Gitiles
Code Review Sign In
gerrit-public.fairphone.software / platform / external / llvm / 3ac1ab835caacdeebbd0d7b4d69160f283928d21 / . / docs / AliasAnalysis.html
blob: a89903d2af81fd5c9d2f0530d15f03d155519f1d [file] [log] [blame]
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
2 "http://www.w3.org/TR/html4/strict.dtd">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]3<html>
4<head>
Misha Brukmana97e6cb2004-07-01 15:33:24 +0000[diff] [blame]5 <title>LLVM Alias Analysis Infrastructure</title>
Misha Brukman6e4afd02004-01-15 19:04:12 +0000[diff] [blame]6 <link rel="stylesheet" href="llvm.css" type="text/css">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]7</head>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]8<body>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]9
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]10<div class="doc_title">
Misha Brukmana97e6cb2004-07-01 15:33:24 +0000[diff] [blame]11 LLVM Alias Analysis Infrastructure
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]12</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]13
14<ol>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]15 <li><a href="#introduction">Introduction</a></li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]16
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]17 <li><a href="#overview"><tt>AliasAnalysis</tt> Class Overview</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]18 <ul>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]19 <li><a href="#pointers">Representation of Pointers</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]20 <li><a href="#alias">The <tt>alias</tt> method</a></li>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]21 <li><a href="#ModRefInfo">The <tt>getModRefInfo</tt> methods</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]22 <li><a href="#OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a></li>
23 </ul>
24 </li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]25
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]26 <li><a href="#writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]27 <ul>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]28 <li><a href="#passsubclasses">Different Pass styles</a></li>
29 <li><a href="#requiredcalls">Required initialization calls</a></li>
30 <li><a href="#interfaces">Interfaces which may be specified</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]31 <li><a href="#chaining"><tt>AliasAnalysis</tt> chaining behavior</a></li>
32 <li><a href="#updating">Updating analysis results for transformations</a></li>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]33 <li><a href="#implefficiency">Efficiency Issues</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]34 </ul>
35 </li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]36
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]37 <li><a href="#using">Using alias analysis results</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]38 <ul>
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]39 <li><a href="#memdep">Using the <tt>MemoryDependenceAnalysis</tt> Pass</a></li>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]40 <li><a href="#ast">Using the <tt>AliasSetTracker</tt> class</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]41 <li><a href="#direct">Using the <tt>AliasAnalysis</tt> interface directly</a></li>
42 </ul>
43 </li>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]44
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]45 <li><a href="#exist">Existing alias analysis implementations and clients</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]46 <ul>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]47 <li><a href="#impls">Available <tt>AliasAnalysis</tt> implementations</a></li>
48 <li><a href="#aliasanalysis-xforms">Alias analysis driven transformations</a></li>
Misha Brukmana97e6cb2004-07-01 15:33:24 +0000[diff] [blame]49 <li><a href="#aliasanalysis-debug">Clients for debugging and evaluation of
50 implementations</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]51 </ul>
52 </li>
Owen Andersond8b47282007-10-02 00:44:20 +0000[diff] [blame]53 <li><a href="#memdep">Memory Dependence Analysis</a></li>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]54</ol>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]55
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]56<div class="doc_author">
57 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]58</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]59
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]60<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]61<div class="doc_section">
62 <a name="introduction">Introduction</a>
63</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]64<!-- *********************************************************************** -->
65
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]66<div class="doc_text">
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]67
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]68<p>Alias Analysis (aka Pointer Analysis) is a class of techniques which attempt
69to determine whether or not two pointers ever can point to the same object in
70memory. There are many different algorithms for alias analysis and many
71different ways of classifying them: flow-sensitive vs flow-insensitive,
72context-sensitive vs context-insensitive, field-sensitive vs field-insensitive,
73unification-based vs subset-based, etc. Traditionally, alias analyses respond
Duncan Sands8036ca42007-03-30 12:22:09 +0000[diff] [blame]74to a query with a <a href="#MustMayNo">Must, May, or No</a> alias response,
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]75indicating that two pointers always point to the same object, might point to the
76same object, or are known to never point to the same object.</p>
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]77
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]78<p>The LLVM <a
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]79href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]80class is the primary interface used by clients and implementations of alias
81analyses in the LLVM system. This class is the common interface between clients
82of alias analysis information and the implementations providing it, and is
83designed to support a wide range of implementations and clients (but currently
84all clients are assumed to be flow-insensitive). In addition to simple alias
85analysis information, this class exposes Mod/Ref information from those
86implementations which can provide it, allowing for powerful analyses and
87transformations to work well together.</p>
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]88
89<p>This document contains information necessary to successfully implement this
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]90interface, use it, and to test both sides. It also explains some of the finer
91points about what exactly results mean. If you feel that something is unclear
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]92or should be added, please <a href="mailto:sabre@nondot.org">let me
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]93know</a>.</p>
94
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]95</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]96
97<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]98<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]99 <a name="overview"><tt>AliasAnalysis</tt> Class Overview</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]100</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]101<!-- *********************************************************************** -->
102
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]103<div class="doc_text">
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]104
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]105<p>The <a
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]106href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]107class defines the interface that the various alias analysis implementations
108should support. This class exports two important enums: <tt>AliasResult</tt>
109and <tt>ModRefResult</tt> which represent the result of an alias query or a
110mod/ref query, respectively.</p>
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]111
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]112<p>The <tt>AliasAnalysis</tt> interface exposes information about memory,
113represented in several different ways. In particular, memory objects are
114represented as a starting address and size, and function calls are represented
115as the actual <tt>call</tt> or <tt>invoke</tt> instructions that performs the
116call. The <tt>AliasAnalysis</tt> interface also exposes some helper methods
117which allow you to get mod/ref information for arbitrary instructions.</p>
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]118
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]119</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]120
121<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]122<div class="doc_subsection">
123 <a name="pointers">Representation of Pointers</a>
124</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]125
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]126<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]127
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]128<p>Most importantly, the <tt>AliasAnalysis</tt> class provides several methods
129which are used to query whether or not two memory objects alias, whether
130function calls can modify or read a memory object, etc. For all of these
131queries, memory objects are represented as a pair of their starting address (a
132symbolic LLVM <tt>Value*</tt>) and a static size.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]133
134<p>Representing memory objects as a starting address and a size is critically
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]135important for correct Alias Analyses. For example, consider this (silly, but
136possible) C code:</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]137
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]138<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]139<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]140int i;
141char C[2];
142char A[10];
143/* ... */
144for (i = 0; i != 10; ++i) {
145 C[0] = A[i]; /* One byte store */
146 C[1] = A[9-i]; /* One byte store */
147}
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]148</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]149</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]150
151<p>In this case, the <tt>basicaa</tt> pass will disambiguate the stores to
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]152<tt>C[0]</tt> and <tt>C[1]</tt> because they are accesses to two distinct
153locations one byte apart, and the accesses are each one byte. In this case, the
154LICM pass can use store motion to remove the stores from the loop. In
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]155constrast, the following code:</p>
156
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]157<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]158<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]159int i;
160char C[2];
161char A[10];
162/* ... */
163for (i = 0; i != 10; ++i) {
164 ((short*)C)[0] = A[i]; /* Two byte store! */
165 C[1] = A[9-i]; /* One byte store */
166}
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]167</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]168</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]169
170<p>In this case, the two stores to C do alias each other, because the access to
171the <tt>&amp;C[0]</tt> element is a two byte access. If size information wasn't
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]172available in the query, even the first case would have to conservatively assume
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]173that the accesses alias.</p>
174
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]175</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]176
177<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]178<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]179 <a name="alias">The <tt>alias</tt> method</a>
180</div>
181
182<div class="doc_text">
183The <tt>alias</tt> method is the primary interface used to determine whether or
184not two memory objects alias each other. It takes two memory objects as input
185and returns MustAlias, MayAlias, or NoAlias as appropriate.
186</div>
187
188<!-- _______________________________________________________________________ -->
189<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]190 <a name="MustMayNo">Must, May, and No Alias Responses</a>
191</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]192
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]193<div class="doc_text">
Nick Lewycky01557ce2008-12-14 21:08:48 +0000[diff] [blame]194<p>The NoAlias response is used when the two pointers refer to distinct objects,
Nick Lewycky78eb8042008-12-18 02:15:05 +0000[diff] [blame]195regardless of whether the pointers compare equal. For example, freed pointers
196don't alias any pointers that were allocated afterwards. As a degenerate case,
197pointers returned by malloc(0) have no bytes for an object, and are considered
198NoAlias even when malloc returns the same pointer. The same rule applies to
199NULL pointers.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]200
Nick Lewycky01557ce2008-12-14 21:08:48 +0000[diff] [blame]201<p>The MayAlias response is used whenever the two pointers might refer to the
202same object. If the two memory objects overlap, but do not start at the same
203location, return MayAlias.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]204
Nick Lewycky01557ce2008-12-14 21:08:48 +0000[diff] [blame]205<p>The MustAlias response may only be returned if the two memory objects are
206guaranteed to always start at exactly the same location. A MustAlias response
207implies that the pointers compare equal.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]208
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]209</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]210
211<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]212<div class="doc_subsection">
213 <a name="ModRefInfo">The <tt>getModRefInfo</tt> methods</a>
214</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]215
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]216<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]217
218<p>The <tt>getModRefInfo</tt> methods return information about whether the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]219execution of an instruction can read or modify a memory location. Mod/Ref
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]220information is always conservative: if an instruction <b>might</b> read or write
221a location, ModRef is returned.</p>
222
223<p>The <tt>AliasAnalysis</tt> class also provides a <tt>getModRefInfo</tt>
224method for testing dependencies between function calls. This method takes two
225call sites (CS1 &amp; CS2), returns NoModRef if the two calls refer to disjoint
226memory locations, Ref if CS1 reads memory written by CS2, Mod if CS1 writes to
227memory read or written by CS2, or ModRef if CS1 might read or write memory
228accessed by CS2. Note that this relation is not commutative. Clients that use
229this method should be predicated on the <tt>hasNoModRefInfoForCalls()</tt>
230method, which indicates whether or not an analysis can provide mod/ref
231information for function call pairs (most can not). If this predicate is false,
232the client shouldn't waste analysis time querying the <tt>getModRefInfo</tt>
233method many times.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]234
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]235</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]236
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]237
238<!-- ======================================================================= -->
239<div class="doc_subsection">
240 <a name="OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a>
241</div>
242
243<div class="doc_text">
244
245<p>
246Several other tidbits of information are often collected by various alias
247analysis implementations and can be put to good use by various clients.
248</p>
249
250</div>
251
252<!-- _______________________________________________________________________ -->
253<div class="doc_subsubsection">
254 The <tt>getMustAliases</tt> method
255</div>
256
257<div class="doc_text">
258
259<p>The <tt>getMustAliases</tt> method returns all values that are known to
260always must alias a pointer. This information can be provided in some cases for
261important objects like the null pointer and global values. Knowing that a
262pointer always points to a particular function allows indirect calls to be
263turned into direct calls, for example.</p>
264
265</div>
266
267<!-- _______________________________________________________________________ -->
268<div class="doc_subsubsection">
269 The <tt>pointsToConstantMemory</tt> method
270</div>
271
272<div class="doc_text">
273
274<p>The <tt>pointsToConstantMemory</tt> method returns true if and only if the
275analysis can prove that the pointer only points to unchanging memory locations
276(functions, constant global variables, and the null pointer). This information
277can be used to refine mod/ref information: it is impossible for an unchanging
278memory location to be modified.</p>
279
280</div>
281
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]282<!-- _______________________________________________________________________ -->
283<div class="doc_subsubsection">
284 <a name="simplemodref">The <tt>doesNotAccessMemory</tt> and
285 <tt>onlyReadsMemory</tt> methods</a>
286</div>
287
288<div class="doc_text">
289
290<p>These methods are used to provide very simple mod/ref information for
291function calls. The <tt>doesNotAccessMemory</tt> method returns true for a
292function if the analysis can prove that the function never reads or writes to
293memory, or if the function only reads from constant memory. Functions with this
294property are side-effect free and only depend on their input arguments, allowing
295them to be eliminated if they form common subexpressions or be hoisted out of
296loops. Many common functions behave this way (e.g., <tt>sin</tt> and
297<tt>cos</tt>) but many others do not (e.g., <tt>acos</tt>, which modifies the
298<tt>errno</tt> variable).</p>
299
300<p>The <tt>onlyReadsMemory</tt> method returns true for a function if analysis
301can prove that (at most) the function only reads from non-volatile memory.
302Functions with this property are side-effect free, only depending on their input
303arguments and the state of memory when they are called. This property allows
304calls to these functions to be eliminated and moved around, as long as there is
305no store instruction that changes the contents of memory. Note that all
306functions that satisfy the <tt>doesNotAccessMemory</tt> method also satisfies
307<tt>onlyReadsMemory</tt>.</p>
308
309</div>
310
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]311<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]312<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]313 <a name="writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]314</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]315<!-- *********************************************************************** -->
316
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]317<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]318
319<p>Writing a new alias analysis implementation for LLVM is quite
320straight-forward. There are already several implementations that you can use
321for examples, and the following information should help fill in any details.
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]322For a examples, take a look at the <a href="#impls">various alias analysis
323implementations</a> included with LLVM.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]324
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]325</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]326
327<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]328<div class="doc_subsection">
329 <a name="passsubclasses">Different Pass styles</a>
330</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]331
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]332<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]333
334<p>The first step to determining what type of <a
335href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias
336Analysis. As is the case with most other analyses and transformations, the
337answer should be fairly obvious from what type of problem you are trying to
338solve:</p>
339
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]340<ol>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]341 <li>If you require interprocedural analysis, it should be a
342 <tt>Pass</tt>.</li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]343 <li>If you are a function-local analysis, subclass <tt>FunctionPass</tt>.</li>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]344 <li>If you don't need to look at the program at all, subclass
345 <tt>ImmutablePass</tt>.</li>
346</ol>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]347
348<p>In addition to the pass that you subclass, you should also inherit from the
Misha Brukman700fd492003-05-07 21:47:16 +0000[diff] [blame]349<tt>AliasAnalysis</tt> interface, of course, and use the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]350<tt>RegisterAnalysisGroup</tt> template to register as an implementation of
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]351<tt>AliasAnalysis</tt>.</p>
352
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]353</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]354
355<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]356<div class="doc_subsection">
357 <a name="requiredcalls">Required initialization calls</a>
358</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]359
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]360<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]361
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]362<p>Your subclass of <tt>AliasAnalysis</tt> is required to invoke two methods on
363the <tt>AliasAnalysis</tt> base class: <tt>getAnalysisUsage</tt> and
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]364<tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
365<tt>getAnalysisUsage</tt> should explicitly call into the
366<tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
367declaring any pass dependencies your pass has. Thus you should have something
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]368like this:</p>
369
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]370<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]371<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]372void getAnalysisUsage(AnalysisUsage &amp;AU) const {
373 AliasAnalysis::getAnalysisUsage(AU);
374 <i>// declare your dependencies here.</i>
375}
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]376</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]377</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]378
379<p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
380from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]381<tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, or <tt>InitializePass</tt>
382for an <tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]383
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]384<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]385<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]386bool run(Module &amp;M) {
387 InitializeAliasAnalysis(this);
388 <i>// Perform analysis here...</i>
389 return false;
390}
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]391</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]392</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]393
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]394</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]395
396<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]397<div class="doc_subsection">
398 <a name="interfaces">Interfaces which may be specified</a>
399</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]400
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]401<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]402
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]403<p>All of the <a
404href="/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
405virtual methods default to providing <a href="#chaining">chaining</a> to another
406alias analysis implementation, which ends up returning conservatively correct
407information (returning "May" Alias and "Mod/Ref" for alias and mod/ref queries
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]408respectively). Depending on the capabilities of the analysis you are
409implementing, you just override the interfaces you can improve.</p>
410
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]411</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]412
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]413
414
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]415<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]416<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]417 <a name="chaining"><tt>AliasAnalysis</tt> chaining behavior</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]418</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]419
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]420<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]421
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]422<p>With only two special exceptions (the <tt><a
423href="#basic-aa">basicaa</a></tt> and <a href="#no-aa"><tt>no-aa</tt></a>
424passes) every alias analysis pass chains to another alias analysis
425implementation (for example, the user can specify "<tt>-basicaa -ds-aa
426-anders-aa -licm</tt>" to get the maximum benefit from the three alias
427analyses). The alias analysis class automatically takes care of most of this
428for methods that you don't override. For methods that you do override, in code
429paths that return a conservative MayAlias or Mod/Ref result, simply return
430whatever the superclass computes. For example:</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]431
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]432<div class="doc_code">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]433<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]434AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
435 const Value *V2, unsigned V2Size) {
436 if (...)
437 return NoAlias;
438 ...
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]439
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]440 <i>// Couldn't determine a must or no-alias result.</i>
441 return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
442}
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]443</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]444</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]445
446<p>In addition to analysis queries, you must make sure to unconditionally pass
447LLVM <a href="#updating">update notification</a> methods to the superclass as
448well if you override them, which allows all alias analyses in a change to be
449updated.</p>
450
451</div>
452
453
454<!-- ======================================================================= -->
455<div class="doc_subsection">
456 <a name="updating">Updating analysis results for transformations</a>
457</div>
458
459<div class="doc_text">
460<p>
461Alias analysis information is initially computed for a static snapshot of the
462program, but clients will use this information to make transformations to the
463code. All but the most trivial forms of alias analysis will need to have their
464analysis results updated to reflect the changes made by these transformations.
465</p>
466
467<p>
468The <tt>AliasAnalysis</tt> interface exposes two methods which are used to
469communicate program changes from the clients to the analysis implementations.
470Various alias analysis implementations should use these methods to ensure that
471their internal data structures are kept up-to-date as the program changes (for
472example, when an instruction is deleted), and clients of alias analysis must be
473sure to call these interfaces appropriately.
474</p>
475</div>
476
477<!-- _______________________________________________________________________ -->
478<div class="doc_subsubsection">The <tt>deleteValue</tt> method</div>
479
480<div class="doc_text">
481The <tt>deleteValue</tt> method is called by transformations when they remove an
482instruction or any other value from the program (including values that do not
483use pointers). Typically alias analyses keep data structures that have entries
484for each value in the program. When this method is called, they should remove
485any entries for the specified value, if they exist.
486</div>
487
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]488<!-- _______________________________________________________________________ -->
489<div class="doc_subsubsection">The <tt>copyValue</tt> method</div>
490
491<div class="doc_text">
492The <tt>copyValue</tt> method is used when a new value is introduced into the
493program. There is no way to introduce a value into the program that did not
494exist before (this doesn't make sense for a safe compiler transformation), so
495this is the only way to introduce a new value. This method indicates that the
496new value has exactly the same properties as the value being copied.
497</div>
498
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]499<!-- _______________________________________________________________________ -->
500<div class="doc_subsubsection">The <tt>replaceWithNewValue</tt> method</div>
501
502<div class="doc_text">
503This method is a simple helper method that is provided to make clients easier to
504use. It is implemented by copying the old analysis information to the new
505value, then deleting the old value. This method cannot be overridden by alias
506analysis implementations.
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]507</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]508
509<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]510<div class="doc_subsection">
511 <a name="implefficiency">Efficiency Issues</a>
512</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]513
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]514<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]515
516<p>From the LLVM perspective, the only thing you need to do to provide an
517efficient alias analysis is to make sure that alias analysis <b>queries</b> are
518serviced quickly. The actual calculation of the alias analysis results (the
519"run" method) is only performed once, but many (perhaps duplicate) queries may
520be performed. Because of this, try to move as much computation to the run
521method as possible (within reason).</p>
522
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]523</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]524
525<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]526<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]527 <a name="using">Using alias analysis results</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]528</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]529<!-- *********************************************************************** -->
530
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]531<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]532
533<p>There are several different ways to use alias analysis results. In order of
534preference, these are...</p>
535
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]536</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]537
538<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]539<div class="doc_subsection">
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]540 <a name="memdep">Using the <tt>MemoryDependenceAnalysis</tt> Pass</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]541</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]542
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]543<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]544
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]545<p>The <tt>memdep</tt> pass uses alias analysis to provide high-level dependence
546information about memory-using instructions. This will tell you which store
547feeds into a load, for example. It uses caching and other techniques to be
548efficient, and is used by Dead Store Elimination, GVN, and memcpy optimizations.
549</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]550
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]551</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]552
553<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]554<div class="doc_subsection">
555 <a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
556</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]557
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]558<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]559
560<p>Many transformations need information about alias <b>sets</b> that are active
561in some scope, rather than information about pairwise aliasing. The <tt><a
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]562href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class
563is used to efficiently build these Alias Sets from the pairwise alias analysis
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]564information provided by the <tt>AliasAnalysis</tt> interface.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]565
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]566<p>First you initialize the AliasSetTracker by using the "<tt>add</tt>" methods
567to add information about various potentially aliasing instructions in the scope
568you are interested in. Once all of the alias sets are completed, your pass
569should simply iterate through the constructed alias sets, using the
570<tt>AliasSetTracker</tt> <tt>begin()</tt>/<tt>end()</tt> methods.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]571
572<p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]573to be disjoint, calculate mod/ref information and volatility for the set, and
574keep track of whether or not all of the pointers in the set are Must aliases.
575The AliasSetTracker also makes sure that sets are properly folded due to call
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]576instructions, and can provide a list of pointers in each set.</p>
577
578<p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]579Invariant Code Motion</a> pass uses <tt>AliasSetTracker</tt>s to calculate alias
580sets for each loop nest. If an <tt>AliasSet</tt> in a loop is not modified,
581then all load instructions from that set may be hoisted out of the loop. If any
582alias sets are stored to <b>and</b> are must alias sets, then the stores may be
583sunk to outside of the loop, promoting the memory location to a register for the
584duration of the loop nest. Both of these transformations only apply if the
585pointer argument is loop-invariant.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]586
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]587</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]588
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]589<!-- _______________________________________________________________________ -->
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]590<div class="doc_subsubsection">
591 The AliasSetTracker implementation
592</div>
593
594<div class="doc_text">
595
596<p>The AliasSetTracker class is implemented to be as efficient as possible. It
597uses the union-find algorithm to efficiently merge AliasSets when a pointer is
598inserted into the AliasSetTracker that aliases multiple sets. The primary data
599structure is a hash table mapping pointers to the AliasSet they are in.</p>
600
601<p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s
602that are in each AliasSet. Since the hash table already has entries for each
603LLVM Value* of interest, the AliasesSets thread the linked list through these
604hash-table nodes to avoid having to allocate memory unnecessarily, and to make
605merging alias sets extremely efficient (the linked list merge is constant time).
606</p>
607
608<p>You shouldn't need to understand these details if you are just a client of
609the AliasSetTracker, but if you look at the code, hopefully this brief
610description will help make sense of why things are designed the way they
611are.</p>
612
613</div>
614
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]615<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]616<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]617 <a name="direct">Using the <tt>AliasAnalysis</tt> interface directly</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]618</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]619
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]620<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]621
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]622<p>If neither of these utility class are what your pass needs, you should use
623the interfaces exposed by the <tt>AliasAnalysis</tt> class directly. Try to use
624the higher-level methods when possible (e.g., use mod/ref information instead of
625the <a href="#alias"><tt>alias</tt></a> method directly if possible) to get the
626best precision and efficiency.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]627
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]628</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]629
630<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]631<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]632 <a name="exist">Existing alias analysis implementations and clients</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]633</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]634<!-- *********************************************************************** -->
635
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]636<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]637
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]638<p>If you're going to be working with the LLVM alias analysis infrastructure,
639you should know what clients and implementations of alias analysis are
640available. In particular, if you are implementing an alias analysis, you should
641be aware of the <a href="#aliasanalysis-debug">the clients</a> that are useful
642for monitoring and evaluating different implementations.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]643
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]644</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]645
646<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]647<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]648 <a name="impls">Available <tt>AliasAnalysis</tt> implementations</a>
649</div>
650
651<div class="doc_text">
652
653<p>This section lists the various implementations of the <tt>AliasAnalysis</tt>
654interface. With the exception of the <a href="#no-aa"><tt>-no-aa</tt></a> and
655<a href="#basic-aa"><tt>-basicaa</tt></a> implementations, all of these <a
Chris Lattnerfcd37252004-06-21 22:52:48 +0000[diff] [blame]656href="#chaining">chain</a> to other alias analysis implementations.</p>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]657
658</div>
659
660<!-- _______________________________________________________________________ -->
661<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]662 <a name="no-aa">The <tt>-no-aa</tt> pass</a>
663</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]664
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]665<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]666
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]667<p>The <tt>-no-aa</tt> pass is just like what it sounds: an alias analysis that
668never returns any useful information. This pass can be useful if you think that
669alias analysis is doing something wrong and are trying to narrow down a
670problem.</p>
671
672</div>
673
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]674<!-- _______________________________________________________________________ -->
675<div class="doc_subsubsection">
676 <a name="basic-aa">The <tt>-basicaa</tt> pass</a>
677</div>
678
679<div class="doc_text">
680
681<p>The <tt>-basicaa</tt> pass is the default LLVM alias analysis. It is an
682aggressive local analysis that "knows" many important facts:</p>
683
684<ul>
685<li>Distinct globals, stack allocations, and heap allocations can never
686 alias.</li>
687<li>Globals, stack allocations, and heap allocations never alias the null
688 pointer.</li>
689<li>Different fields of a structure do not alias.</li>
690<li>Indexes into arrays with statically differing subscripts cannot alias.</li>
691<li>Many common standard C library functions <a
692 href="#simplemodref">never access memory or only read memory</a>.</li>
693<li>Pointers that obviously point to constant globals
694 "<tt>pointToConstantMemory</tt>".</li>
695<li>Function calls can not modify or references stack allocations if they never
696 escape from the function that allocates them (a common case for automatic
697 arrays).</li>
698</ul>
699
700</div>
701
Chris Lattner100a4f82004-06-28 19:19:47 +0000[diff] [blame]702<!-- _______________________________________________________________________ -->
703<div class="doc_subsubsection">
704 <a name="globalsmodref">The <tt>-globalsmodref-aa</tt> pass</a>
705</div>
706
707<div class="doc_text">
708
709<p>This pass implements a simple context-sensitive mod/ref and alias analysis
Chris Lattnerccb354b2004-07-27 07:50:07 +0000[diff] [blame]710for internal global variables that don't "have their address taken". If a
711global does not have its address taken, the pass knows that no pointers alias
712the global. This pass also keeps track of functions that it knows never access
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]713memory or never read memory. This allows certain optimizations (e.g. GVN) to
Chris Lattnerccb354b2004-07-27 07:50:07 +0000[diff] [blame]714eliminate call instructions entirely.
Chris Lattner100a4f82004-06-28 19:19:47 +0000[diff] [blame]715</p>
716
717<p>The real power of this pass is that it provides context-sensitive mod/ref
718information for call instructions. This allows the optimizer to know that
719calls to a function do not clobber or read the value of the global, allowing
720loads and stores to be eliminated.</p>
721
722<p>Note that this pass is somewhat limited in its scope (only support
723non-address taken globals), but is very quick analysis.</p>
724</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]725
726<!-- _______________________________________________________________________ -->
727<div class="doc_subsubsection">
728 <a name="anders-aa">The <tt>-anders-aa</tt> pass</a>
729</div>
730
731<div class="doc_text">
732
733<p>The <tt>-anders-aa</tt> pass implements the well-known "Andersen's algorithm"
734for interprocedural alias analysis. This algorithm is a subset-based,
735flow-insensitive, context-insensitive, and field-insensitive alias analysis that
736is widely believed to be fairly precise. Unfortunately, this algorithm is also
737O(N<sup>3</sup>). The LLVM implementation currently does not implement any of
738the refinements (such as "online cycle elimination" or "offline variable
739substitution") to improve its efficiency, so it can be quite slow in common
740cases.
741</p>
742
743</div>
744
745<!-- _______________________________________________________________________ -->
746<div class="doc_subsubsection">
747 <a name="steens-aa">The <tt>-steens-aa</tt> pass</a>
748</div>
749
750<div class="doc_text">
751
752<p>The <tt>-steens-aa</tt> pass implements a variation on the well-known
753"Steensgaard's algorithm" for interprocedural alias analysis. Steensgaard's
754algorithm is a unification-based, flow-insensitive, context-insensitive, and
755field-insensitive alias analysis that is also very scalable (effectively linear
756time).</p>
757
758<p>The LLVM <tt>-steens-aa</tt> pass implements a "speculatively
759field-<b>sensitive</b>" version of Steensgaard's algorithm using the Data
760Structure Analysis framework. This gives it substantially more precision than
761the standard algorithm while maintaining excellent analysis scalability.</p>
762
Chris Lattnerd6e0dd12007-07-03 04:41:50 +0000[diff] [blame]763<p>Note that <tt>-steens-aa</tt> is available in the optional "poolalloc"
764module, it is not part of the LLVM core.</p>
765
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]766</div>
767
768<!-- _______________________________________________________________________ -->
769<div class="doc_subsubsection">
770 <a name="ds-aa">The <tt>-ds-aa</tt> pass</a>
771</div>
772
773<div class="doc_text">
774
775<p>The <tt>-ds-aa</tt> pass implements the full Data Structure Analysis
776algorithm. Data Structure Analysis is a modular unification-based,
777flow-insensitive, context-<b>sensitive</b>, and speculatively
778field-<b>sensitive</b> alias analysis that is also quite scalable, usually at
779O(n*log(n)).</p>
780
781<p>This algorithm is capable of responding to a full variety of alias analysis
782queries, and can provide context-sensitive mod/ref information as well. The
783only major facility not implemented so far is support for must-alias
784information.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]785
Chris Lattnerd6e0dd12007-07-03 04:41:50 +0000[diff] [blame]786<p>Note that <tt>-ds-aa</tt> is available in the optional "poolalloc"
787module, it is not part of the LLVM core.</p>
788
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]789</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]790
791
792<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]793<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]794 <a name="aliasanalysis-xforms">Alias analysis driven transformations</a>
795</div>
796
797<div class="doc_text">
798LLVM includes several alias-analysis driven transformations which can be used
799with any of the implementations above.
800</div>
801
802<!-- _______________________________________________________________________ -->
803<div class="doc_subsubsection">
804 <a name="adce">The <tt>-adce</tt> pass</a>
805</div>
806
807<div class="doc_text">
808
809<p>The <tt>-adce</tt> pass, which implements Aggressive Dead Code Elimination
810uses the <tt>AliasAnalysis</tt> interface to delete calls to functions that do
811not have side-effects and are not used.</p>
812
813</div>
814
815
816<!-- _______________________________________________________________________ -->
817<div class="doc_subsubsection">
818 <a name="licm">The <tt>-licm</tt> pass</a>
819</div>
820
821<div class="doc_text">
822
823<p>The <tt>-licm</tt> pass implements various Loop Invariant Code Motion related
824transformations. It uses the <tt>AliasAnalysis</tt> interface for several
825different transformations:</p>
826
827<ul>
828<li>It uses mod/ref information to hoist or sink load instructions out of loops
829if there are no instructions in the loop that modifies the memory loaded.</li>
830
831<li>It uses mod/ref information to hoist function calls out of loops that do not
832write to memory and are loop-invariant.</li>
833
834<li>If uses alias information to promote memory objects that are loaded and
835stored to in loops to live in a register instead. It can do this if there are
836no may aliases to the loaded/stored memory location.</li>
837</ul>
838
839</div>
840
841<!-- _______________________________________________________________________ -->
842<div class="doc_subsubsection">
843 <a name="argpromotion">The <tt>-argpromotion</tt> pass</a>
844</div>
845
846<div class="doc_text">
847<p>
848The <tt>-argpromotion</tt> pass promotes by-reference arguments to be passed in
849by-value instead. In particular, if pointer arguments are only loaded from it
850passes in the value loaded instead of the address to the function. This pass
851uses alias information to make sure that the value loaded from the argument
852pointer is not modified between the entry of the function and any load of the
853pointer.</p>
854</div>
855
856<!-- _______________________________________________________________________ -->
857<div class="doc_subsubsection">
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]858 <a name="gvn">The <tt>-gvn</tt>, <tt>-memcpyopt</tt>, and <tt>-dse</tt>
859 passes</a>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]860</div>
861
862<div class="doc_text">
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]863
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]864<p>These passes use AliasAnalysis information to reason about loads and stores.
865</p>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]866
867</div>
868
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]869<!-- ======================================================================= -->
870<div class="doc_subsection">
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]871 <a name="aliasanalysis-debug">Clients for debugging and evaluation of
872 implementations</a>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]873</div>
874
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]875<div class="doc_text">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]876
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]877<p>These passes are useful for evaluating the various alias analysis
878implementations. You can use them with commands like '<tt>opt -anders-aa -ds-aa
879-aa-eval foo.bc -disable-output -stats</tt>'.</p>
880
881</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]882
883<!-- _______________________________________________________________________ -->
884<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]885 <a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
886</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]887
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]888<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]889
890<p>The <tt>-print-alias-sets</tt> pass is exposed as part of the
Chris Lattner05d5c9d2006-01-03 06:04:48 +0000[diff] [blame]891<tt>opt</tt> tool to print out the Alias Sets formed by the <a
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]892href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
Chris Lattner05d5c9d2006-01-03 06:04:48 +0000[diff] [blame]893the <tt>AliasSetTracker</tt> class. To use it, use something like:</p>
894
895<div class="doc_code">
896<pre>
897% opt -ds-aa -print-alias-sets -disable-output
898</pre>
899</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]900
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]901</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]902
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]903
904<!-- _______________________________________________________________________ -->
905<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]906 <a name="count-aa">The <tt>-count-aa</tt> pass</a>
907</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]908
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]909<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]910
911<p>The <tt>-count-aa</tt> pass is useful to see how many queries a particular
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]912pass is making and what responses are returned by the alias analysis. As an
913example,</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]914
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]915<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]916<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]917% opt -basicaa -count-aa -ds-aa -count-aa -licm
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]918</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]919</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]920
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]921<p>will print out how many queries (and what responses are returned) by the
922<tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are made
923of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be useful
924when debugging a transformation or an alias analysis implementation.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]925
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]926</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]927
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]928<!-- _______________________________________________________________________ -->
929<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]930 <a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
931</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]932
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]933<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]934
935<p>The <tt>-aa-eval</tt> pass simply iterates through all pairs of pointers in a
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]936function and asks an alias analysis whether or not the pointers alias. This
937gives an indication of the precision of the alias analysis. Statistics are
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]938printed indicating the percent of no/may/must aliases found (a more precise
939algorithm will have a lower number of may aliases).</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]940
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]941</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]942
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]943<!-- *********************************************************************** -->
Owen Anderson5a726b82007-10-02 00:43:25 +0000[diff] [blame]944<div class="doc_section">
945 <a name="memdep">Memory Dependence Analysis</a>
946</div>
947<!-- *********************************************************************** -->
948
949<div class="doc_text">
950
951<p>If you're just looking to be a client of alias analysis information, consider
952using the Memory Dependence Analysis interface instead. MemDep is a lazy,
953caching layer on top of alias analysis that is able to answer the question of
954what preceding memory operations a given instruction depends on, either at an
955intra- or inter-block level. Because of its laziness and caching
956policy, using MemDep can be a significant performance win over accessing alias
957analysis directly.</p>
958
959</div>
960
961<!-- *********************************************************************** -->
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]962
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]963<hr>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]964<address>
Misha Brukman38847d52003-12-21 22:53:21 +0000[diff] [blame]965 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]966 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]967 <a href="http://validator.w3.org/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]968 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]969
970 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]971 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]972 Last modified: $Date$
973</address>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]974
975</body>
976</html>