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4<head>
Misha Brukmana97e6cb2004-07-01 15:33:24 +0000[diff] [blame]5 <title>LLVM Alias Analysis Infrastructure</title>
Misha Brukman6e4afd02004-01-15 19:04:12 +0000[diff] [blame]6 <link rel="stylesheet" href="llvm.css" type="text/css">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]7</head>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]8<body>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]9
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]10<div class="doc_title">
Misha Brukmana97e6cb2004-07-01 15:33:24 +0000[diff] [blame]11 LLVM Alias Analysis Infrastructure
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]12</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]13
14<ol>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]15 <li><a href="#introduction">Introduction</a></li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]16
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]17 <li><a href="#overview"><tt>AliasAnalysis</tt> Class Overview</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]18 <ul>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]19 <li><a href="#pointers">Representation of Pointers</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]20 <li><a href="#alias">The <tt>alias</tt> method</a></li>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]21 <li><a href="#ModRefInfo">The <tt>getModRefInfo</tt> methods</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]22 <li><a href="#OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a></li>
23 </ul>
24 </li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]25
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]26 <li><a href="#writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]27 <ul>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]28 <li><a href="#passsubclasses">Different Pass styles</a></li>
29 <li><a href="#requiredcalls">Required initialization calls</a></li>
30 <li><a href="#interfaces">Interfaces which may be specified</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]31 <li><a href="#chaining"><tt>AliasAnalysis</tt> chaining behavior</a></li>
32 <li><a href="#updating">Updating analysis results for transformations</a></li>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]33 <li><a href="#implefficiency">Efficiency Issues</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]34 </ul>
35 </li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]36
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]37 <li><a href="#using">Using alias analysis results</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]38 <ul>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]39 <li><a href="#loadvn">Using the <tt>-load-vn</tt> Pass</a></li>
40 <li><a href="#ast">Using the <tt>AliasSetTracker</tt> class</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]41 <li><a href="#direct">Using the <tt>AliasAnalysis</tt> interface directly</a></li>
42 </ul>
43 </li>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]44
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]45 <li><a href="#exist">Existing alias analysis implementations and clients</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]46 <ul>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]47 <li><a href="#impls">Available <tt>AliasAnalysis</tt> implementations</a></li>
48 <li><a href="#aliasanalysis-xforms">Alias analysis driven transformations</a></li>
Misha Brukmana97e6cb2004-07-01 15:33:24 +0000[diff] [blame]49 <li><a href="#aliasanalysis-debug">Clients for debugging and evaluation of
50 implementations</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]51 </ul>
52 </li>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]53</ol>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]54
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]55<div class="doc_author">
56 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]57</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]58
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]59<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]60<div class="doc_section">
61 <a name="introduction">Introduction</a>
62</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]63<!-- *********************************************************************** -->
64
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]65<div class="doc_text">
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]66
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]67<p>Alias Analysis (aka Pointer Analysis) is a class of techniques which attempt
68to determine whether or not two pointers ever can point to the same object in
69memory. There are many different algorithms for alias analysis and many
70different ways of classifying them: flow-sensitive vs flow-insensitive,
71context-sensitive vs context-insensitive, field-sensitive vs field-insensitive,
72unification-based vs subset-based, etc. Traditionally, alias analyses respond
73to a query with a <a href="#MustNoMay">Must, May, or No</a> alias response,
74indicating that two pointers always point to the same object, might point to the
75same object, or are known to never point to the same object.</p>
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]76
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]77<p>The LLVM <a
78href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
79class is the primary interface used by clients and implementations of alias
80analyses in the LLVM system. This class is the common interface between clients
81of alias analysis information and the implementations providing it, and is
82designed to support a wide range of implementations and clients (but currently
83all clients are assumed to be flow-insensitive). In addition to simple alias
84analysis information, this class exposes Mod/Ref information from those
85implementations which can provide it, allowing for powerful analyses and
86transformations to work well together.</p>
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]87
88<p>This document contains information necessary to successfully implement this
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]89interface, use it, and to test both sides. It also explains some of the finer
90points about what exactly results mean. If you feel that something is unclear
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]91or should be added, please <a href="mailto:sabre@nondot.org">let me
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]92know</a>.</p>
93
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]94</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]95
96<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]97<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]98 <a name="overview"><tt>AliasAnalysis</tt> Class Overview</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]99</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]100<!-- *********************************************************************** -->
101
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]102<div class="doc_text">
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]103
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]104<p>The <a
105href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
106class defines the interface that the various alias analysis implementations
107should support. This class exports two important enums: <tt>AliasResult</tt>
108and <tt>ModRefResult</tt> which represent the result of an alias query or a
109mod/ref query, respectively.</p>
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]110
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]111<p>The <tt>AliasAnalysis</tt> interface exposes information about memory,
112represented in several different ways. In particular, memory objects are
113represented as a starting address and size, and function calls are represented
114as the actual <tt>call</tt> or <tt>invoke</tt> instructions that performs the
115call. The <tt>AliasAnalysis</tt> interface also exposes some helper methods
116which allow you to get mod/ref information for arbitrary instructions.</p>
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]117
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]118</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]119
120<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]121<div class="doc_subsection">
122 <a name="pointers">Representation of Pointers</a>
123</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]124
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]125<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]126
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]127<p>Most importantly, the <tt>AliasAnalysis</tt> class provides several methods
128which are used to query whether or not two memory objects alias, whether
129function calls can modify or read a memory object, etc. For all of these
130queries, memory objects are represented as a pair of their starting address (a
131symbolic LLVM <tt>Value*</tt>) and a static size.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]132
133<p>Representing memory objects as a starting address and a size is critically
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]134important for correct Alias Analyses. For example, consider this (silly, but
135possible) C code:</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]136
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]137<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]138<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]139int i;
140char C[2];
141char A[10];
142/* ... */
143for (i = 0; i != 10; ++i) {
144 C[0] = A[i]; /* One byte store */
145 C[1] = A[9-i]; /* One byte store */
146}
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]147</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]148</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]149
150<p>In this case, the <tt>basicaa</tt> pass will disambiguate the stores to
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]151<tt>C[0]</tt> and <tt>C[1]</tt> because they are accesses to two distinct
152locations one byte apart, and the accesses are each one byte. In this case, the
153LICM pass can use store motion to remove the stores from the loop. In
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]154constrast, the following code:</p>
155
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]156<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]157<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]158int i;
159char C[2];
160char A[10];
161/* ... */
162for (i = 0; i != 10; ++i) {
163 ((short*)C)[0] = A[i]; /* Two byte store! */
164 C[1] = A[9-i]; /* One byte store */
165}
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]166</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]167</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]168
169<p>In this case, the two stores to C do alias each other, because the access to
170the <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]171available in the query, even the first case would have to conservatively assume
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]172that the accesses alias.</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">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]178 <a name="alias">The <tt>alias</tt> method</a>
179</div>
180
181<div class="doc_text">
182The <tt>alias</tt> method is the primary interface used to determine whether or
183not two memory objects alias each other. It takes two memory objects as input
184and returns MustAlias, MayAlias, or NoAlias as appropriate.
185</div>
186
187<!-- _______________________________________________________________________ -->
188<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]189 <a name="MustMayNo">Must, May, and No Alias Responses</a>
190</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]191
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]192<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]193
194<p>An Alias Analysis implementation can return one of three responses:
195MustAlias, MayAlias, and NoAlias. The No and May alias results are obvious: if
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]196the two pointers can never equal each other, return NoAlias, if they might,
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]197return MayAlias.</p>
198
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]199<p>The MustAlias response is trickier though. In LLVM, the Must Alias response
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]200may only be returned if the two memory objects are guaranteed to always start at
201exactly the same location. If two memory objects overlap, but do not start at
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]202the same location, return MayAlias.</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="ModRefInfo">The <tt>getModRefInfo</tt> methods</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 <tt>getModRefInfo</tt> methods return information about whether the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]214execution of an instruction can read or modify a memory location. Mod/Ref
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]215information is always conservative: if an instruction <b>might</b> read or write
216a location, ModRef is returned.</p>
217
218<p>The <tt>AliasAnalysis</tt> class also provides a <tt>getModRefInfo</tt>
219method for testing dependencies between function calls. This method takes two
220call sites (CS1 &amp; CS2), returns NoModRef if the two calls refer to disjoint
221memory locations, Ref if CS1 reads memory written by CS2, Mod if CS1 writes to
222memory read or written by CS2, or ModRef if CS1 might read or write memory
223accessed by CS2. Note that this relation is not commutative. Clients that use
224this method should be predicated on the <tt>hasNoModRefInfoForCalls()</tt>
225method, which indicates whether or not an analysis can provide mod/ref
226information for function call pairs (most can not). If this predicate is false,
227the client shouldn't waste analysis time querying the <tt>getModRefInfo</tt>
228method many times.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]229
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]230</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]231
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]232
233<!-- ======================================================================= -->
234<div class="doc_subsection">
235 <a name="OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a>
236</div>
237
238<div class="doc_text">
239
240<p>
241Several other tidbits of information are often collected by various alias
242analysis implementations and can be put to good use by various clients.
243</p>
244
245</div>
246
247<!-- _______________________________________________________________________ -->
248<div class="doc_subsubsection">
249 The <tt>getMustAliases</tt> method
250</div>
251
252<div class="doc_text">
253
254<p>The <tt>getMustAliases</tt> method returns all values that are known to
255always must alias a pointer. This information can be provided in some cases for
256important objects like the null pointer and global values. Knowing that a
257pointer always points to a particular function allows indirect calls to be
258turned into direct calls, for example.</p>
259
260</div>
261
262<!-- _______________________________________________________________________ -->
263<div class="doc_subsubsection">
264 The <tt>pointsToConstantMemory</tt> method
265</div>
266
267<div class="doc_text">
268
269<p>The <tt>pointsToConstantMemory</tt> method returns true if and only if the
270analysis can prove that the pointer only points to unchanging memory locations
271(functions, constant global variables, and the null pointer). This information
272can be used to refine mod/ref information: it is impossible for an unchanging
273memory location to be modified.</p>
274
275</div>
276
277
278<!-- _______________________________________________________________________ -->
279<div class="doc_subsubsection">
280 <a name="simplemodref">The <tt>doesNotAccessMemory</tt> and
281 <tt>onlyReadsMemory</tt> methods</a>
282</div>
283
284<div class="doc_text">
285
286<p>These methods are used to provide very simple mod/ref information for
287function calls. The <tt>doesNotAccessMemory</tt> method returns true for a
288function if the analysis can prove that the function never reads or writes to
289memory, or if the function only reads from constant memory. Functions with this
290property are side-effect free and only depend on their input arguments, allowing
291them to be eliminated if they form common subexpressions or be hoisted out of
292loops. Many common functions behave this way (e.g., <tt>sin</tt> and
293<tt>cos</tt>) but many others do not (e.g., <tt>acos</tt>, which modifies the
294<tt>errno</tt> variable).</p>
295
296<p>The <tt>onlyReadsMemory</tt> method returns true for a function if analysis
297can prove that (at most) the function only reads from non-volatile memory.
298Functions with this property are side-effect free, only depending on their input
299arguments and the state of memory when they are called. This property allows
300calls to these functions to be eliminated and moved around, as long as there is
301no store instruction that changes the contents of memory. Note that all
302functions that satisfy the <tt>doesNotAccessMemory</tt> method also satisfies
303<tt>onlyReadsMemory</tt>.</p>
304
305</div>
306
307
308
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_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]311 <a name="writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]312</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]313<!-- *********************************************************************** -->
314
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]315<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]316
317<p>Writing a new alias analysis implementation for LLVM is quite
318straight-forward. There are already several implementations that you can use
319for examples, and the following information should help fill in any details.
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]320For a examples, take a look at the <a href="#impls">various alias analysis
321implementations</a> included with LLVM.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]322
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]323</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]324
325<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]326<div class="doc_subsection">
327 <a name="passsubclasses">Different Pass styles</a>
328</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]329
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]330<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]331
332<p>The first step to determining what type of <a
333href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias
334Analysis. As is the case with most other analyses and transformations, the
335answer should be fairly obvious from what type of problem you are trying to
336solve:</p>
337
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]338<ol>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]339 <li>If you require interprocedural analysis, it should be a
340 <tt>Pass</tt>.</li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]341 <li>If you are a function-local analysis, subclass <tt>FunctionPass</tt>.</li>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]342 <li>If you don't need to look at the program at all, subclass
343 <tt>ImmutablePass</tt>.</li>
344</ol>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]345
346<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]347<tt>AliasAnalysis</tt> interface, of course, and use the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]348<tt>RegisterAnalysisGroup</tt> template to register as an implementation of
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]349<tt>AliasAnalysis</tt>.</p>
350
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]351</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]352
353<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]354<div class="doc_subsection">
355 <a name="requiredcalls">Required initialization calls</a>
356</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]357
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]358<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]359
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]360<p>Your subclass of <tt>AliasAnalysis</tt> is required to invoke two methods on
361the <tt>AliasAnalysis</tt> base class: <tt>getAnalysisUsage</tt> and
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]362<tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
363<tt>getAnalysisUsage</tt> should explicitly call into the
364<tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
365declaring any pass dependencies your pass has. Thus you should have something
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]366like this:</p>
367
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]368<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]369<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]370void getAnalysisUsage(AnalysisUsage &amp;AU) const {
371 AliasAnalysis::getAnalysisUsage(AU);
372 <i>// declare your dependencies here.</i>
373}
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]374</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]375</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]376
377<p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
378from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]379<tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, or <tt>InitializePass</tt>
380for an <tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]381
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]382<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]383<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]384bool run(Module &amp;M) {
385 InitializeAliasAnalysis(this);
386 <i>// Perform analysis here...</i>
387 return false;
388}
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]389</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]390</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]391
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]392</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]393
394<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]395<div class="doc_subsection">
396 <a name="interfaces">Interfaces which may be specified</a>
397</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]398
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]399<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]400
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]401<p>All of the <a
402href="/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
403virtual methods default to providing <a href="#chaining">chaining</a> to another
404alias analysis implementation, which ends up returning conservatively correct
405information (returning "May" Alias and "Mod/Ref" for alias and mod/ref queries
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]406respectively). Depending on the capabilities of the analysis you are
407implementing, you just override the interfaces you can improve.</p>
408
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]409</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]410
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]411
412
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]413<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]414<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]415 <a name="chaining"><tt>AliasAnalysis</tt> chaining behavior</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]416</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]417
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]418<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]419
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]420<p>With only two special exceptions (the <tt><a
421href="#basic-aa">basicaa</a></tt> and <a href="#no-aa"><tt>no-aa</tt></a>
422passes) every alias analysis pass chains to another alias analysis
423implementation (for example, the user can specify "<tt>-basicaa -ds-aa
424-anders-aa -licm</tt>" to get the maximum benefit from the three alias
425analyses). The alias analysis class automatically takes care of most of this
426for methods that you don't override. For methods that you do override, in code
427paths that return a conservative MayAlias or Mod/Ref result, simply return
428whatever the superclass computes. For example:</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]429
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]430<div class="doc_code">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]431<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]432AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
433 const Value *V2, unsigned V2Size) {
434 if (...)
435 return NoAlias;
436 ...
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]437
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]438 <i>// Couldn't determine a must or no-alias result.</i>
439 return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
440}
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]441</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]442</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]443
444<p>In addition to analysis queries, you must make sure to unconditionally pass
445LLVM <a href="#updating">update notification</a> methods to the superclass as
446well if you override them, which allows all alias analyses in a change to be
447updated.</p>
448
449</div>
450
451
452<!-- ======================================================================= -->
453<div class="doc_subsection">
454 <a name="updating">Updating analysis results for transformations</a>
455</div>
456
457<div class="doc_text">
458<p>
459Alias analysis information is initially computed for a static snapshot of the
460program, but clients will use this information to make transformations to the
461code. All but the most trivial forms of alias analysis will need to have their
462analysis results updated to reflect the changes made by these transformations.
463</p>
464
465<p>
466The <tt>AliasAnalysis</tt> interface exposes two methods which are used to
467communicate program changes from the clients to the analysis implementations.
468Various alias analysis implementations should use these methods to ensure that
469their internal data structures are kept up-to-date as the program changes (for
470example, when an instruction is deleted), and clients of alias analysis must be
471sure to call these interfaces appropriately.
472</p>
473</div>
474
475<!-- _______________________________________________________________________ -->
476<div class="doc_subsubsection">The <tt>deleteValue</tt> method</div>
477
478<div class="doc_text">
479The <tt>deleteValue</tt> method is called by transformations when they remove an
480instruction or any other value from the program (including values that do not
481use pointers). Typically alias analyses keep data structures that have entries
482for each value in the program. When this method is called, they should remove
483any entries for the specified value, if they exist.
484</div>
485
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]486<!-- _______________________________________________________________________ -->
487<div class="doc_subsubsection">The <tt>copyValue</tt> method</div>
488
489<div class="doc_text">
490The <tt>copyValue</tt> method is used when a new value is introduced into the
491program. There is no way to introduce a value into the program that did not
492exist before (this doesn't make sense for a safe compiler transformation), so
493this is the only way to introduce a new value. This method indicates that the
494new value has exactly the same properties as the value being copied.
495</div>
496
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]497<!-- _______________________________________________________________________ -->
498<div class="doc_subsubsection">The <tt>replaceWithNewValue</tt> method</div>
499
500<div class="doc_text">
501This method is a simple helper method that is provided to make clients easier to
502use. It is implemented by copying the old analysis information to the new
503value, then deleting the old value. This method cannot be overridden by alias
504analysis implementations.
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]505</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]506
507<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]508<div class="doc_subsection">
509 <a name="implefficiency">Efficiency Issues</a>
510</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]511
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]512<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]513
514<p>From the LLVM perspective, the only thing you need to do to provide an
515efficient alias analysis is to make sure that alias analysis <b>queries</b> are
516serviced quickly. The actual calculation of the alias analysis results (the
517"run" method) is only performed once, but many (perhaps duplicate) queries may
518be performed. Because of this, try to move as much computation to the run
519method as possible (within reason).</p>
520
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]521</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]522
523<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]524<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]525 <a name="using">Using alias analysis results</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]526</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]527<!-- *********************************************************************** -->
528
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]529<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]530
531<p>There are several different ways to use alias analysis results. In order of
532preference, these are...</p>
533
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]534</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]535
536<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]537<div class="doc_subsection">
538 <a name="loadvn">Using the <tt>-load-vn</tt> Pass</a>
539</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]540
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]541<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]542
543<p>The <tt>load-vn</tt> pass uses alias analysis to provide value numbering
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]544information for <tt>load</tt> instructions and pointer values. If your analysis
545or transformation can be modeled in a form that uses value numbering
546information, you don't have to do anything special to handle load instructions:
547just use the <tt>load-vn</tt> pass, which uses alias analysis.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]548
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]549</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]550
551<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]552<div class="doc_subsection">
553 <a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
554</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]555
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]556<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]557
558<p>Many transformations need information about alias <b>sets</b> that are active
559in some scope, rather than information about pairwise aliasing. The <tt><a
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]560href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class
561is used to efficiently build these Alias Sets from the pairwise alias analysis
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]562information provided by the <tt>AliasAnalysis</tt> interface.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]563
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]564<p>First you initialize the AliasSetTracker by using the "<tt>add</tt>" methods
565to add information about various potentially aliasing instructions in the scope
566you are interested in. Once all of the alias sets are completed, your pass
567should simply iterate through the constructed alias sets, using the
568<tt>AliasSetTracker</tt> <tt>begin()</tt>/<tt>end()</tt> methods.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]569
570<p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]571to be disjoint, calculate mod/ref information and volatility for the set, and
572keep track of whether or not all of the pointers in the set are Must aliases.
573The AliasSetTracker also makes sure that sets are properly folded due to call
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]574instructions, and can provide a list of pointers in each set.</p>
575
576<p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]577Invariant Code Motion</a> pass uses <tt>AliasSetTracker</tt>s to calculate alias
578sets for each loop nest. If an <tt>AliasSet</tt> in a loop is not modified,
579then all load instructions from that set may be hoisted out of the loop. If any
580alias sets are stored to <b>and</b> are must alias sets, then the stores may be
581sunk to outside of the loop, promoting the memory location to a register for the
582duration of the loop nest. Both of these transformations only apply if the
583pointer argument is loop-invariant.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]584
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]585</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]586
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]587<!-- _______________________________________________________________________ -->
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]588<div class="doc_subsubsection">
589 The AliasSetTracker implementation
590</div>
591
592<div class="doc_text">
593
594<p>The AliasSetTracker class is implemented to be as efficient as possible. It
595uses the union-find algorithm to efficiently merge AliasSets when a pointer is
596inserted into the AliasSetTracker that aliases multiple sets. The primary data
597structure is a hash table mapping pointers to the AliasSet they are in.</p>
598
599<p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s
600that are in each AliasSet. Since the hash table already has entries for each
601LLVM Value* of interest, the AliasesSets thread the linked list through these
602hash-table nodes to avoid having to allocate memory unnecessarily, and to make
603merging alias sets extremely efficient (the linked list merge is constant time).
604</p>
605
606<p>You shouldn't need to understand these details if you are just a client of
607the AliasSetTracker, but if you look at the code, hopefully this brief
608description will help make sense of why things are designed the way they
609are.</p>
610
611</div>
612
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]613<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]614<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]615 <a name="direct">Using the <tt>AliasAnalysis</tt> interface directly</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]616</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]617
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]618<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]619
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]620<p>If neither of these utility class are what your pass needs, you should use
621the interfaces exposed by the <tt>AliasAnalysis</tt> class directly. Try to use
622the higher-level methods when possible (e.g., use mod/ref information instead of
623the <a href="#alias"><tt>alias</tt></a> method directly if possible) to get the
624best precision and efficiency.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]625
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]626</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]627
628<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]629<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]630 <a name="exist">Existing alias analysis implementations and clients</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]631</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]632<!-- *********************************************************************** -->
633
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]634<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]635
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]636<p>If you're going to be working with the LLVM alias analysis infrastructure,
637you should know what clients and implementations of alias analysis are
638available. In particular, if you are implementing an alias analysis, you should
639be aware of the <a href="#aliasanalysis-debug">the clients</a> that are useful
640for monitoring and evaluating different implementations.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]641
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]642</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]643
644<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]645<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]646 <a name="impls">Available <tt>AliasAnalysis</tt> implementations</a>
647</div>
648
649<div class="doc_text">
650
651<p>This section lists the various implementations of the <tt>AliasAnalysis</tt>
652interface. With the exception of the <a href="#no-aa"><tt>-no-aa</tt></a> and
653<a href="#basic-aa"><tt>-basicaa</tt></a> implementations, all of these <a
Chris Lattnerfcd37252004-06-21 22:52:48 +0000[diff] [blame]654href="#chaining">chain</a> to other alias analysis implementations.</p>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]655
656</div>
657
658<!-- _______________________________________________________________________ -->
659<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]660 <a name="no-aa">The <tt>-no-aa</tt> pass</a>
661</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]662
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]663<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]664
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]665<p>The <tt>-no-aa</tt> pass is just like what it sounds: an alias analysis that
666never returns any useful information. This pass can be useful if you think that
667alias analysis is doing something wrong and are trying to narrow down a
668problem.</p>
669
670</div>
671
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]672<!-- _______________________________________________________________________ -->
673<div class="doc_subsubsection">
674 <a name="basic-aa">The <tt>-basicaa</tt> pass</a>
675</div>
676
677<div class="doc_text">
678
679<p>The <tt>-basicaa</tt> pass is the default LLVM alias analysis. It is an
680aggressive local analysis that "knows" many important facts:</p>
681
682<ul>
683<li>Distinct globals, stack allocations, and heap allocations can never
684 alias.</li>
685<li>Globals, stack allocations, and heap allocations never alias the null
686 pointer.</li>
687<li>Different fields of a structure do not alias.</li>
688<li>Indexes into arrays with statically differing subscripts cannot alias.</li>
689<li>Many common standard C library functions <a
690 href="#simplemodref">never access memory or only read memory</a>.</li>
691<li>Pointers that obviously point to constant globals
692 "<tt>pointToConstantMemory</tt>".</li>
693<li>Function calls can not modify or references stack allocations if they never
694 escape from the function that allocates them (a common case for automatic
695 arrays).</li>
696</ul>
697
698</div>
699
Chris Lattner100a4f82004-06-28 19:19:47 +0000[diff] [blame]700<!-- _______________________________________________________________________ -->
701<div class="doc_subsubsection">
702 <a name="globalsmodref">The <tt>-globalsmodref-aa</tt> pass</a>
703</div>
704
705<div class="doc_text">
706
707<p>This pass implements a simple context-sensitive mod/ref and alias analysis
Chris Lattnerccb354b2004-07-27 07:50:07 +0000[diff] [blame]708for internal global variables that don't "have their address taken". If a
709global does not have its address taken, the pass knows that no pointers alias
710the global. This pass also keeps track of functions that it knows never access
711memory or never read memory. This allows certain optimizations (e.g. GCSE) to
712eliminate call instructions entirely.
Chris Lattner100a4f82004-06-28 19:19:47 +0000[diff] [blame]713</p>
714
715<p>The real power of this pass is that it provides context-sensitive mod/ref
716information for call instructions. This allows the optimizer to know that
717calls to a function do not clobber or read the value of the global, allowing
718loads and stores to be eliminated.</p>
719
720<p>Note that this pass is somewhat limited in its scope (only support
721non-address taken globals), but is very quick analysis.</p>
722</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]723
724<!-- _______________________________________________________________________ -->
725<div class="doc_subsubsection">
726 <a name="anders-aa">The <tt>-anders-aa</tt> pass</a>
727</div>
728
729<div class="doc_text">
730
731<p>The <tt>-anders-aa</tt> pass implements the well-known "Andersen's algorithm"
732for interprocedural alias analysis. This algorithm is a subset-based,
733flow-insensitive, context-insensitive, and field-insensitive alias analysis that
734is widely believed to be fairly precise. Unfortunately, this algorithm is also
735O(N<sup>3</sup>). The LLVM implementation currently does not implement any of
736the refinements (such as "online cycle elimination" or "offline variable
737substitution") to improve its efficiency, so it can be quite slow in common
738cases.
739</p>
740
741</div>
742
743<!-- _______________________________________________________________________ -->
744<div class="doc_subsubsection">
745 <a name="steens-aa">The <tt>-steens-aa</tt> pass</a>
746</div>
747
748<div class="doc_text">
749
750<p>The <tt>-steens-aa</tt> pass implements a variation on the well-known
751"Steensgaard's algorithm" for interprocedural alias analysis. Steensgaard's
752algorithm is a unification-based, flow-insensitive, context-insensitive, and
753field-insensitive alias analysis that is also very scalable (effectively linear
754time).</p>
755
756<p>The LLVM <tt>-steens-aa</tt> pass implements a "speculatively
757field-<b>sensitive</b>" version of Steensgaard's algorithm using the Data
758Structure Analysis framework. This gives it substantially more precision than
759the standard algorithm while maintaining excellent analysis scalability.</p>
760
761</div>
762
763<!-- _______________________________________________________________________ -->
764<div class="doc_subsubsection">
765 <a name="ds-aa">The <tt>-ds-aa</tt> pass</a>
766</div>
767
768<div class="doc_text">
769
770<p>The <tt>-ds-aa</tt> pass implements the full Data Structure Analysis
771algorithm. Data Structure Analysis is a modular unification-based,
772flow-insensitive, context-<b>sensitive</b>, and speculatively
773field-<b>sensitive</b> alias analysis that is also quite scalable, usually at
774O(n*log(n)).</p>
775
776<p>This algorithm is capable of responding to a full variety of alias analysis
777queries, and can provide context-sensitive mod/ref information as well. The
778only major facility not implemented so far is support for must-alias
779information.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]780
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]781</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]782
783
784<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]785<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]786 <a name="aliasanalysis-xforms">Alias analysis driven transformations</a>
787</div>
788
789<div class="doc_text">
790LLVM includes several alias-analysis driven transformations which can be used
791with any of the implementations above.
792</div>
793
794<!-- _______________________________________________________________________ -->
795<div class="doc_subsubsection">
796 <a name="adce">The <tt>-adce</tt> pass</a>
797</div>
798
799<div class="doc_text">
800
801<p>The <tt>-adce</tt> pass, which implements Aggressive Dead Code Elimination
802uses the <tt>AliasAnalysis</tt> interface to delete calls to functions that do
803not have side-effects and are not used.</p>
804
805</div>
806
807
808<!-- _______________________________________________________________________ -->
809<div class="doc_subsubsection">
810 <a name="licm">The <tt>-licm</tt> pass</a>
811</div>
812
813<div class="doc_text">
814
815<p>The <tt>-licm</tt> pass implements various Loop Invariant Code Motion related
816transformations. It uses the <tt>AliasAnalysis</tt> interface for several
817different transformations:</p>
818
819<ul>
820<li>It uses mod/ref information to hoist or sink load instructions out of loops
821if there are no instructions in the loop that modifies the memory loaded.</li>
822
823<li>It uses mod/ref information to hoist function calls out of loops that do not
824write to memory and are loop-invariant.</li>
825
826<li>If uses alias information to promote memory objects that are loaded and
827stored to in loops to live in a register instead. It can do this if there are
828no may aliases to the loaded/stored memory location.</li>
829</ul>
830
831</div>
832
833<!-- _______________________________________________________________________ -->
834<div class="doc_subsubsection">
835 <a name="argpromotion">The <tt>-argpromotion</tt> pass</a>
836</div>
837
838<div class="doc_text">
839<p>
840The <tt>-argpromotion</tt> pass promotes by-reference arguments to be passed in
841by-value instead. In particular, if pointer arguments are only loaded from it
842passes in the value loaded instead of the address to the function. This pass
843uses alias information to make sure that the value loaded from the argument
844pointer is not modified between the entry of the function and any load of the
845pointer.</p>
846</div>
847
848<!-- _______________________________________________________________________ -->
849<div class="doc_subsubsection">
850 <a name="gcseloadvn">The <tt>-load-vn</tt> &amp; <tt>-gcse</tt> passes</a>
851</div>
852
853<div class="doc_text">
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]854
855<p>The <tt>-load-vn</tt> pass uses alias analysis to "<a href="#loadvn">value
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]856number</a>" loads and pointers values, which is used by the GCSE pass to
857eliminate instructions. The <tt>-load-vn</tt> pass relies on alias information
858and must-alias information. This combination of passes can make the following
859transformations:</p>
860
861<ul>
862<li>Redundant load instructions are eliminated.</li>
863<li>Load instructions that follow a store to the same location are replaced with
864the stored value ("store forwarding").</li>
865<li>Pointers values (e.g. formal arguments) that must-alias simpler expressions
866(e.g. global variables or the null pointer) are replaced. Note that this
867implements transformations like "virtual method resolution", turning indirect
868calls into direct calls.</li>
869</ul>
870
871</div>
872
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]873<!-- ======================================================================= -->
874<div class="doc_subsection">
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]875 <a name="aliasanalysis-debug">Clients for debugging and evaluation of
876 implementations</a>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]877</div>
878
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]879<div class="doc_text">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]880
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]881<p>These passes are useful for evaluating the various alias analysis
882implementations. You can use them with commands like '<tt>opt -anders-aa -ds-aa
883-aa-eval foo.bc -disable-output -stats</tt>'.</p>
884
885</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]886
887<!-- _______________________________________________________________________ -->
888<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]889 <a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
890</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]891
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]892<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]893
894<p>The <tt>-print-alias-sets</tt> pass is exposed as part of the
895<tt>analyze</tt> tool to print out the Alias Sets formed by the <a
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]896href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]897the <tt>AliasSetTracker</tt> class.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]898
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]899</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]900
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]901
902<!-- _______________________________________________________________________ -->
903<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]904 <a name="count-aa">The <tt>-count-aa</tt> pass</a>
905</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]906
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]907<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]908
909<p>The <tt>-count-aa</tt> pass is useful to see how many queries a particular
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]910pass is making and what responses are returned by the alias analysis. As an
911example,</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]912
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]913<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]914<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]915% opt -basicaa -count-aa -ds-aa -count-aa -licm
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]916</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]917</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]918
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame^]919<p>will print out how many queries (and what responses are returned) by the
920<tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are made
921of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be useful
922when debugging a transformation or an alias analysis implementation.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]923
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]924</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]925
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]926<!-- _______________________________________________________________________ -->
927<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]928 <a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
929</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]930
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]931<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]932
933<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]934function and asks an alias analysis whether or not the pointers alias. This
935gives an indication of the precision of the alias analysis. Statistics are
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]936printed indicating the percent of no/may/must aliases found (a more precise
937algorithm will have a lower number of may aliases).</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]938
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]939</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]940
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]941<!-- *********************************************************************** -->
942
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]943<hr>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]944<address>
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Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]949
950 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Misha Brukmanf6acb512004-05-12 18:02:00 +0000[diff] [blame]951 <a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]952 Last modified: $Date$
953</address>
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