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4<head>
Misha Brukman3eef5d42004-07-01 15:33:24 +0000[diff] [blame]5 <title>LLVM Alias Analysis Infrastructure</title>
Misha Brukmanc6ef5592004-01-15 19:04:12 +0000[diff] [blame]6 <link rel="stylesheet" href="llvm.css" type="text/css">
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]7</head>
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]8<body>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]9
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]10<div class="doc_title">
Misha Brukman3eef5d42004-07-01 15:33:24 +0000[diff] [blame]11 LLVM Alias Analysis Infrastructure
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]12</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]13
14<ol>
Misha Brukman9782b032003-10-23 02:29:42 +0000[diff] [blame]15 <li><a href="#introduction">Introduction</a></li>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]16
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]17 <li><a href="#overview"><tt>AliasAnalysis</tt> Class Overview</a>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]18 <ul>
Misha Brukman9782b032003-10-23 02:29:42 +0000[diff] [blame]19 <li><a href="#pointers">Representation of Pointers</a></li>
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]20 <li><a href="#alias">The <tt>alias</tt> method</a></li>
Misha Brukman9782b032003-10-23 02:29:42 +0000[diff] [blame]21 <li><a href="#ModRefInfo">The <tt>getModRefInfo</tt> methods</a></li>
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]22 <li><a href="#OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a></li>
23 </ul>
24 </li>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]25
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]26 <li><a href="#writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]27 <ul>
Misha Brukman9782b032003-10-23 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 Lattner2a19d832004-05-23 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 Brukman9782b032003-10-23 02:29:42 +0000[diff] [blame]33 <li><a href="#implefficiency">Efficiency Issues</a></li>
Dan Gohmana3de0182010-06-24 19:34:03 +0000[diff] [blame]34 <li><a href="#passmanager">Pass Manager Issues</a></li>
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]35 </ul>
36 </li>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]37
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]38 <li><a href="#using">Using alias analysis results</a>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]39 <ul>
Chris Lattner6a6d3cc2009-04-25 21:11:37 +0000[diff] [blame]40 <li><a href="#memdep">Using the <tt>MemoryDependenceAnalysis</tt> Pass</a></li>
Misha Brukman9782b032003-10-23 02:29:42 +0000[diff] [blame]41 <li><a href="#ast">Using the <tt>AliasSetTracker</tt> class</a></li>
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]42 <li><a href="#direct">Using the <tt>AliasAnalysis</tt> interface directly</a></li>
43 </ul>
44 </li>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]45
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]46 <li><a href="#exist">Existing alias analysis implementations and clients</a>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]47 <ul>
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]48 <li><a href="#impls">Available <tt>AliasAnalysis</tt> implementations</a></li>
49 <li><a href="#aliasanalysis-xforms">Alias analysis driven transformations</a></li>
Misha Brukman3eef5d42004-07-01 15:33:24 +0000[diff] [blame]50 <li><a href="#aliasanalysis-debug">Clients for debugging and evaluation of
51 implementations</a></li>
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]52 </ul>
53 </li>
Owen Anderson88847732007-10-02 00:44:20 +0000[diff] [blame]54 <li><a href="#memdep">Memory Dependence Analysis</a></li>
Misha Brukman9782b032003-10-23 02:29:42 +0000[diff] [blame]55</ol>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]56
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]57<div class="doc_author">
58 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]59</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]60
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]61<!-- *********************************************************************** -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]62<div class="doc_section">
63 <a name="introduction">Introduction</a>
64</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]65<!-- *********************************************************************** -->
66
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]67<div class="doc_text">
Misha Brukmanbc2d3302003-11-21 22:30:25 +0000[diff] [blame]68
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]69<p>Alias Analysis (aka Pointer Analysis) is a class of techniques which attempt
70to determine whether or not two pointers ever can point to the same object in
71memory. There are many different algorithms for alias analysis and many
72different ways of classifying them: flow-sensitive vs flow-insensitive,
73context-sensitive vs context-insensitive, field-sensitive vs field-insensitive,
74unification-based vs subset-based, etc. Traditionally, alias analyses respond
Duncan Sands16f122e2007-03-30 12:22:09 +0000[diff] [blame]75to a query with a <a href="#MustMayNo">Must, May, or No</a> alias response,
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]76indicating that two pointers always point to the same object, might point to the
77same object, or are known to never point to the same object.</p>
Misha Brukmanbc2d3302003-11-21 22:30:25 +0000[diff] [blame]78
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]79<p>The LLVM <a
Reid Spencerca058542006-03-14 05:39:39 +0000[diff] [blame]80href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]81class is the primary interface used by clients and implementations of alias
82analyses in the LLVM system. This class is the common interface between clients
83of alias analysis information and the implementations providing it, and is
84designed to support a wide range of implementations and clients (but currently
85all clients are assumed to be flow-insensitive). In addition to simple alias
86analysis information, this class exposes Mod/Ref information from those
87implementations which can provide it, allowing for powerful analyses and
88transformations to work well together.</p>
Misha Brukmanbc2d3302003-11-21 22:30:25 +0000[diff] [blame]89
90<p>This document contains information necessary to successfully implement this
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]91interface, use it, and to test both sides. It also explains some of the finer
92points about what exactly results mean. If you feel that something is unclear
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]93or should be added, please <a href="mailto:sabre@nondot.org">let me
Misha Brukmanbc2d3302003-11-21 22:30:25 +0000[diff] [blame]94know</a>.</p>
95
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]96</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]97
98<!-- *********************************************************************** -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]99<div class="doc_section">
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]100 <a name="overview"><tt>AliasAnalysis</tt> Class Overview</a>
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]101</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]102<!-- *********************************************************************** -->
103
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]104<div class="doc_text">
Misha Brukmanbc2d3302003-11-21 22:30:25 +0000[diff] [blame]105
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]106<p>The <a
Reid Spencerca058542006-03-14 05:39:39 +0000[diff] [blame]107href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]108class defines the interface that the various alias analysis implementations
109should support. This class exports two important enums: <tt>AliasResult</tt>
110and <tt>ModRefResult</tt> which represent the result of an alias query or a
111mod/ref query, respectively.</p>
Misha Brukmanbc2d3302003-11-21 22:30:25 +0000[diff] [blame]112
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]113<p>The <tt>AliasAnalysis</tt> interface exposes information about memory,
114represented in several different ways. In particular, memory objects are
115represented as a starting address and size, and function calls are represented
116as the actual <tt>call</tt> or <tt>invoke</tt> instructions that performs the
117call. The <tt>AliasAnalysis</tt> interface also exposes some helper methods
118which allow you to get mod/ref information for arbitrary instructions.</p>
Misha Brukmanbc2d3302003-11-21 22:30:25 +0000[diff] [blame]119
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]120</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]121
122<!-- ======================================================================= -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]123<div class="doc_subsection">
124 <a name="pointers">Representation of Pointers</a>
125</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]126
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]127<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]128
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]129<p>Most importantly, the <tt>AliasAnalysis</tt> class provides several methods
130which are used to query whether or not two memory objects alias, whether
131function calls can modify or read a memory object, etc. For all of these
132queries, memory objects are represented as a pair of their starting address (a
133symbolic LLVM <tt>Value*</tt>) and a static size.</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]134
135<p>Representing memory objects as a starting address and a size is critically
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]136important for correct Alias Analyses. For example, consider this (silly, but
137possible) C code:</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]138
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]139<div class="doc_code">
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]140<pre>
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]141int i;
142char C[2];
143char A[10];
144/* ... */
145for (i = 0; i != 10; ++i) {
146 C[0] = A[i]; /* One byte store */
147 C[1] = A[9-i]; /* One byte store */
148}
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]149</pre>
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]150</div>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]151
152<p>In this case, the <tt>basicaa</tt> pass will disambiguate the stores to
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]153<tt>C[0]</tt> and <tt>C[1]</tt> because they are accesses to two distinct
154locations one byte apart, and the accesses are each one byte. In this case, the
155LICM pass can use store motion to remove the stores from the loop. In
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]156constrast, the following code:</p>
157
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]158<div class="doc_code">
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]159<pre>
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]160int i;
161char C[2];
162char A[10];
163/* ... */
164for (i = 0; i != 10; ++i) {
165 ((short*)C)[0] = A[i]; /* Two byte store! */
166 C[1] = A[9-i]; /* One byte store */
167}
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]168</pre>
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]169</div>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]170
171<p>In this case, the two stores to C do alias each other, because the access to
172the <tt>&amp;C[0]</tt> element is a two byte access. If size information wasn't
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]173available in the query, even the first case would have to conservatively assume
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]174that the accesses alias.</p>
175
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]176</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]177
178<!-- ======================================================================= -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]179<div class="doc_subsection">
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]180 <a name="alias">The <tt>alias</tt> method</a>
181</div>
182
183<div class="doc_text">
184The <tt>alias</tt> method is the primary interface used to determine whether or
185not two memory objects alias each other. It takes two memory objects as input
186and returns MustAlias, MayAlias, or NoAlias as appropriate.
187</div>
188
189<!-- _______________________________________________________________________ -->
190<div class="doc_subsubsection">
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]191 <a name="MustMayNo">Must, May, and No Alias Responses</a>
192</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]193
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]194<div class="doc_text">
Dan Gohmandf12d082010-07-02 18:41:32 +0000[diff] [blame]195<p>The NoAlias response may be used when there is never an immediate dependence
196between any memory reference <i>based</i> on one pointer and any memory
197reference <i>based</i> the other. The most obvious example is when the two
198pointers point to non-overlapping memory ranges. Another is when the two
199pointers are only ever used for reading memory. Another is when the memory is
200freed and reallocated between accesses through one pointer and accesses through
201the other -- in this case, there is a dependence, but it's mediated by the free
202and reallocation.</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]203
Nick Lewycky4048ca52008-12-14 21:08:48 +0000[diff] [blame]204<p>The MayAlias response is used whenever the two pointers might refer to the
205same object. If the two memory objects overlap, but do not start at the same
206location, return MayAlias.</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]207
Nick Lewycky4048ca52008-12-14 21:08:48 +0000[diff] [blame]208<p>The MustAlias response may only be returned if the two memory objects are
209guaranteed to always start at exactly the same location. A MustAlias response
210implies that the pointers compare equal.</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]211
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]212</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]213
214<!-- ======================================================================= -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]215<div class="doc_subsection">
216 <a name="ModRefInfo">The <tt>getModRefInfo</tt> methods</a>
217</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]218
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]219<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]220
221<p>The <tt>getModRefInfo</tt> methods return information about whether the
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]222execution of an instruction can read or modify a memory location. Mod/Ref
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]223information is always conservative: if an instruction <b>might</b> read or write
224a location, ModRef is returned.</p>
225
226<p>The <tt>AliasAnalysis</tt> class also provides a <tt>getModRefInfo</tt>
227method for testing dependencies between function calls. This method takes two
228call sites (CS1 &amp; CS2), returns NoModRef if the two calls refer to disjoint
229memory locations, Ref if CS1 reads memory written by CS2, Mod if CS1 writes to
230memory read or written by CS2, or ModRef if CS1 might read or write memory
Chris Lattner88874ec2009-11-22 16:01:44 +0000[diff] [blame]231accessed by CS2. Note that this relation is not commutative.</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]232
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]233</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]234
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]235
236<!-- ======================================================================= -->
237<div class="doc_subsection">
238 <a name="OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a>
239</div>
240
241<div class="doc_text">
242
243<p>
244Several other tidbits of information are often collected by various alias
245analysis implementations and can be put to good use by various clients.
246</p>
247
248</div>
249
250<!-- _______________________________________________________________________ -->
251<div class="doc_subsubsection">
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]252 The <tt>pointsToConstantMemory</tt> method
253</div>
254
255<div class="doc_text">
256
257<p>The <tt>pointsToConstantMemory</tt> method returns true if and only if the
258analysis can prove that the pointer only points to unchanging memory locations
259(functions, constant global variables, and the null pointer). This information
260can be used to refine mod/ref information: it is impossible for an unchanging
261memory location to be modified.</p>
262
263</div>
264
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]265<!-- _______________________________________________________________________ -->
266<div class="doc_subsubsection">
267 <a name="simplemodref">The <tt>doesNotAccessMemory</tt> and
268 <tt>onlyReadsMemory</tt> methods</a>
269</div>
270
271<div class="doc_text">
272
273<p>These methods are used to provide very simple mod/ref information for
274function calls. The <tt>doesNotAccessMemory</tt> method returns true for a
275function if the analysis can prove that the function never reads or writes to
276memory, or if the function only reads from constant memory. Functions with this
277property are side-effect free and only depend on their input arguments, allowing
278them to be eliminated if they form common subexpressions or be hoisted out of
279loops. Many common functions behave this way (e.g., <tt>sin</tt> and
280<tt>cos</tt>) but many others do not (e.g., <tt>acos</tt>, which modifies the
281<tt>errno</tt> variable).</p>
282
283<p>The <tt>onlyReadsMemory</tt> method returns true for a function if analysis
284can prove that (at most) the function only reads from non-volatile memory.
285Functions with this property are side-effect free, only depending on their input
286arguments and the state of memory when they are called. This property allows
287calls to these functions to be eliminated and moved around, as long as there is
288no store instruction that changes the contents of memory. Note that all
289functions that satisfy the <tt>doesNotAccessMemory</tt> method also satisfies
290<tt>onlyReadsMemory</tt>.</p>
291
292</div>
293
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]294<!-- *********************************************************************** -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]295<div class="doc_section">
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]296 <a name="writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]297</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]298<!-- *********************************************************************** -->
299
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]300<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]301
302<p>Writing a new alias analysis implementation for LLVM is quite
303straight-forward. There are already several implementations that you can use
304for examples, and the following information should help fill in any details.
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]305For a examples, take a look at the <a href="#impls">various alias analysis
306implementations</a> included with LLVM.</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]307
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]308</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]309
310<!-- ======================================================================= -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]311<div class="doc_subsection">
312 <a name="passsubclasses">Different Pass styles</a>
313</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]314
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]315<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]316
317<p>The first step to determining what type of <a
318href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias
319Analysis. As is the case with most other analyses and transformations, the
320answer should be fairly obvious from what type of problem you are trying to
321solve:</p>
322
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]323<ol>
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]324 <li>If you require interprocedural analysis, it should be a
325 <tt>Pass</tt>.</li>
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]326 <li>If you are a function-local analysis, subclass <tt>FunctionPass</tt>.</li>
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]327 <li>If you don't need to look at the program at all, subclass
328 <tt>ImmutablePass</tt>.</li>
329</ol>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]330
331<p>In addition to the pass that you subclass, you should also inherit from the
Misha Brukmanb4dd5392003-05-07 21:47:16 +0000[diff] [blame]332<tt>AliasAnalysis</tt> interface, of course, and use the
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]333<tt>RegisterAnalysisGroup</tt> template to register as an implementation of
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]334<tt>AliasAnalysis</tt>.</p>
335
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]336</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]337
338<!-- ======================================================================= -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]339<div class="doc_subsection">
340 <a name="requiredcalls">Required initialization calls</a>
341</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]342
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]343<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]344
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]345<p>Your subclass of <tt>AliasAnalysis</tt> is required to invoke two methods on
346the <tt>AliasAnalysis</tt> base class: <tt>getAnalysisUsage</tt> and
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]347<tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
348<tt>getAnalysisUsage</tt> should explicitly call into the
349<tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
350declaring any pass dependencies your pass has. Thus you should have something
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]351like this:</p>
352
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]353<div class="doc_code">
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]354<pre>
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]355void getAnalysisUsage(AnalysisUsage &amp;AU) const {
356 AliasAnalysis::getAnalysisUsage(AU);
357 <i>// declare your dependencies here.</i>
358}
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]359</pre>
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]360</div>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]361
362<p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
363from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]364<tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, or <tt>InitializePass</tt>
365for an <tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]366
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]367<div class="doc_code">
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]368<pre>
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]369bool run(Module &amp;M) {
370 InitializeAliasAnalysis(this);
371 <i>// Perform analysis here...</i>
372 return false;
373}
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]374</pre>
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]375</div>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]376
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]377</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]378
379<!-- ======================================================================= -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]380<div class="doc_subsection">
381 <a name="interfaces">Interfaces which may be specified</a>
382</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]383
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]384<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]385
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]386<p>All of the <a
387href="/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
388virtual methods default to providing <a href="#chaining">chaining</a> to another
389alias analysis implementation, which ends up returning conservatively correct
390information (returning "May" Alias and "Mod/Ref" for alias and mod/ref queries
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]391respectively). Depending on the capabilities of the analysis you are
392implementing, you just override the interfaces you can improve.</p>
393
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]394</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]395
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]396
397
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]398<!-- ======================================================================= -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]399<div class="doc_subsection">
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]400 <a name="chaining"><tt>AliasAnalysis</tt> chaining behavior</a>
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]401</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]402
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]403<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]404
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]405<p>With only two special exceptions (the <tt><a
406href="#basic-aa">basicaa</a></tt> and <a href="#no-aa"><tt>no-aa</tt></a>
407passes) every alias analysis pass chains to another alias analysis
408implementation (for example, the user can specify "<tt>-basicaa -ds-aa
Chris Lattner5ea3e652010-03-01 19:24:17 +0000[diff] [blame]409-licm</tt>" to get the maximum benefit from both alias
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]410analyses). The alias analysis class automatically takes care of most of this
411for methods that you don't override. For methods that you do override, in code
412paths that return a conservative MayAlias or Mod/Ref result, simply return
413whatever the superclass computes. For example:</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]414
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]415<div class="doc_code">
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]416<pre>
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]417AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
418 const Value *V2, unsigned V2Size) {
419 if (...)
420 return NoAlias;
421 ...
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]422
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]423 <i>// Couldn't determine a must or no-alias result.</i>
424 return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
425}
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]426</pre>
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]427</div>
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]428
429<p>In addition to analysis queries, you must make sure to unconditionally pass
430LLVM <a href="#updating">update notification</a> methods to the superclass as
431well if you override them, which allows all alias analyses in a change to be
432updated.</p>
433
434</div>
435
436
437<!-- ======================================================================= -->
438<div class="doc_subsection">
439 <a name="updating">Updating analysis results for transformations</a>
440</div>
441
442<div class="doc_text">
443<p>
444Alias analysis information is initially computed for a static snapshot of the
445program, but clients will use this information to make transformations to the
446code. All but the most trivial forms of alias analysis will need to have their
447analysis results updated to reflect the changes made by these transformations.
448</p>
449
450<p>
451The <tt>AliasAnalysis</tt> interface exposes two methods which are used to
452communicate program changes from the clients to the analysis implementations.
453Various alias analysis implementations should use these methods to ensure that
454their internal data structures are kept up-to-date as the program changes (for
455example, when an instruction is deleted), and clients of alias analysis must be
456sure to call these interfaces appropriately.
457</p>
458</div>
459
460<!-- _______________________________________________________________________ -->
461<div class="doc_subsubsection">The <tt>deleteValue</tt> method</div>
462
463<div class="doc_text">
464The <tt>deleteValue</tt> method is called by transformations when they remove an
465instruction or any other value from the program (including values that do not
466use pointers). Typically alias analyses keep data structures that have entries
467for each value in the program. When this method is called, they should remove
468any entries for the specified value, if they exist.
469</div>
470
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]471<!-- _______________________________________________________________________ -->
472<div class="doc_subsubsection">The <tt>copyValue</tt> method</div>
473
474<div class="doc_text">
475The <tt>copyValue</tt> method is used when a new value is introduced into the
476program. There is no way to introduce a value into the program that did not
477exist before (this doesn't make sense for a safe compiler transformation), so
478this is the only way to introduce a new value. This method indicates that the
479new value has exactly the same properties as the value being copied.
480</div>
481
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]482<!-- _______________________________________________________________________ -->
483<div class="doc_subsubsection">The <tt>replaceWithNewValue</tt> method</div>
484
485<div class="doc_text">
486This method is a simple helper method that is provided to make clients easier to
487use. It is implemented by copying the old analysis information to the new
488value, then deleting the old value. This method cannot be overridden by alias
489analysis implementations.
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]490</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]491
492<!-- ======================================================================= -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]493<div class="doc_subsection">
494 <a name="implefficiency">Efficiency Issues</a>
495</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]496
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]497<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]498
499<p>From the LLVM perspective, the only thing you need to do to provide an
500efficient alias analysis is to make sure that alias analysis <b>queries</b> are
501serviced quickly. The actual calculation of the alias analysis results (the
502"run" method) is only performed once, but many (perhaps duplicate) queries may
503be performed. Because of this, try to move as much computation to the run
504method as possible (within reason).</p>
505
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]506</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]507
Dan Gohmana3de0182010-06-24 19:34:03 +0000[diff] [blame]508<!-- ======================================================================= -->
509<div class="doc_subsection">
510 <a name="passmanager">Pass Manager Issues</a>
511</div>
512
513<div class="doc_text">
514
515<p>PassManager support for alternative AliasAnalysis implementation
516has some issues.</p>
517
518<p>There is no way to override the default alias analysis. It would
519be very useful to be able to do something like "opt -my-aa -O2" and
520have it use -my-aa for all passes which need AliasAnalysis, but there
521is currently no support for that, short of changing the source code
522and recompiling. Similarly, there is also no way of setting a chain
523of analyses as the default.</p>
524
525<p>There is no way for transform passes to declare that they preserve
526<tt>AliasAnalysis</tt> implementations. The <tt>AliasAnalysis</tt>
527interface includes <tt>deleteValue</tt> and <tt>copyValue</tt> methods
528which are intended to allow a pass to keep an AliasAnalysis consistent,
529however there's no way for a pass to declare in its
530<tt>getAnalysisUsage</tt> that it does so. Some passes attempt to use
531<tt>AU.addPreserved&lt;AliasAnalysis&gt;</tt>, however this doesn't
532actually have any effect.</tt>
533
534<p><tt>AliasAnalysisCounter</tt> (<tt>-count-aa</tt>) and <tt>AliasDebugger</tt>
535(<tt>-debug-aa</tt>) are implemented as <tt>ModulePass</tt> classes, so if your
536alias analysis uses <tt>FunctionPass</tt>, it won't be able to use
537these utilities. If you try to use them, the pass manager will
538silently route alias analysis queries directly to
539<tt>BasicAliasAnalysis</tt> instead.</p>
540
541<p>Similarly, the <tt>opt -p</tt> option introduces <tt>ModulePass</tt>
542passes between each pass, which prevents the use of <tt>FunctionPass</tt>
543alias analysis passes.</p>
544
545</div>
546
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]547<!-- *********************************************************************** -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]548<div class="doc_section">
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]549 <a name="using">Using alias analysis results</a>
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]550</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]551<!-- *********************************************************************** -->
552
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]553<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]554
555<p>There are several different ways to use alias analysis results. In order of
556preference, these are...</p>
557
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]558</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]559
560<!-- ======================================================================= -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]561<div class="doc_subsection">
Chris Lattner6a6d3cc2009-04-25 21:11:37 +0000[diff] [blame]562 <a name="memdep">Using the <tt>MemoryDependenceAnalysis</tt> Pass</a>
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]563</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]564
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]565<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]566
Chris Lattner6a6d3cc2009-04-25 21:11:37 +0000[diff] [blame]567<p>The <tt>memdep</tt> pass uses alias analysis to provide high-level dependence
568information about memory-using instructions. This will tell you which store
569feeds into a load, for example. It uses caching and other techniques to be
570efficient, and is used by Dead Store Elimination, GVN, and memcpy optimizations.
571</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]572
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]573</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]574
575<!-- ======================================================================= -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]576<div class="doc_subsection">
577 <a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
578</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]579
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]580<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]581
582<p>Many transformations need information about alias <b>sets</b> that are active
583in some scope, rather than information about pairwise aliasing. The <tt><a
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]584href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class
585is used to efficiently build these Alias Sets from the pairwise alias analysis
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]586information provided by the <tt>AliasAnalysis</tt> interface.</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]587
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]588<p>First you initialize the AliasSetTracker by using the "<tt>add</tt>" methods
589to add information about various potentially aliasing instructions in the scope
590you are interested in. Once all of the alias sets are completed, your pass
591should simply iterate through the constructed alias sets, using the
592<tt>AliasSetTracker</tt> <tt>begin()</tt>/<tt>end()</tt> methods.</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]593
594<p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed
Chris Lattnerb8b9163a2003-12-19 08:43:07 +0000[diff] [blame]595to be disjoint, calculate mod/ref information and volatility for the set, and
596keep track of whether or not all of the pointers in the set are Must aliases.
597The AliasSetTracker also makes sure that sets are properly folded due to call
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]598instructions, and can provide a list of pointers in each set.</p>
599
600<p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]601Invariant Code Motion</a> pass uses <tt>AliasSetTracker</tt>s to calculate alias
602sets for each loop nest. If an <tt>AliasSet</tt> in a loop is not modified,
603then all load instructions from that set may be hoisted out of the loop. If any
604alias sets are stored to <b>and</b> are must alias sets, then the stores may be
605sunk to outside of the loop, promoting the memory location to a register for the
606duration of the loop nest. Both of these transformations only apply if the
607pointer argument is loop-invariant.</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]608
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]609</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]610
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]611<!-- _______________________________________________________________________ -->
Chris Lattnerb8b9163a2003-12-19 08:43:07 +0000[diff] [blame]612<div class="doc_subsubsection">
613 The AliasSetTracker implementation
614</div>
615
616<div class="doc_text">
617
618<p>The AliasSetTracker class is implemented to be as efficient as possible. It
619uses the union-find algorithm to efficiently merge AliasSets when a pointer is
620inserted into the AliasSetTracker that aliases multiple sets. The primary data
621structure is a hash table mapping pointers to the AliasSet they are in.</p>
622
623<p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s
624that are in each AliasSet. Since the hash table already has entries for each
625LLVM Value* of interest, the AliasesSets thread the linked list through these
626hash-table nodes to avoid having to allocate memory unnecessarily, and to make
627merging alias sets extremely efficient (the linked list merge is constant time).
628</p>
629
630<p>You shouldn't need to understand these details if you are just a client of
631the AliasSetTracker, but if you look at the code, hopefully this brief
632description will help make sense of why things are designed the way they
633are.</p>
634
635</div>
636
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]637<!-- ======================================================================= -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]638<div class="doc_subsection">
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]639 <a name="direct">Using the <tt>AliasAnalysis</tt> interface directly</a>
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]640</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]641
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]642<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]643
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]644<p>If neither of these utility class are what your pass needs, you should use
645the interfaces exposed by the <tt>AliasAnalysis</tt> class directly. Try to use
646the higher-level methods when possible (e.g., use mod/ref information instead of
647the <a href="#alias"><tt>alias</tt></a> method directly if possible) to get the
648best precision and efficiency.</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]649
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]650</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]651
652<!-- *********************************************************************** -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]653<div class="doc_section">
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]654 <a name="exist">Existing alias analysis implementations and clients</a>
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]655</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]656<!-- *********************************************************************** -->
657
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]658<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]659
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]660<p>If you're going to be working with the LLVM alias analysis infrastructure,
661you should know what clients and implementations of alias analysis are
662available. In particular, if you are implementing an alias analysis, you should
663be aware of the <a href="#aliasanalysis-debug">the clients</a> that are useful
664for monitoring and evaluating different implementations.</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]665
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]666</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]667
668<!-- ======================================================================= -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]669<div class="doc_subsection">
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]670 <a name="impls">Available <tt>AliasAnalysis</tt> implementations</a>
671</div>
672
673<div class="doc_text">
674
675<p>This section lists the various implementations of the <tt>AliasAnalysis</tt>
676interface. With the exception of the <a href="#no-aa"><tt>-no-aa</tt></a> and
677<a href="#basic-aa"><tt>-basicaa</tt></a> implementations, all of these <a
Chris Lattner53e5e2a2004-06-21 22:52:48 +0000[diff] [blame]678href="#chaining">chain</a> to other alias analysis implementations.</p>
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]679
680</div>
681
682<!-- _______________________________________________________________________ -->
683<div class="doc_subsubsection">
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]684 <a name="no-aa">The <tt>-no-aa</tt> pass</a>
685</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]686
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]687<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]688
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]689<p>The <tt>-no-aa</tt> pass is just like what it sounds: an alias analysis that
690never returns any useful information. This pass can be useful if you think that
691alias analysis is doing something wrong and are trying to narrow down a
692problem.</p>
693
694</div>
695
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]696<!-- _______________________________________________________________________ -->
697<div class="doc_subsubsection">
698 <a name="basic-aa">The <tt>-basicaa</tt> pass</a>
699</div>
700
701<div class="doc_text">
702
703<p>The <tt>-basicaa</tt> pass is the default LLVM alias analysis. It is an
704aggressive local analysis that "knows" many important facts:</p>
705
706<ul>
707<li>Distinct globals, stack allocations, and heap allocations can never
708 alias.</li>
709<li>Globals, stack allocations, and heap allocations never alias the null
710 pointer.</li>
711<li>Different fields of a structure do not alias.</li>
712<li>Indexes into arrays with statically differing subscripts cannot alias.</li>
713<li>Many common standard C library functions <a
714 href="#simplemodref">never access memory or only read memory</a>.</li>
715<li>Pointers that obviously point to constant globals
716 "<tt>pointToConstantMemory</tt>".</li>
717<li>Function calls can not modify or references stack allocations if they never
718 escape from the function that allocates them (a common case for automatic
719 arrays).</li>
720</ul>
721
722</div>
723
Chris Lattner9475b962004-06-28 19:19:47 +0000[diff] [blame]724<!-- _______________________________________________________________________ -->
725<div class="doc_subsubsection">
726 <a name="globalsmodref">The <tt>-globalsmodref-aa</tt> pass</a>
727</div>
728
729<div class="doc_text">
730
731<p>This pass implements a simple context-sensitive mod/ref and alias analysis
Chris Lattnerd1384372004-07-27 07:50:07 +0000[diff] [blame]732for internal global variables that don't "have their address taken". If a
733global does not have its address taken, the pass knows that no pointers alias
734the global. This pass also keeps track of functions that it knows never access
Chris Lattner6a6d3cc2009-04-25 21:11:37 +0000[diff] [blame]735memory or never read memory. This allows certain optimizations (e.g. GVN) to
Chris Lattnerd1384372004-07-27 07:50:07 +0000[diff] [blame]736eliminate call instructions entirely.
Chris Lattner9475b962004-06-28 19:19:47 +0000[diff] [blame]737</p>
738
739<p>The real power of this pass is that it provides context-sensitive mod/ref
740information for call instructions. This allows the optimizer to know that
741calls to a function do not clobber or read the value of the global, allowing
742loads and stores to be eliminated.</p>
743
744<p>Note that this pass is somewhat limited in its scope (only support
745non-address taken globals), but is very quick analysis.</p>
746</div>
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]747
748<!-- _______________________________________________________________________ -->
749<div class="doc_subsubsection">
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]750 <a name="steens-aa">The <tt>-steens-aa</tt> pass</a>
751</div>
752
753<div class="doc_text">
754
755<p>The <tt>-steens-aa</tt> pass implements a variation on the well-known
756"Steensgaard's algorithm" for interprocedural alias analysis. Steensgaard's
757algorithm is a unification-based, flow-insensitive, context-insensitive, and
758field-insensitive alias analysis that is also very scalable (effectively linear
759time).</p>
760
761<p>The LLVM <tt>-steens-aa</tt> pass implements a "speculatively
762field-<b>sensitive</b>" version of Steensgaard's algorithm using the Data
763Structure Analysis framework. This gives it substantially more precision than
764the standard algorithm while maintaining excellent analysis scalability.</p>
765
Chris Lattner4b325282007-07-03 04:41:50 +0000[diff] [blame]766<p>Note that <tt>-steens-aa</tt> is available in the optional "poolalloc"
767module, it is not part of the LLVM core.</p>
768
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]769</div>
770
771<!-- _______________________________________________________________________ -->
772<div class="doc_subsubsection">
773 <a name="ds-aa">The <tt>-ds-aa</tt> pass</a>
774</div>
775
776<div class="doc_text">
777
778<p>The <tt>-ds-aa</tt> pass implements the full Data Structure Analysis
779algorithm. Data Structure Analysis is a modular unification-based,
780flow-insensitive, context-<b>sensitive</b>, and speculatively
781field-<b>sensitive</b> alias analysis that is also quite scalable, usually at
782O(n*log(n)).</p>
783
784<p>This algorithm is capable of responding to a full variety of alias analysis
785queries, and can provide context-sensitive mod/ref information as well. The
786only major facility not implemented so far is support for must-alias
787information.</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]788
Chris Lattner4b325282007-07-03 04:41:50 +0000[diff] [blame]789<p>Note that <tt>-ds-aa</tt> is available in the optional "poolalloc"
790module, it is not part of the LLVM core.</p>
791
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]792</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]793
Dan Gohmanb160ba42010-06-28 22:09:52 +0000[diff] [blame]794<!-- _______________________________________________________________________ -->
795<div class="doc_subsubsection">
796 <a name="scev-aa">The <tt>-scev-aa</tt> pass</a>
797</div>
798
799<div class="doc_text">
800
801<p>The <tt>-scev-aa</tt> pass implements AliasAnalysis queries by
802translating them into ScalarEvolution queries. This gives it a
803more complete understanding of <tt>getelementptr</tt> instructions
804and loop induction variables than other alias analyses have.</p>
805
806</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]807
808<!-- ======================================================================= -->
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]809<div class="doc_subsection">
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]810 <a name="aliasanalysis-xforms">Alias analysis driven transformations</a>
811</div>
812
813<div class="doc_text">
814LLVM includes several alias-analysis driven transformations which can be used
815with any of the implementations above.
816</div>
817
818<!-- _______________________________________________________________________ -->
819<div class="doc_subsubsection">
820 <a name="adce">The <tt>-adce</tt> pass</a>
821</div>
822
823<div class="doc_text">
824
825<p>The <tt>-adce</tt> pass, which implements Aggressive Dead Code Elimination
826uses the <tt>AliasAnalysis</tt> interface to delete calls to functions that do
827not have side-effects and are not used.</p>
828
829</div>
830
831
832<!-- _______________________________________________________________________ -->
833<div class="doc_subsubsection">
834 <a name="licm">The <tt>-licm</tt> pass</a>
835</div>
836
837<div class="doc_text">
838
839<p>The <tt>-licm</tt> pass implements various Loop Invariant Code Motion related
840transformations. It uses the <tt>AliasAnalysis</tt> interface for several
841different transformations:</p>
842
843<ul>
844<li>It uses mod/ref information to hoist or sink load instructions out of loops
845if there are no instructions in the loop that modifies the memory loaded.</li>
846
847<li>It uses mod/ref information to hoist function calls out of loops that do not
848write to memory and are loop-invariant.</li>
849
850<li>If uses alias information to promote memory objects that are loaded and
851stored to in loops to live in a register instead. It can do this if there are
852no may aliases to the loaded/stored memory location.</li>
853</ul>
854
855</div>
856
857<!-- _______________________________________________________________________ -->
858<div class="doc_subsubsection">
859 <a name="argpromotion">The <tt>-argpromotion</tt> pass</a>
860</div>
861
862<div class="doc_text">
863<p>
864The <tt>-argpromotion</tt> pass promotes by-reference arguments to be passed in
865by-value instead. In particular, if pointer arguments are only loaded from it
866passes in the value loaded instead of the address to the function. This pass
867uses alias information to make sure that the value loaded from the argument
868pointer is not modified between the entry of the function and any load of the
869pointer.</p>
870</div>
871
872<!-- _______________________________________________________________________ -->
873<div class="doc_subsubsection">
Chris Lattner6a6d3cc2009-04-25 21:11:37 +0000[diff] [blame]874 <a name="gvn">The <tt>-gvn</tt>, <tt>-memcpyopt</tt>, and <tt>-dse</tt>
875 passes</a>
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]876</div>
877
878<div class="doc_text">
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]879
Chris Lattner6a6d3cc2009-04-25 21:11:37 +0000[diff] [blame]880<p>These passes use AliasAnalysis information to reason about loads and stores.
881</p>
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]882
883</div>
884
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]885<!-- ======================================================================= -->
886<div class="doc_subsection">
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]887 <a name="aliasanalysis-debug">Clients for debugging and evaluation of
888 implementations</a>
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]889</div>
890
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]891<div class="doc_text">
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]892
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]893<p>These passes are useful for evaluating the various alias analysis
Chris Lattner5ea3e652010-03-01 19:24:17 +0000[diff] [blame]894implementations. You can use them with commands like '<tt>opt -ds-aa
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]895-aa-eval foo.bc -disable-output -stats</tt>'.</p>
896
897</div>
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]898
899<!-- _______________________________________________________________________ -->
900<div class="doc_subsubsection">
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]901 <a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
902</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]903
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]904<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]905
906<p>The <tt>-print-alias-sets</tt> pass is exposed as part of the
Chris Lattner2e6f4482006-01-03 06:04:48 +0000[diff] [blame]907<tt>opt</tt> tool to print out the Alias Sets formed by the <a
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]908href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
Chris Lattner2e6f4482006-01-03 06:04:48 +0000[diff] [blame]909the <tt>AliasSetTracker</tt> class. To use it, use something like:</p>
910
911<div class="doc_code">
912<pre>
913% opt -ds-aa -print-alias-sets -disable-output
914</pre>
915</div>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]916
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]917</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]918
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]919
920<!-- _______________________________________________________________________ -->
921<div class="doc_subsubsection">
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]922 <a name="count-aa">The <tt>-count-aa</tt> pass</a>
923</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]924
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]925<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]926
927<p>The <tt>-count-aa</tt> pass is useful to see how many queries a particular
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]928pass is making and what responses are returned by the alias analysis. As an
929example,</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]930
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]931<div class="doc_code">
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]932<pre>
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]933% opt -basicaa -count-aa -ds-aa -count-aa -licm
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]934</pre>
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]935</div>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]936
Misha Brukman2361fcf2004-07-28 22:18:33 +0000[diff] [blame]937<p>will print out how many queries (and what responses are returned) by the
938<tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are made
939of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be useful
940when debugging a transformation or an alias analysis implementation.</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]941
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]942</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]943
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]944<!-- _______________________________________________________________________ -->
945<div class="doc_subsubsection">
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]946 <a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
947</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]948
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]949<div class="doc_text">
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]950
951<p>The <tt>-aa-eval</tt> pass simply iterates through all pairs of pointers in a
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]952function and asks an alias analysis whether or not the pointers alias. This
953gives an indication of the precision of the alias analysis. Statistics are
Chris Lattner2a19d832004-05-23 21:04:01 +0000[diff] [blame]954printed indicating the percent of no/may/must aliases found (a more precise
955algorithm will have a lower number of may aliases).</p>
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]956
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]957</div>
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]958
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]959<!-- *********************************************************************** -->
Owen Andersond263cfa2007-10-02 00:43:25 +0000[diff] [blame]960<div class="doc_section">
961 <a name="memdep">Memory Dependence Analysis</a>
962</div>
963<!-- *********************************************************************** -->
964
965<div class="doc_text">
966
967<p>If you're just looking to be a client of alias analysis information, consider
968using the Memory Dependence Analysis interface instead. MemDep is a lazy,
969caching layer on top of alias analysis that is able to answer the question of
970what preceding memory operations a given instruction depends on, either at an
971intra- or inter-block level. Because of its laziness and caching
972policy, using MemDep can be a significant performance win over accessing alias
973analysis directly.</p>
974
975</div>
976
977<!-- *********************************************************************** -->
Chris Lattnerb512e9f2003-03-04 19:37:49 +0000[diff] [blame]978
Misha Brukman084a3a072003-10-24 18:06:11 +0000[diff] [blame]979<hr>
Misha Brukmane8e932a2003-11-22 01:26:21 +0000[diff] [blame]980<address>
Misha Brukman8834af92003-12-21 22:53:21 +0000[diff] [blame]981 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman86242e12008-12-11 17:34:48 +0000[diff] [blame]982 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Misha Brukmane8e932a2003-11-22 01:26:21 +0000[diff] [blame]983 <a href="http://validator.w3.org/check/referer"><img
Misha Brukman86242e12008-12-11 17:34:48 +0000[diff] [blame]984 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
Misha Brukmane8e932a2003-11-22 01:26:21 +0000[diff] [blame]985
986 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Reid Spencerca058542006-03-14 05:39:39 +0000[diff] [blame]987 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukmane8e932a2003-11-22 01:26:21 +0000[diff] [blame]988 Last modified: $Date$
989</address>
Misha Brukman283ff692003-10-22 23:27:16 +0000[diff] [blame]990
991</body>
992</html>