a4f8	2004-06-28 19:19:47 +0000	[diff] [blame]	797	<a name="globalsmodref">The <tt>-globalsmodref-aa</tt> pass</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	798	</h4>
Chris Lattner	100a4f8	2004-06-28 19:19:47 +0000	[diff] [blame]	799
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	800	<div>
Chris Lattner	100a4f8	2004-06-28 19:19:47 +0000	[diff] [blame]	801
				802	<p>This pass implements a simple context-sensitive mod/ref and alias analysis
Chris Lattner	ccb354b	2004-07-27 07:50:07 +0000	[diff] [blame]	803	for internal global variables that don't "have their address taken". If a
				804	global does not have its address taken, the pass knows that no pointers alias
				805	the global. This pass also keeps track of functions that it knows never access
Chris Lattner	4f4365e	2009-04-25 21:11:37 +0000	[diff] [blame]	806	memory or never read memory. This allows certain optimizations (e.g. GVN) to
Chris Lattner	ccb354b	2004-07-27 07:50:07 +0000	[diff] [blame]	807	eliminate call instructions entirely.
Chris Lattner	100a4f8	2004-06-28 19:19:47 +0000	[diff] [blame]	808	</p>
				809
				810	<p>The real power of this pass is that it provides context-sensitive mod/ref
				811	information for call instructions. This allows the optimizer to know that
				812	calls to a function do not clobber or read the value of the global, allowing
				813	loads and stores to be eliminated.</p>
				814
				815	<p>Note that this pass is somewhat limited in its scope (only support
				816	non-address taken globals), but is very quick analysis.</p>
				817	</div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	818
				819	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	820	<h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	821	<a name="steens-aa">The <tt>-steens-aa</tt> pass</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	822	</h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	823
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	824	<div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	825
				826	<p>The <tt>-steens-aa</tt> pass implements a variation on the well-known
				827	"Steensgaard's algorithm" for interprocedural alias analysis. Steensgaard's
				828	algorithm is a unification-based, flow-insensitive, context-insensitive, and
				829	field-insensitive alias analysis that is also very scalable (effectively linear
				830	time).</p>
				831
				832	<p>The LLVM <tt>-steens-aa</tt> pass implements a "speculatively
				833	field-<b>sensitive</b>" version of Steensgaard's algorithm using the Data
				834	Structure Analysis framework. This gives it substantially more precision than
				835	the standard algorithm while maintaining excellent analysis scalability.</p>
				836
Chris Lattner	d6e0dd1	2007-07-03 04:41:50 +0000	[diff] [blame]	837	<p>Note that <tt>-steens-aa</tt> is available in the optional "poolalloc"
				838	module, it is not part of the LLVM core.</p>
				839
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	840	</div>
				841
				842	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	843	<h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	844	<a name="ds-aa">The <tt>-ds-aa</tt> pass</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	845	</h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	846
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	847	<div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	848
				849	<p>The <tt>-ds-aa</tt> pass implements the full Data Structure Analysis
				850	algorithm. Data Structure Analysis is a modular unification-based,
				851	flow-insensitive, context-<b>sensitive</b>, and speculatively
				852	field-<b>sensitive</b> alias analysis that is also quite scalable, usually at
				853	O(n*log(n)).</p>
				854
				855	<p>This algorithm is capable of responding to a full variety of alias analysis
				856	queries, and can provide context-sensitive mod/ref information as well. The
				857	only major facility not implemented so far is support for must-alias
				858	information.</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	859
Chris Lattner	d6e0dd1	2007-07-03 04:41:50 +0000	[diff] [blame]	860	<p>Note that <tt>-ds-aa</tt> is available in the optional "poolalloc"
				861	module, it is not part of the LLVM core.</p>
				862
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	863	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	864
Dan Gohman	85dfca6	2010-06-28 22:09:52 +0000	[diff] [blame]	865	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	866	<h4>
Dan Gohman	85dfca6	2010-06-28 22:09:52 +0000	[diff] [blame]	867	<a name="scev-aa">The <tt>-scev-aa</tt> pass</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	868	</h4>
Dan Gohman	85dfca6	2010-06-28 22:09:52 +0000	[diff] [blame]	869
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	870	<div>
Dan Gohman	85dfca6	2010-06-28 22:09:52 +0000	[diff] [blame]	871
				872	<p>The <tt>-scev-aa</tt> pass implements AliasAnalysis queries by
				873	translating them into ScalarEvolution queries. This gives it a
				874	more complete understanding of <tt>getelementptr</tt> instructions
				875	and loop induction variables than other alias analyses have.</p>
				876
				877	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	878
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	879	</div>
				880
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	881	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	882	<h3>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	883	<a name="aliasanalysis-xforms">Alias analysis driven transformations</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	884	</h3>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	885
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	886	<div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	887	LLVM includes several alias-analysis driven transformations which can be used
				888	with any of the implementations above.
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	889
				890	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	891	<h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	892	<a name="adce">The <tt>-adce</tt> pass</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	893	</h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	894
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	895	<div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	896
				897	<p>The <tt>-adce</tt> pass, which implements Aggressive Dead Code Elimination
				898	uses the <tt>AliasAnalysis</tt> interface to delete calls to functions that do
				899	not have side-effects and are not used.</p>
				900
				901	</div>
				902
				903
				904	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	905	<h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	906	<a name="licm">The <tt>-licm</tt> pass</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	907	</h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	908
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	909	<div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	910
				911	<p>The <tt>-licm</tt> pass implements various Loop Invariant Code Motion related
				912	transformations. It uses the <tt>AliasAnalysis</tt> interface for several
				913	different transformations:</p>
				914
				915	<ul>
				916	<li>It uses mod/ref information to hoist or sink load instructions out of loops
				917	if there are no instructions in the loop that modifies the memory loaded.</li>
				918
				919	<li>It uses mod/ref information to hoist function calls out of loops that do not
				920	write to memory and are loop-invariant.</li>
				921
				922	<li>If uses alias information to promote memory objects that are loaded and
				923	stored to in loops to live in a register instead. It can do this if there are
				924	no may aliases to the loaded/stored memory location.</li>
				925	</ul>
				926
				927	</div>
				928
				929	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	930	<h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	931	<a name="argpromotion">The <tt>-argpromotion</tt> pass</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	932	</h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	933
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	934	<div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	935	<p>
				936	The <tt>-argpromotion</tt> pass promotes by-reference arguments to be passed in
				937	by-value instead. In particular, if pointer arguments are only loaded from it
				938	passes in the value loaded instead of the address to the function. This pass
				939	uses alias information to make sure that the value loaded from the argument
				940	pointer is not modified between the entry of the function and any load of the
				941	pointer.</p>
				942	</div>
				943
				944	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	945	<h4>
Chris Lattner	4f4365e	2009-04-25 21:11:37 +0000	[diff] [blame]	946	<a name="gvn">The <tt>-gvn</tt>, <tt>-memcpyopt</tt>, and <tt>-dse</tt>
				947	passes</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	948	</h4>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	949
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	950	<div>
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	951
Chris Lattner	4f4365e	2009-04-25 21:11:37 +0000	[diff] [blame]	952	<p>These passes use AliasAnalysis information to reason about loads and stores.
				953	</p>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	954
				955	</div>
				956
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	957	</div>
				958
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	959	<!-- ======================================================================= -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	960	<h3>
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	961	<a name="aliasanalysis-debug">Clients for debugging and evaluation of
				962	implementations</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	963	</h3>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	964
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	965	<div>
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	966
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	967	<p>These passes are useful for evaluating the various alias analysis
Chris Lattner	a9cf196	2010-03-01 19:24:17 +0000	[diff] [blame]	968	implementations. You can use them with commands like '<tt>opt -ds-aa
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	969	-aa-eval foo.bc -disable-output -stats</tt>'.</p>
				970
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	971	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	972	<h4>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	973	<a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	974	</h4>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	975
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	976	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	977
				978	<p>The <tt>-print-alias-sets</tt> pass is exposed as part of the
Chris Lattner	05d5c9d	2006-01-03 06:04:48 +0000	[diff] [blame]	979	<tt>opt</tt> tool to print out the Alias Sets formed by the <a
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	980	href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
Chris Lattner	05d5c9d	2006-01-03 06:04:48 +0000	[diff] [blame]	981	the <tt>AliasSetTracker</tt> class. To use it, use something like:</p>
				982
				983	<div class="doc_code">
				984	<pre>
				985	% opt -ds-aa -print-alias-sets -disable-output
				986	</pre>
				987	</div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	988
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	989	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	990
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	991
				992	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	993	<h4>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	994	<a name="count-aa">The <tt>-count-aa</tt> pass</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	995	</h4>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	996
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	997	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	998
				999	<p>The <tt>-count-aa</tt> pass is useful to see how many queries a particular
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	1000	pass is making and what responses are returned by the alias analysis. As an
				1001	example,</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	1002
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	1003	<div class="doc_code">
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	1004	<pre>
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	1005	% opt -basicaa -count-aa -ds-aa -count-aa -licm
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	1006	</pre>
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	1007	</div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	1008
Misha Brukman	294611a	2004-07-28 22:18:33 +0000	[diff] [blame]	1009	<p>will print out how many queries (and what responses are returned) by the
				1010	<tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are made
				1011	of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be useful
				1012	when debugging a transformation or an alias analysis implementation.</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	1013
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	1014	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	1015
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	1016	<!-- _______________________________________________________________________ -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	1017	<h4>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	1018	<a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	1019	</h4>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	1020
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	1021	<div>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	1022
				1023	<p>The <tt>-aa-eval</tt> pass simply iterates through all pairs of pointers in a
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	1024	function and asks an alias analysis whether or not the pointers alias. This
				1025	gives an indication of the precision of the alias analysis. Statistics are
Chris Lattner	ccc8065	2004-05-23 21:04:01 +0000	[diff] [blame]	1026	printed indicating the percent of no/may/must aliases found (a more precise
				1027	algorithm will have a lower number of may aliases).</p>
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	1028
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	1029	</div>
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	1030
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	1031	</div>
				1032
				1033	</div>
				1034
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	1035	<!-- *********************************************************************** -->
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	1036	<h2>
Owen Anderson	5a726b8	2007-10-02 00:43:25 +0000	[diff] [blame]	1037	<a name="memdep">Memory Dependence Analysis</a>
NAKAMURA Takumi	05d0265	2011-04-18 23:59:50 +0000	[diff] [blame]	1038	</h2>
Owen Anderson	5a726b8	2007-10-02 00:43:25 +0000	[diff] [blame]	1039	<!-- *********************************************************************** -->
				1040
NAKAMURA Takumi	074eeaa	2011-04-21 01:52:00 +0000	[diff] [blame]	1041	<div>
Owen Anderson	5a726b8	2007-10-02 00:43:25 +0000	[diff] [blame]	1042
				1043	<p>If you're just looking to be a client of alias analysis information, consider
				1044	using the Memory Dependence Analysis interface instead. MemDep is a lazy,
				1045	caching layer on top of alias analysis that is able to answer the question of
				1046	what preceding memory operations a given instruction depends on, either at an
				1047	intra- or inter-block level. Because of its laziness and caching
				1048	policy, using MemDep can be a significant performance win over accessing alias
				1049	analysis directly.</p>
				1050
				1051	</div>
				1052
				1053	<!-- *********************************************************************** -->
Chris Lattner	9f64875	2003-03-04 19:37:49 +0000	[diff] [blame]	1054
Misha Brukman	3896be2	2003-10-24 18:06:11 +0000	[diff] [blame]	1055	<hr>
Misha Brukman	915cab2	2003-11-22 01:26:21 +0000	[diff] [blame]	1056	<address>
Misha Brukman	38847d5	2003-12-21 22:53:21 +0000	[diff] [blame]	1057	<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman	4440870	2008-12-11 17:34:48 +0000	[diff] [blame]	1058	src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Misha Brukman	915cab2	2003-11-22 01:26:21 +0000	[diff] [blame]	1059	<a href="http://validator.w3.org/check/referer"><img
Misha Brukman	4440870	2008-12-11 17:34:48 +0000	[diff] [blame]	1060	src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
Misha Brukman	915cab2	2003-11-22 01:26:21 +0000	[diff] [blame]	1061
				1062	<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
NAKAMURA Takumi	b9a3363	2011-04-09 02:13:37 +0000	[diff] [blame]	1063	<a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
Misha Brukman	915cab2	2003-11-22 01:26:21 +0000	[diff] [blame]	1064	Last modified: $Date$
				1065	</address>
Misha Brukman	cd11e45	2003-10-22 23:27:16 +0000	[diff] [blame]	1066
				1067	</body>
				1068	</html>