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gerrit-public.fairphone.software / fp2-dev / platform / external / llvm / 75f258ebffac583b61e245b34d6d371659f9f284 / . / docs / AliasAnalysis.html
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Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]2<html>
3<head>
4 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
5 <link rel="stylesheet" href="llvm.css" type="text/css" media="screen" />
6 <title>Alias Analysis Infrastructure in LLVM</title>
7</head>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]8
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]9<body>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]10
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]11<div class="doc_title">
12 Alias Analysis Infrastructure in LLVM
13</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]14
15<ol>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]16 <li><a href="#introduction">Introduction</a></li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]17
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]18 <li><a href="#overview">AliasAnalysis Overview</a></li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]19 <ul>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]20 <li><a href="#pointers">Representation of Pointers</a></li>
21 <li><a href="#MustMayNo">Must, May, and No Alias Responses</a></li>
22 <li><a href="#ModRefInfo">The <tt>getModRefInfo</tt> methods</a></li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]23 </ul>
24
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]25 <li><a href="#writingnew">Writing a new AliasAnalysis Implementation</a></li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]26 <ul>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]27 <li><a href="#passsubclasses">Different Pass styles</a></li>
28 <li><a href="#requiredcalls">Required initialization calls</a></li>
29 <li><a href="#interfaces">Interfaces which may be specified</a></li>
30 <li><a href="#chaining">The AliasAnalysis chaining behavior</a></li>
31 <li><a href="#implefficiency">Efficiency Issues</a></li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]32 </ul>
33
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]34 <li><a href="#using">Using AliasAnalysis results</a></li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]35 <ul>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]36 <li><a href="#loadvn">Using the <tt>-load-vn</tt> Pass</a></li>
37 <li><a href="#ast">Using the <tt>AliasSetTracker</tt> class</a></li>
38 <li><a href="#direct">Using the AliasAnalysis interface directly</a></li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]39 </ul>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]40 <li><a href="#tools">Helpful alias analysis related tools</a></li>
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>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]46 </ul>
47 </ul>
48
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]49 <p><b>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></b></p>
50</ol>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]51
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">
60<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]61Alias Analysis (or Pointer Analysis) is a technique which attempts to determine
62whether or not two pointers ever can point to the same object in memory.
63Traditionally, Alias Analyses respond to a query with either a <a
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 Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]66known not to point to the same object.
67</p>
68<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]69The <a href="/doxygen/classAliasAnalysis.html">AliasAnalysis</a> class is the
70centerpiece of the LLVM Alias Analysis related infrastructure. This class is
71the common interface between clients of alias analysis information and the
72implementations providing it. In addition to simple alias analysis information,
73this class exposes Mod/Ref information from those implementations which can
74provide it, allowing for powerful analyses and transformations to work well
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]75together.
76</p>
77<p>
Misha Brukman5560c9d2003-08-18 14:43:39 +0000[diff] [blame]78This document contains information necessary to successfully implement this
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]79interface, use it, and to test both sides. It also explains some of the finer
80points about what exactly results mean. If you feel that something is unclear
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]81or should be added, please <a href="mailto:sabre@nondot.org">let me
82know</a>.
83</p>
84</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]85
86<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]87<div class="doc_section">
88 <a name="overview">AliasAnalysis Overview</a>
89</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]90<!-- *********************************************************************** -->
91
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]92<div class="doc_text">
93<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]94The <a href="/doxygen/classAliasAnalysis.html">AliasAnalysis</a> class defines
95the interface that Alias Analysis implementations should support. This class
96exports two important enums: <tt>AliasResult</tt> and <tt>ModRefResult</tt>
97which represent the result of an alias query or a mod/ref query,
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]98respectively.
99</p>
100<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]101The AliasAnalysis interface exposes information about memory, represented in
102several different ways. In particular, memory objects are represented as a
103starting address and size, and function calls are represented as the actual
104<tt>call</tt> or <tt>invoke</tt> instructions that performs the call. The
105AliasAnalysis interface also exposes some helper methods which allow you to get
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]106mod/ref information for arbitrary instructions.
107</p>
108</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]109
110<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]111<div class="doc_subsection">
112 <a name="pointers">Representation of Pointers</a>
113</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]114
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]115<div class="doc_text">
116<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]117Most importantly, the AliasAnalysis class provides several methods which are
118used to query whether or not pointers alias, whether function calls can modify
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]119or read memory, etc.
120</p>
121<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]122Representing memory objects as a starting address and a size is critically
123important for precise Alias Analyses. For example, consider this (silly) C
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]124code:
125</p>
126<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]127<pre>
128 int i;
129 char C[2];
130 char A[10];
131 /* ... */
132 for (i = 0; i != 10; ++i) {
133 C[0] = A[i]; /* One byte store */
134 C[1] = A[9-i]; /* One byte store */
135 }
136</pre>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]137</p>
138<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]139In this case, the <tt>basicaa</tt> pass will disambiguate the stores to
140<tt>C[0]</tt> and <tt>C[1]</tt> because they are accesses to two distinct
141locations one byte apart, and the accesses are each one byte. In this case, the
142LICM pass can use store motion to remove the stores from the loop. In
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]143constrast, the following code:
144</p>
145<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]146<pre>
147 int i;
148 char C[2];
149 char A[10];
150 /* ... */
151 for (i = 0; i != 10; ++i) {
152 ((short*)C)[0] = A[i]; /* Two byte store! */
153 C[1] = A[9-i]; /* One byte store */
154 }
155</pre>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]156</p>
157<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]158In this case, the two stores to C do alias each other, because the access to the
159<tt>&amp;C[0]</tt> element is a two byte access. If size information wasn't
160available in the query, even the first case would have to conservatively assume
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]161that the accesses alias.
162</p>
163</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]164
165<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]166<div class="doc_subsection">
167 <a name="MustMayNo">Must, May, and No Alias Responses</a>
168</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]169
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]170<div class="doc_text">
171<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]172An Alias Analysis implementation can return one of three responses: MustAlias,
173MayAlias, and NoAlias. The No and May alias results are obvious: if the two
174pointers may never equal each other, return NoAlias, if they might, return
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]175MayAlias.
176</p>
177<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]178The Must Alias response is trickier though. In LLVM, the Must Alias response
179may only be returned if the two memory objects are guaranteed to always start at
180exactly the same location. If two memory objects overlap, but do not start at
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]181the same location, MayAlias must be returned.
182</p>
183</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]184
185<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]186<div class="doc_subsection">
187 <a name="ModRefInfo">The <tt>getModRefInfo</tt> methods</a>
188</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]189
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]190<div class="doc_text">
191<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]192The <tt>getModRefInfo</tt> methods return information about whether the
193execution of an instruction can read or modify a memory location. Mod/Ref
194information is always conservative: if an action <b>may</b> read a location, Ref
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]195is returned.
196</p>
197</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]198
199<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]200<div class="doc_section">
201 <a name="writingnew">Writing a new AliasAnalysis Implementation</a>
202</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]203<!-- *********************************************************************** -->
204
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]205<div class="doc_text">
206<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]207Writing a new alias analysis implementation for LLVM is quite straight-forward.
208There are already several implementations that you can use for examples, and the
209following information should help fill in any details. For a minimal example,
210take a look at the <a href="/doxygen/structNoAA.html"><tt>no-aa</tt></a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]211implementation.
212</p>
213</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]214
215<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]216<div class="doc_subsection">
217 <a name="passsubclasses">Different Pass styles</a>
218</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]219
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]220<div class="doc_text">
221<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]222The first step to determining what type of <a href="WritingAnLLVMPass.html">LLVM
223pass</a> you need to use for your Alias Analysis. As is the case with most
224other analyses and transformations, the answer should be fairly obvious from
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]225what type of problem you are trying to solve:
226</p>
227<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]228<ol>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]229 <li>If you require interprocedural analysis, it should be a
230 <tt>Pass</tt>.</li>
231 <li>If you are a global analysis, subclass <tt>FunctionPass</tt>.</li>
232 <li>If you are a local pass, subclass <tt>BasicBlockPass</tt>.</li>
233 <li>If you don't need to look at the program at all, subclass
234 <tt>ImmutablePass</tt>.</li>
235</ol>
236</p>
237<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]238In addition to the pass that you subclass, you should also inherit from the
Misha Brukman700fd492003-05-07 21:47:16 +0000[diff] [blame]239<tt>AliasAnalysis</tt> interface, of course, and use the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]240<tt>RegisterAnalysisGroup</tt> template to register as an implementation of
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]241<tt>AliasAnalysis</tt>.
242</p>
243</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]244
245<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]246<div class="doc_subsection">
247 <a name="requiredcalls">Required initialization calls</a>
248</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]249
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]250<div class="doc_text">
251<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]252Your subclass of AliasAnalysis is required to invoke two methods on the
253AliasAnalysis base class: <tt>getAnalysisUsage</tt> and
254<tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
255<tt>getAnalysisUsage</tt> should explicitly call into the
256<tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
257declaring any pass dependencies your pass has. Thus you should have something
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]258like this:
259</p>
260<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]261<pre>
262 void getAnalysisUsage(AnalysisUsage &amp;AU) const {
263 AliasAnalysis::getAnalysisUsage(AU);
264 <i>// declare your dependencies here.</i>
265 }
266</pre>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]267</p>
268<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]269Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method from
270your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
271<tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, <tt>runOnBasicBlock</tt> for
272a <tt>BasicBlockPass</tt>, or <tt>InitializeAliasAnalysis</tt> for an
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]273<tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):
274</p>
275<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]276<pre>
277 bool run(Module &amp;M) {
278 InitializeAliasAnalysis(this);
279 <i>// Perform analysis here...</i>
280 return false;
281 }
282</pre>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]283</p>
284</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]285
286<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]287<div class="doc_subsection">
288 <a name="interfaces">Interfaces which may be specified</a>
289</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]290
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]291<div class="doc_text">
292<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]293All of the <a href="/doxygen/classAliasAnalysis.html">AliasAnalysis</a> virtual
294methods default to providing conservatively correct information (returning "May"
295Alias and "Mod/Ref" for alias and mod/ref queries respectively). Depending on
296the capabilities of the analysis you are implementing, you just override the
297interfaces you can improve.
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]298</p>
299</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]300
301<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]302<div class="doc_subsection">
303 <a name="chaining">The AliasAnalysis chaining behavior</a>
304</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]305
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]306<div class="doc_text">
307<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]308With only two special exceptions (the <tt>basicaa</tt> and <a
309href="#no-aa"><tt>no-aa</tt></a> passes) every alias analysis pass should chain
310to another alias analysis implementation (for example, you could specify
311"<tt>-basic-aa -ds-aa -andersens-aa -licm</tt>" to get the maximum benefit from
312the three alias analyses). To do this, simply "Require" AliasAnalysis in your
313<tt>getAnalysisUsage</tt> method, and if you need to return a conservative
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]314MayAlias or Mod/Ref result, simply chain to a lower analysis.
315</p>
316</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]317
318<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]319<div class="doc_subsection">
320 <a name="implefficiency">Efficiency Issues</a>
321</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]322
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]323<div class="doc_text">
324<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]325From the LLVM perspective, the only thing you need to do to provide an efficient
326alias analysis is to make sure that alias analysis <b>queries</b> are serviced
327quickly. The actual calculation of the alias analysis results (the "run"
328method) is only performed once, but many (perhaps duplicate) queries may be
329performed. Because of this, try to move as much computation to the run method
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]330as possible (within reason).
331</p>
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_section">
336 <a name="using">Using AliasAnalysis results</a>
337</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]338<!-- *********************************************************************** -->
339
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]340<div class="doc_text">
341<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]342There are several different ways to use alias analysis results. In order of
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]343preference, these are...
344</p>
345</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]346
347<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]348<div class="doc_subsection">
349 <a name="loadvn">Using the <tt>-load-vn</tt> Pass</a>
350</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]351
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]352<div class="doc_text">
353<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]354The <tt>load-vn</tt> pass uses alias analysis to provide value numbering
355information for <tt>load</tt> instructions. If your analysis or transformation
356can be modelled in a form that uses value numbering information, you don't have
357to do anything special to handle load instructions: just use the
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]358<tt>load-vn</tt> pass, which uses alias analysis.
359</p>
360</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]361
362<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]363<div class="doc_subsection">
364 <a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
365</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]366
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]367<div class="doc_text">
368<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]369Many transformations need information about alias <b>sets</b> that are active in
370some scope, rather than information about pairwise aliasing. The <tt><a
371href="/doxygen/classAliasSetTracker.html">AliasSetTracker</a></tt> class is used
372to efficiently build these Alias Sets from the pairwise alias analysis
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]373information provided by the AliasAnalysis interface.
374</p>
375<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]376First you initialize the AliasSetTracker by use the "<tt>add</tt>" methods to
377add information about various potentially aliasing instructions in the scope you
378are interested in. Once all of the alias sets are completed, your pass should
379simply iterate through the constructed alias sets, using the AliasSetTracker
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]380<tt>begin()</tt>/<tt>end()</tt> methods.
381</p>
382<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]383The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed to
384be disjoint, calculate mod/ref information for the set, and keep track of
385whether or not all of the pointers in the set are Must aliases. The
386AliasSetTracker also makes sure that sets are properly folded due to call
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]387instructions, and can provide a list of pointers in each set.
388</p>
389<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]390As an example user of this, the <a href="/doxygen/structLICM.html">Loop
391Invariant Code Motion</a> pass uses AliasSetTrackers to build alias information
392about each loop nest. If an AliasSet in a loop is not modified, then all load
393instructions from that set may be hoisted out of the loop. If any alias sets
394are stored <b>and</b> are must alias sets, then the stores may be sunk to
395outside of the loop. Both of these transformations obviously only apply if the
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]396pointer argument is loop-invariant.
397</p>
398</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]399
400<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]401<div class="doc_subsection">
402 <a name="direct">Using the AliasAnalysis interface directly</a>
403</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]404
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]405<div class="doc_text">
406<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]407As a last resort, your pass could use the AliasAnalysis interface directly to
408service your pass. If you find the need to do this, please <a
409href="mailto:sabre@nondot.org">let me know</a> so I can see if something new
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]410needs to be added to LLVM.
411</p>
412</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]413
414<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]415<div class="doc_section">
416 <a name="tools">Helpful alias-analysis-related tools</a>
417</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]418<!-- *********************************************************************** -->
419
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]420<div class="doc_text">
421<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]422If you're going to be working with the AliasAnalysis infrastructure, there are
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]423several nice tools that may be useful for you and are worth knowing about...
424</p>
425</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]426
427<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]428<div class="doc_subsection">
429 <a name="no-aa">The <tt>-no-aa</tt> pass</a>
430</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]431
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]432<div class="doc_text">
433<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]434The <tt>-no-aa</tt> analysis is just like what it sounds: an alias analysis that
435never returns any useful information. This pass can be useful if you think that
436alias analysis is doing something wrong and are trying to narrow down a problem.
437If you don't specify an alias analysis, the default will be to use the
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]438<tt>basicaa</tt> pass which does quite a bit of disambiguation on its own.
439</p>
440</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]441
442
443<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]444<div class="doc_subsection">
445 <a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
446</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]447
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]448<div class="doc_text">
449<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]450The <tt>-print-alias-sets</tt> pass is exposed as part of the <tt>analyze</tt>
451tool to print out the Alias Sets formed by the <a
452href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]453the <tt>AliasSetTracker</tt>.
454</p>
455</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]456
457<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]458<div class="doc_subsection">
459 <a name="count-aa">The <tt>-count-aa</tt> pass</a>
460</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]461
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]462<div class="doc_text">
463<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]464The <tt>-count-aa</tt> pass is useful to see how many queries a particular pass
465is making and what kinds of responses are returned by the alias analysis. An
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]466example usage is:
467</p>
468<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]469<pre>
470 $ opt -basicaa -count-aa -ds-aa -count-aa -licm
471</pre>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]472</p>
473<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]474Which will print out how many queries (and what responses are returned) by the
475<tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are made
476of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be useful
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]477when evaluating an alias analysis for precision.
478</p>
479</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]480
481<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]482<div class="doc_subsection">
483 <a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
484</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]485
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]486<div class="doc_text">
487<p>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]488The <tt>-aa-eval</tt> pass simply iterates through all pairs of pointers in a
489function and asks an alias analysis whether or not the pointers alias. This
490gives an indication of the precision of the alias analysis. Statistics are
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]491printed.
492</p>
493</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]494
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]495<!-- *********************************************************************** -->
496
Misha Brukman21b892c2003-10-23 01:49:47 +0000[diff] [blame]497<hr><font size="-1">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]498<address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
Misha Brukman21b892c2003-10-23 01:49:47 +0000[diff] [blame]499Last modified: $Date$
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]500</font>
501
502</body>
503</html>