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4<head>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]5 <title>Alias Analysis Infrastructure in LLVM</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">
11 Alias Analysis Infrastructure in LLVM
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
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]17 <li><a href="#overview">AliasAnalysis 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>
20 <li><a href="#MustMayNo">Must, May, and No Alias Responses</a></li>
21 <li><a href="#ModRefInfo">The <tt>getModRefInfo</tt> methods</a></li>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]22 </ul></li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]23
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]24 <li><a href="#writingnew">Writing a new AliasAnalysis Implementation</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]25 <ul>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]26 <li><a href="#passsubclasses">Different Pass styles</a></li>
27 <li><a href="#requiredcalls">Required initialization calls</a></li>
28 <li><a href="#interfaces">Interfaces which may be specified</a></li>
29 <li><a href="#chaining">The AliasAnalysis chaining behavior</a></li>
30 <li><a href="#implefficiency">Efficiency Issues</a></li>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]31 </ul></li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]32
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]33 <li><a href="#using">Using AliasAnalysis results</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]34 <ul>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]35 <li><a href="#loadvn">Using the <tt>-load-vn</tt> Pass</a></li>
36 <li><a href="#ast">Using the <tt>AliasSetTracker</tt> class</a></li>
37 <li><a href="#direct">Using the AliasAnalysis interface directly</a></li>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]38 </ul></li>
39
40 <li><a href="#tools">Helpful alias analysis related tools</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]41 <ul>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]42 <li><a href="#no-aa">The <tt>-no-aa</tt> pass</a></li>
43 <li><a href="#print-alias-sets">The <tt>-print-alias-sets</tt> pass</a></li>
44 <li><a href="#count-aa">The <tt>-count-aa</tt> pass</a></li>
45 <li><a href="#aa-eval">The <tt>-aa-eval</tt> pass</a></li>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]46 </ul></li>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]47</ol>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]48
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]49<div class="doc_text">
50 <p><b>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></b></p>
51</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]52
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]53<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]54<div class="doc_section">
55 <a name="introduction">Introduction</a>
56</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]57<!-- *********************************************************************** -->
58
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]59<div class="doc_text">
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]60
61<p>Alias Analysis (or Pointer Analysis) is a technique which attempts to
62determine whether or not two pointers ever can point to the same object in
63memory. Traditionally, Alias Analyses respond to a query with either a <a
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]64href="#MustNoMay">Must, May, or No</a> alias response, indicating that two
65pointers do point to the same object, might point to the same object, or are
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]66known not to point to the same object.</p>
67
Misha Brukmane12e5f32003-12-17 23:10:49 +0000[diff] [blame]68<p>The <a href="/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a> class is the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]69centerpiece of the LLVM Alias Analysis related infrastructure. This class is
70the common interface between clients of alias analysis information and the
71implementations providing it. In addition to simple alias analysis information,
72this class exposes Mod/Ref information from those implementations which can
73provide it, allowing for powerful analyses and transformations to work well
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]74together.</p>
75
76<p>This document contains information necessary to successfully implement this
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]77interface, use it, and to test both sides. It also explains some of the finer
78points about what exactly results mean. If you feel that something is unclear
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]79or should be added, please <a href="mailto:sabre@nondot.org">let me
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]80know</a>.</p>
81
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]82</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]83
84<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]85<div class="doc_section">
86 <a name="overview">AliasAnalysis Overview</a>
87</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]88<!-- *********************************************************************** -->
89
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]90<div class="doc_text">
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]91
Misha Brukmane12e5f32003-12-17 23:10:49 +0000[diff] [blame]92<p>The <a href="/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a> class
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]93defines the interface that Alias Analysis implementations should support. This
94class exports two important enums: <tt>AliasResult</tt> and
95<tt>ModRefResult</tt> which represent the result of an alias query or a mod/ref
96query, respectively.</p>
97
98<p>The AliasAnalysis interface exposes information about memory, represented in
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]99several different ways. In particular, memory objects are represented as a
100starting address and size, and function calls are represented as the actual
101<tt>call</tt> or <tt>invoke</tt> instructions that performs the call. The
102AliasAnalysis interface also exposes some helper methods which allow you to get
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]103mod/ref information for arbitrary instructions.</p>
104
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]105</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]106
107<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]108<div class="doc_subsection">
109 <a name="pointers">Representation of Pointers</a>
110</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]111
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]112<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]113
114<p>Most importantly, the AliasAnalysis class provides several methods which are
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]115used to query whether or not pointers alias, whether function calls can modify
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]116or read memory, etc.</p>
117
118<p>Representing memory objects as a starting address and a size is critically
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]119important for precise Alias Analyses. For example, consider this (silly) C
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]120code:</p>
121
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]122<pre>
123 int i;
124 char C[2];
125 char A[10];
126 /* ... */
127 for (i = 0; i != 10; ++i) {
128 C[0] = A[i]; /* One byte store */
129 C[1] = A[9-i]; /* One byte store */
130 }
131</pre>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]132
133<p>In this case, the <tt>basicaa</tt> pass will disambiguate the stores to
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]134<tt>C[0]</tt> and <tt>C[1]</tt> because they are accesses to two distinct
135locations one byte apart, and the accesses are each one byte. In this case, the
136LICM pass can use store motion to remove the stores from the loop. In
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]137constrast, the following code:</p>
138
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]139<pre>
140 int i;
141 char C[2];
142 char A[10];
143 /* ... */
144 for (i = 0; i != 10; ++i) {
145 ((short*)C)[0] = A[i]; /* Two byte store! */
146 C[1] = A[9-i]; /* One byte store */
147 }
148</pre>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]149
150<p>In this case, the two stores to C do alias each other, because the access to
151the <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]152available in the query, even the first case would have to conservatively assume
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]153that the accesses alias.</p>
154
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]155</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]156
157<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]158<div class="doc_subsection">
159 <a name="MustMayNo">Must, May, and No Alias Responses</a>
160</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]161
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]162<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]163
164<p>An Alias Analysis implementation can return one of three responses:
165MustAlias, MayAlias, and NoAlias. The No and May alias results are obvious: if
166the two pointers may never equal each other, return NoAlias, if they might,
167return MayAlias.</p>
168
169<p>The Must Alias response is trickier though. In LLVM, the Must Alias response
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]170may only be returned if the two memory objects are guaranteed to always start at
171exactly the same location. If two memory objects overlap, but do not start at
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]172the same location, MayAlias must be returned.</p>
173
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]174</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]175
176<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]177<div class="doc_subsection">
178 <a name="ModRefInfo">The <tt>getModRefInfo</tt> methods</a>
179</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]180
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]181<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]182
183<p>The <tt>getModRefInfo</tt> methods return information about whether the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]184execution of an instruction can read or modify a memory location. Mod/Ref
185information is always conservative: if an action <b>may</b> read a location, Ref
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]186is returned.</p>
187
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]188</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]189
190<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]191<div class="doc_section">
192 <a name="writingnew">Writing a new AliasAnalysis Implementation</a>
193</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]194<!-- *********************************************************************** -->
195
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]196<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]197
198<p>Writing a new alias analysis implementation for LLVM is quite
199straight-forward. There are already several implementations that you can use
200for examples, and the following information should help fill in any details.
201For a minimal example, take a look at the <a
Misha Brukman3e7ada62003-12-17 18:46:49 +0000[diff] [blame]202href="/doxygen/structllvm_1_1NoAA.html"><tt>no-aa</tt></a> implementation.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]203
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]204</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]205
206<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]207<div class="doc_subsection">
208 <a name="passsubclasses">Different Pass styles</a>
209</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]210
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]211<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]212
213<p>The first step to determining what type of <a
214href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias
215Analysis. As is the case with most other analyses and transformations, the
216answer should be fairly obvious from what type of problem you are trying to
217solve:</p>
218
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]219<ol>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]220 <li>If you require interprocedural analysis, it should be a
221 <tt>Pass</tt>.</li>
222 <li>If you are a global analysis, subclass <tt>FunctionPass</tt>.</li>
223 <li>If you are a local pass, subclass <tt>BasicBlockPass</tt>.</li>
224 <li>If you don't need to look at the program at all, subclass
225 <tt>ImmutablePass</tt>.</li>
226</ol>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]227
228<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]229<tt>AliasAnalysis</tt> interface, of course, and use the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]230<tt>RegisterAnalysisGroup</tt> template to register as an implementation of
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]231<tt>AliasAnalysis</tt>.</p>
232
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]233</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]234
235<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]236<div class="doc_subsection">
237 <a name="requiredcalls">Required initialization calls</a>
238</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]239
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]240<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]241
242<p>Your subclass of AliasAnalysis is required to invoke two methods on the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]243AliasAnalysis base class: <tt>getAnalysisUsage</tt> and
244<tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
245<tt>getAnalysisUsage</tt> should explicitly call into the
246<tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
247declaring any pass dependencies your pass has. Thus you should have something
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]248like this:</p>
249
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]250<pre>
251 void getAnalysisUsage(AnalysisUsage &amp;AU) const {
252 AliasAnalysis::getAnalysisUsage(AU);
253 <i>// declare your dependencies here.</i>
254 }
255</pre>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]256
257<p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
258from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]259<tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, <tt>runOnBasicBlock</tt> for
260a <tt>BasicBlockPass</tt>, or <tt>InitializeAliasAnalysis</tt> for an
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]261<tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
262
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]263<pre>
264 bool run(Module &amp;M) {
265 InitializeAliasAnalysis(this);
266 <i>// Perform analysis here...</i>
267 return false;
268 }
269</pre>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]270
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]271</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]272
273<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]274<div class="doc_subsection">
275 <a name="interfaces">Interfaces which may be specified</a>
276</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]277
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]278<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]279
Misha Brukmane12e5f32003-12-17 23:10:49 +0000[diff] [blame]280<p>All of the <a href="/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]281virtual methods default to providing conservatively correct information
282(returning "May" Alias and "Mod/Ref" for alias and mod/ref queries
283respectively). Depending on the capabilities of the analysis you are
284implementing, you just override the interfaces you can improve.</p>
285
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]286</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]287
288<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]289<div class="doc_subsection">
290 <a name="chaining">The AliasAnalysis chaining behavior</a>
291</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]292
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]293<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]294
295<p>With only two special exceptions (the <tt>basicaa</tt> and <a
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]296href="#no-aa"><tt>no-aa</tt></a> passes) every alias analysis pass should chain
297to another alias analysis implementation (for example, you could specify
298"<tt>-basic-aa -ds-aa -andersens-aa -licm</tt>" to get the maximum benefit from
299the three alias analyses). To do this, simply "Require" AliasAnalysis in your
300<tt>getAnalysisUsage</tt> method, and if you need to return a conservative
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]301MayAlias or Mod/Ref result, simply chain to a lower analysis.</p>
302
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]303</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]304
305<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]306<div class="doc_subsection">
307 <a name="implefficiency">Efficiency Issues</a>
308</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]309
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]310<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]311
312<p>From the LLVM perspective, the only thing you need to do to provide an
313efficient alias analysis is to make sure that alias analysis <b>queries</b> are
314serviced quickly. The actual calculation of the alias analysis results (the
315"run" method) is only performed once, but many (perhaps duplicate) queries may
316be performed. Because of this, try to move as much computation to the run
317method as possible (within reason).</p>
318
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]319</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]320
321<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]322<div class="doc_section">
323 <a name="using">Using AliasAnalysis results</a>
324</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]325<!-- *********************************************************************** -->
326
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]327<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]328
329<p>There are several different ways to use alias analysis results. In order of
330preference, these are...</p>
331
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]332</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]333
334<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]335<div class="doc_subsection">
336 <a name="loadvn">Using the <tt>-load-vn</tt> Pass</a>
337</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]338
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]339<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]340
341<p>The <tt>load-vn</tt> pass uses alias analysis to provide value numbering
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]342information for <tt>load</tt> instructions. If your analysis or transformation
343can be modelled in a form that uses value numbering information, you don't have
344to do anything special to handle load instructions: just use the
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]345<tt>load-vn</tt> pass, which uses alias analysis.</p>
346
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]347</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]348
349<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]350<div class="doc_subsection">
351 <a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
352</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]353
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]354<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]355
356<p>Many transformations need information about alias <b>sets</b> that are active
357in some scope, rather than information about pairwise aliasing. The <tt><a
Misha Brukmane12e5f32003-12-17 23:10:49 +0000[diff] [blame]358href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class is used
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]359to efficiently build these Alias Sets from the pairwise alias analysis
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]360information provided by the AliasAnalysis interface.</p>
361
362<p>First you initialize the AliasSetTracker by use the "<tt>add</tt>" methods to
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]363add information about various potentially aliasing instructions in the scope you
364are interested in. Once all of the alias sets are completed, your pass should
365simply iterate through the constructed alias sets, using the AliasSetTracker
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]366<tt>begin()</tt>/<tt>end()</tt> methods.</p>
367
368<p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]369to be disjoint, calculate mod/ref information and volatility for the set, and
370keep track of whether or not all of the pointers in the set are Must aliases.
371The AliasSetTracker also makes sure that sets are properly folded due to call
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]372instructions, and can provide a list of pointers in each set.</p>
373
374<p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]375Invariant Code Motion</a> pass uses AliasSetTrackers to build alias information
376about each loop nest. If an AliasSet in a loop is not modified, then all load
377instructions from that set may be hoisted out of the loop. If any alias sets
378are stored <b>and</b> are must alias sets, then the stores may be sunk to
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]379outside of the loop, promoting the memory location to a register for the
380duration of the loop nest. Both of these transformations obviously only apply
381if the pointer argument is loop-invariant.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]382
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]383</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]384
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]385<div class="doc_subsubsection">
386 The AliasSetTracker implementation
387</div>
388
389<div class="doc_text">
390
391<p>The AliasSetTracker class is implemented to be as efficient as possible. It
392uses the union-find algorithm to efficiently merge AliasSets when a pointer is
393inserted into the AliasSetTracker that aliases multiple sets. The primary data
394structure is a hash table mapping pointers to the AliasSet they are in.</p>
395
396<p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s
397that are in each AliasSet. Since the hash table already has entries for each
398LLVM Value* of interest, the AliasesSets thread the linked list through these
399hash-table nodes to avoid having to allocate memory unnecessarily, and to make
400merging alias sets extremely efficient (the linked list merge is constant time).
401</p>
402
403<p>You shouldn't need to understand these details if you are just a client of
404the AliasSetTracker, but if you look at the code, hopefully this brief
405description will help make sense of why things are designed the way they
406are.</p>
407
408</div>
409
410
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]411<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]412<div class="doc_subsection">
413 <a name="direct">Using the AliasAnalysis interface directly</a>
414</div>
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_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]417
418<p>As a last resort, your pass could use the AliasAnalysis interface directly to
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]419service your pass. If you find the need to do this, please <a
420href="mailto:sabre@nondot.org">let me know</a> so I can see if something new
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]421needs to be added to LLVM.</p>
422
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]423</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]424
425<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]426<div class="doc_section">
427 <a name="tools">Helpful alias-analysis-related tools</a>
428</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]429<!-- *********************************************************************** -->
430
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]431<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]432
433<p>If you're going to be working with the AliasAnalysis infrastructure, there
434are several nice tools that may be useful for you and are worth knowing
435about...</p>
436
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]437</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]438
439<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]440<div class="doc_subsection">
441 <a name="no-aa">The <tt>-no-aa</tt> pass</a>
442</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]443
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]444<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]445
446<p>The <tt>-no-aa</tt> analysis is just like what it sounds: an alias analysis
447that never returns any useful information. This pass can be useful if you think
448that alias analysis is doing something wrong and are trying to narrow down a
449problem. If you don't specify an alias analysis, the default will be to use the
450<tt>basicaa</tt> pass which does quite a bit of disambiguation on its own.</p>
451
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]452</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]453
454
455<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]456<div class="doc_subsection">
457 <a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
458</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]459
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]460<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]461
462<p>The <tt>-print-alias-sets</tt> pass is exposed as part of the
463<tt>analyze</tt> tool to print out the Alias Sets formed by the <a
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]464href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]465the <tt>AliasSetTracker</tt>.</p>
466
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]467</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]468
469<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]470<div class="doc_subsection">
471 <a name="count-aa">The <tt>-count-aa</tt> pass</a>
472</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]473
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]474<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]475
476<p>The <tt>-count-aa</tt> pass is useful to see how many queries a particular
477pass is making and what kinds of responses are returned by the alias analysis.
478An example usage is:</p>
479
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]480<pre>
481 $ opt -basicaa -count-aa -ds-aa -count-aa -licm
482</pre>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]483
484<p>Which will print out how many queries (and what responses are returned) by
485the <tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are
486made of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be
487useful when evaluating an alias analysis for precision.</p>
488
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]489</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]490
491<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]492<div class="doc_subsection">
493 <a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
494</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]495
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]496<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]497
498<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]499function and asks an alias analysis whether or not the pointers alias. This
500gives an indication of the precision of the alias analysis. Statistics are
Misha Brukman5feba4e2003-11-22 01:07:30 +0000[diff] [blame]501printed.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]502
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]503</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]504
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]505<!-- *********************************************************************** -->
506
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]507<hr>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]508<address>
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513
514 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
515 <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a><br>
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