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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
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame^]78href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]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
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame^]105href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]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
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]277<!-- _______________________________________________________________________ -->
278<div class="doc_subsubsection">
279 <a name="simplemodref">The <tt>doesNotAccessMemory</tt> and
280 <tt>onlyReadsMemory</tt> methods</a>
281</div>
282
283<div class="doc_text">
284
285<p>These methods are used to provide very simple mod/ref information for
286function calls. The <tt>doesNotAccessMemory</tt> method returns true for a
287function if the analysis can prove that the function never reads or writes to
288memory, or if the function only reads from constant memory. Functions with this
289property are side-effect free and only depend on their input arguments, allowing
290them to be eliminated if they form common subexpressions or be hoisted out of
291loops. Many common functions behave this way (e.g., <tt>sin</tt> and
292<tt>cos</tt>) but many others do not (e.g., <tt>acos</tt>, which modifies the
293<tt>errno</tt> variable).</p>
294
295<p>The <tt>onlyReadsMemory</tt> method returns true for a function if analysis
296can prove that (at most) the function only reads from non-volatile memory.
297Functions with this property are side-effect free, only depending on their input
298arguments and the state of memory when they are called. This property allows
299calls to these functions to be eliminated and moved around, as long as there is
300no store instruction that changes the contents of memory. Note that all
301functions that satisfy the <tt>doesNotAccessMemory</tt> method also satisfies
302<tt>onlyReadsMemory</tt>.</p>
303
304</div>
305
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]306<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]307<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]308 <a name="writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]309</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]310<!-- *********************************************************************** -->
311
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]312<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]313
314<p>Writing a new alias analysis implementation for LLVM is quite
315straight-forward. There are already several implementations that you can use
316for examples, and the following information should help fill in any details.
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]317For a examples, take a look at the <a href="#impls">various alias analysis
318implementations</a> included with LLVM.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]319
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]320</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]321
322<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]323<div class="doc_subsection">
324 <a name="passsubclasses">Different Pass styles</a>
325</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]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>The first step to determining what type of <a
330href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias
331Analysis. As is the case with most other analyses and transformations, the
332answer should be fairly obvious from what type of problem you are trying to
333solve:</p>
334
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]335<ol>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]336 <li>If you require interprocedural analysis, it should be a
337 <tt>Pass</tt>.</li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]338 <li>If you are a function-local analysis, subclass <tt>FunctionPass</tt>.</li>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]339 <li>If you don't need to look at the program at all, subclass
340 <tt>ImmutablePass</tt>.</li>
341</ol>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]342
343<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]344<tt>AliasAnalysis</tt> interface, of course, and use the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]345<tt>RegisterAnalysisGroup</tt> template to register as an implementation of
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]346<tt>AliasAnalysis</tt>.</p>
347
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]348</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]349
350<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]351<div class="doc_subsection">
352 <a name="requiredcalls">Required initialization calls</a>
353</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]354
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]355<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]356
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]357<p>Your subclass of <tt>AliasAnalysis</tt> is required to invoke two methods on
358the <tt>AliasAnalysis</tt> base class: <tt>getAnalysisUsage</tt> and
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]359<tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
360<tt>getAnalysisUsage</tt> should explicitly call into the
361<tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
362declaring any pass dependencies your pass has. Thus you should have something
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]363like this:</p>
364
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]365<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]366<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]367void getAnalysisUsage(AnalysisUsage &amp;AU) const {
368 AliasAnalysis::getAnalysisUsage(AU);
369 <i>// declare your dependencies here.</i>
370}
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]371</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]372</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]373
374<p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
375from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]376<tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, or <tt>InitializePass</tt>
377for an <tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]378
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]379<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]380<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]381bool run(Module &amp;M) {
382 InitializeAliasAnalysis(this);
383 <i>// Perform analysis here...</i>
384 return false;
385}
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]386</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]387</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]388
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]389</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]390
391<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]392<div class="doc_subsection">
393 <a name="interfaces">Interfaces which may be specified</a>
394</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]395
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]396<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]397
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]398<p>All of the <a
399href="/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
400virtual methods default to providing <a href="#chaining">chaining</a> to another
401alias analysis implementation, which ends up returning conservatively correct
402information (returning "May" Alias and "Mod/Ref" for alias and mod/ref queries
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]403respectively). Depending on the capabilities of the analysis you are
404implementing, you just override the interfaces you can improve.</p>
405
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]406</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]407
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]408
409
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]410<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]411<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]412 <a name="chaining"><tt>AliasAnalysis</tt> chaining behavior</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]413</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]414
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]415<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]416
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]417<p>With only two special exceptions (the <tt><a
418href="#basic-aa">basicaa</a></tt> and <a href="#no-aa"><tt>no-aa</tt></a>
419passes) every alias analysis pass chains to another alias analysis
420implementation (for example, the user can specify "<tt>-basicaa -ds-aa
421-anders-aa -licm</tt>" to get the maximum benefit from the three alias
422analyses). The alias analysis class automatically takes care of most of this
423for methods that you don't override. For methods that you do override, in code
424paths that return a conservative MayAlias or Mod/Ref result, simply return
425whatever the superclass computes. For example:</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]426
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]427<div class="doc_code">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]428<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]429AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
430 const Value *V2, unsigned V2Size) {
431 if (...)
432 return NoAlias;
433 ...
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]434
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]435 <i>// Couldn't determine a must or no-alias result.</i>
436 return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
437}
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]438</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]439</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]440
441<p>In addition to analysis queries, you must make sure to unconditionally pass
442LLVM <a href="#updating">update notification</a> methods to the superclass as
443well if you override them, which allows all alias analyses in a change to be
444updated.</p>
445
446</div>
447
448
449<!-- ======================================================================= -->
450<div class="doc_subsection">
451 <a name="updating">Updating analysis results for transformations</a>
452</div>
453
454<div class="doc_text">
455<p>
456Alias analysis information is initially computed for a static snapshot of the
457program, but clients will use this information to make transformations to the
458code. All but the most trivial forms of alias analysis will need to have their
459analysis results updated to reflect the changes made by these transformations.
460</p>
461
462<p>
463The <tt>AliasAnalysis</tt> interface exposes two methods which are used to
464communicate program changes from the clients to the analysis implementations.
465Various alias analysis implementations should use these methods to ensure that
466their internal data structures are kept up-to-date as the program changes (for
467example, when an instruction is deleted), and clients of alias analysis must be
468sure to call these interfaces appropriately.
469</p>
470</div>
471
472<!-- _______________________________________________________________________ -->
473<div class="doc_subsubsection">The <tt>deleteValue</tt> method</div>
474
475<div class="doc_text">
476The <tt>deleteValue</tt> method is called by transformations when they remove an
477instruction or any other value from the program (including values that do not
478use pointers). Typically alias analyses keep data structures that have entries
479for each value in the program. When this method is called, they should remove
480any entries for the specified value, if they exist.
481</div>
482
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]483<!-- _______________________________________________________________________ -->
484<div class="doc_subsubsection">The <tt>copyValue</tt> method</div>
485
486<div class="doc_text">
487The <tt>copyValue</tt> method is used when a new value is introduced into the
488program. There is no way to introduce a value into the program that did not
489exist before (this doesn't make sense for a safe compiler transformation), so
490this is the only way to introduce a new value. This method indicates that the
491new value has exactly the same properties as the value being copied.
492</div>
493
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]494<!-- _______________________________________________________________________ -->
495<div class="doc_subsubsection">The <tt>replaceWithNewValue</tt> method</div>
496
497<div class="doc_text">
498This method is a simple helper method that is provided to make clients easier to
499use. It is implemented by copying the old analysis information to the new
500value, then deleting the old value. This method cannot be overridden by alias
501analysis implementations.
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]502</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]503
504<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]505<div class="doc_subsection">
506 <a name="implefficiency">Efficiency Issues</a>
507</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]508
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]509<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]510
511<p>From the LLVM perspective, the only thing you need to do to provide an
512efficient alias analysis is to make sure that alias analysis <b>queries</b> are
513serviced quickly. The actual calculation of the alias analysis results (the
514"run" method) is only performed once, but many (perhaps duplicate) queries may
515be performed. Because of this, try to move as much computation to the run
516method as possible (within reason).</p>
517
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]518</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]519
520<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]521<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]522 <a name="using">Using alias analysis results</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]523</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]524<!-- *********************************************************************** -->
525
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]526<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]527
528<p>There are several different ways to use alias analysis results. In order of
529preference, these are...</p>
530
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]531</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]532
533<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]534<div class="doc_subsection">
535 <a name="loadvn">Using the <tt>-load-vn</tt> Pass</a>
536</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]537
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]538<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]539
540<p>The <tt>load-vn</tt> pass uses alias analysis to provide value numbering
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]541information for <tt>load</tt> instructions and pointer values. If your analysis
542or transformation can be modeled in a form that uses value numbering
543information, you don't have to do anything special to handle load instructions:
544just use the <tt>load-vn</tt> pass, which uses alias analysis.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]545
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]546</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]547
548<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]549<div class="doc_subsection">
550 <a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
551</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]552
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]553<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]554
555<p>Many transformations need information about alias <b>sets</b> that are active
556in some scope, rather than information about pairwise aliasing. The <tt><a
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]557href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class
558is used to efficiently build these Alias Sets from the pairwise alias analysis
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]559information provided by the <tt>AliasAnalysis</tt> interface.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]560
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]561<p>First you initialize the AliasSetTracker by using the "<tt>add</tt>" methods
562to add information about various potentially aliasing instructions in the scope
563you are interested in. Once all of the alias sets are completed, your pass
564should simply iterate through the constructed alias sets, using the
565<tt>AliasSetTracker</tt> <tt>begin()</tt>/<tt>end()</tt> methods.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]566
567<p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]568to be disjoint, calculate mod/ref information and volatility for the set, and
569keep track of whether or not all of the pointers in the set are Must aliases.
570The AliasSetTracker also makes sure that sets are properly folded due to call
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]571instructions, and can provide a list of pointers in each set.</p>
572
573<p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]574Invariant Code Motion</a> pass uses <tt>AliasSetTracker</tt>s to calculate alias
575sets for each loop nest. If an <tt>AliasSet</tt> in a loop is not modified,
576then all load instructions from that set may be hoisted out of the loop. If any
577alias sets are stored to <b>and</b> are must alias sets, then the stores may be
578sunk to outside of the loop, promoting the memory location to a register for the
579duration of the loop nest. Both of these transformations only apply if the
580pointer argument is loop-invariant.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]581
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]582</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]583
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]584<!-- _______________________________________________________________________ -->
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]585<div class="doc_subsubsection">
586 The AliasSetTracker implementation
587</div>
588
589<div class="doc_text">
590
591<p>The AliasSetTracker class is implemented to be as efficient as possible. It
592uses the union-find algorithm to efficiently merge AliasSets when a pointer is
593inserted into the AliasSetTracker that aliases multiple sets. The primary data
594structure is a hash table mapping pointers to the AliasSet they are in.</p>
595
596<p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s
597that are in each AliasSet. Since the hash table already has entries for each
598LLVM Value* of interest, the AliasesSets thread the linked list through these
599hash-table nodes to avoid having to allocate memory unnecessarily, and to make
600merging alias sets extremely efficient (the linked list merge is constant time).
601</p>
602
603<p>You shouldn't need to understand these details if you are just a client of
604the AliasSetTracker, but if you look at the code, hopefully this brief
605description will help make sense of why things are designed the way they
606are.</p>
607
608</div>
609
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]610<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]611<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]612 <a name="direct">Using the <tt>AliasAnalysis</tt> interface directly</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]613</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]614
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]615<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]616
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]617<p>If neither of these utility class are what your pass needs, you should use
618the interfaces exposed by the <tt>AliasAnalysis</tt> class directly. Try to use
619the higher-level methods when possible (e.g., use mod/ref information instead of
620the <a href="#alias"><tt>alias</tt></a> method directly if possible) to get the
621best precision and efficiency.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]622
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]623</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]624
625<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]626<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]627 <a name="exist">Existing alias analysis implementations and clients</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]628</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]629<!-- *********************************************************************** -->
630
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]631<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]632
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]633<p>If you're going to be working with the LLVM alias analysis infrastructure,
634you should know what clients and implementations of alias analysis are
635available. In particular, if you are implementing an alias analysis, you should
636be aware of the <a href="#aliasanalysis-debug">the clients</a> that are useful
637for monitoring and evaluating different implementations.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]638
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]639</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]640
641<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]642<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]643 <a name="impls">Available <tt>AliasAnalysis</tt> implementations</a>
644</div>
645
646<div class="doc_text">
647
648<p>This section lists the various implementations of the <tt>AliasAnalysis</tt>
649interface. With the exception of the <a href="#no-aa"><tt>-no-aa</tt></a> and
650<a href="#basic-aa"><tt>-basicaa</tt></a> implementations, all of these <a
Chris Lattnerfcd37252004-06-21 22:52:48 +0000[diff] [blame]651href="#chaining">chain</a> to other alias analysis implementations.</p>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]652
653</div>
654
655<!-- _______________________________________________________________________ -->
656<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]657 <a name="no-aa">The <tt>-no-aa</tt> pass</a>
658</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]659
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]660<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]661
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]662<p>The <tt>-no-aa</tt> pass is just like what it sounds: an alias analysis that
663never returns any useful information. This pass can be useful if you think that
664alias analysis is doing something wrong and are trying to narrow down a
665problem.</p>
666
667</div>
668
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]669<!-- _______________________________________________________________________ -->
670<div class="doc_subsubsection">
671 <a name="basic-aa">The <tt>-basicaa</tt> pass</a>
672</div>
673
674<div class="doc_text">
675
676<p>The <tt>-basicaa</tt> pass is the default LLVM alias analysis. It is an
677aggressive local analysis that "knows" many important facts:</p>
678
679<ul>
680<li>Distinct globals, stack allocations, and heap allocations can never
681 alias.</li>
682<li>Globals, stack allocations, and heap allocations never alias the null
683 pointer.</li>
684<li>Different fields of a structure do not alias.</li>
685<li>Indexes into arrays with statically differing subscripts cannot alias.</li>
686<li>Many common standard C library functions <a
687 href="#simplemodref">never access memory or only read memory</a>.</li>
688<li>Pointers that obviously point to constant globals
689 "<tt>pointToConstantMemory</tt>".</li>
690<li>Function calls can not modify or references stack allocations if they never
691 escape from the function that allocates them (a common case for automatic
692 arrays).</li>
693</ul>
694
695</div>
696
Chris Lattner100a4f82004-06-28 19:19:47 +0000[diff] [blame]697<!-- _______________________________________________________________________ -->
698<div class="doc_subsubsection">
699 <a name="globalsmodref">The <tt>-globalsmodref-aa</tt> pass</a>
700</div>
701
702<div class="doc_text">
703
704<p>This pass implements a simple context-sensitive mod/ref and alias analysis
Chris Lattnerccb354b2004-07-27 07:50:07 +0000[diff] [blame]705for internal global variables that don't "have their address taken". If a
706global does not have its address taken, the pass knows that no pointers alias
707the global. This pass also keeps track of functions that it knows never access
708memory or never read memory. This allows certain optimizations (e.g. GCSE) to
709eliminate call instructions entirely.
Chris Lattner100a4f82004-06-28 19:19:47 +0000[diff] [blame]710</p>
711
712<p>The real power of this pass is that it provides context-sensitive mod/ref
713information for call instructions. This allows the optimizer to know that
714calls to a function do not clobber or read the value of the global, allowing
715loads and stores to be eliminated.</p>
716
717<p>Note that this pass is somewhat limited in its scope (only support
718non-address taken globals), but is very quick analysis.</p>
719</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]720
721<!-- _______________________________________________________________________ -->
722<div class="doc_subsubsection">
723 <a name="anders-aa">The <tt>-anders-aa</tt> pass</a>
724</div>
725
726<div class="doc_text">
727
728<p>The <tt>-anders-aa</tt> pass implements the well-known "Andersen's algorithm"
729for interprocedural alias analysis. This algorithm is a subset-based,
730flow-insensitive, context-insensitive, and field-insensitive alias analysis that
731is widely believed to be fairly precise. Unfortunately, this algorithm is also
732O(N<sup>3</sup>). The LLVM implementation currently does not implement any of
733the refinements (such as "online cycle elimination" or "offline variable
734substitution") to improve its efficiency, so it can be quite slow in common
735cases.
736</p>
737
738</div>
739
740<!-- _______________________________________________________________________ -->
741<div class="doc_subsubsection">
742 <a name="steens-aa">The <tt>-steens-aa</tt> pass</a>
743</div>
744
745<div class="doc_text">
746
747<p>The <tt>-steens-aa</tt> pass implements a variation on the well-known
748"Steensgaard's algorithm" for interprocedural alias analysis. Steensgaard's
749algorithm is a unification-based, flow-insensitive, context-insensitive, and
750field-insensitive alias analysis that is also very scalable (effectively linear
751time).</p>
752
753<p>The LLVM <tt>-steens-aa</tt> pass implements a "speculatively
754field-<b>sensitive</b>" version of Steensgaard's algorithm using the Data
755Structure Analysis framework. This gives it substantially more precision than
756the standard algorithm while maintaining excellent analysis scalability.</p>
757
758</div>
759
760<!-- _______________________________________________________________________ -->
761<div class="doc_subsubsection">
762 <a name="ds-aa">The <tt>-ds-aa</tt> pass</a>
763</div>
764
765<div class="doc_text">
766
767<p>The <tt>-ds-aa</tt> pass implements the full Data Structure Analysis
768algorithm. Data Structure Analysis is a modular unification-based,
769flow-insensitive, context-<b>sensitive</b>, and speculatively
770field-<b>sensitive</b> alias analysis that is also quite scalable, usually at
771O(n*log(n)).</p>
772
773<p>This algorithm is capable of responding to a full variety of alias analysis
774queries, and can provide context-sensitive mod/ref information as well. The
775only major facility not implemented so far is support for must-alias
776information.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]777
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]778</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]779
780
781<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]782<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]783 <a name="aliasanalysis-xforms">Alias analysis driven transformations</a>
784</div>
785
786<div class="doc_text">
787LLVM includes several alias-analysis driven transformations which can be used
788with any of the implementations above.
789</div>
790
791<!-- _______________________________________________________________________ -->
792<div class="doc_subsubsection">
793 <a name="adce">The <tt>-adce</tt> pass</a>
794</div>
795
796<div class="doc_text">
797
798<p>The <tt>-adce</tt> pass, which implements Aggressive Dead Code Elimination
799uses the <tt>AliasAnalysis</tt> interface to delete calls to functions that do
800not have side-effects and are not used.</p>
801
802</div>
803
804
805<!-- _______________________________________________________________________ -->
806<div class="doc_subsubsection">
807 <a name="licm">The <tt>-licm</tt> pass</a>
808</div>
809
810<div class="doc_text">
811
812<p>The <tt>-licm</tt> pass implements various Loop Invariant Code Motion related
813transformations. It uses the <tt>AliasAnalysis</tt> interface for several
814different transformations:</p>
815
816<ul>
817<li>It uses mod/ref information to hoist or sink load instructions out of loops
818if there are no instructions in the loop that modifies the memory loaded.</li>
819
820<li>It uses mod/ref information to hoist function calls out of loops that do not
821write to memory and are loop-invariant.</li>
822
823<li>If uses alias information to promote memory objects that are loaded and
824stored to in loops to live in a register instead. It can do this if there are
825no may aliases to the loaded/stored memory location.</li>
826</ul>
827
828</div>
829
830<!-- _______________________________________________________________________ -->
831<div class="doc_subsubsection">
832 <a name="argpromotion">The <tt>-argpromotion</tt> pass</a>
833</div>
834
835<div class="doc_text">
836<p>
837The <tt>-argpromotion</tt> pass promotes by-reference arguments to be passed in
838by-value instead. In particular, if pointer arguments are only loaded from it
839passes in the value loaded instead of the address to the function. This pass
840uses alias information to make sure that the value loaded from the argument
841pointer is not modified between the entry of the function and any load of the
842pointer.</p>
843</div>
844
845<!-- _______________________________________________________________________ -->
846<div class="doc_subsubsection">
847 <a name="gcseloadvn">The <tt>-load-vn</tt> &amp; <tt>-gcse</tt> passes</a>
848</div>
849
850<div class="doc_text">
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]851
852<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]853number</a>" loads and pointers values, which is used by the GCSE pass to
854eliminate instructions. The <tt>-load-vn</tt> pass relies on alias information
855and must-alias information. This combination of passes can make the following
856transformations:</p>
857
858<ul>
859<li>Redundant load instructions are eliminated.</li>
860<li>Load instructions that follow a store to the same location are replaced with
861the stored value ("store forwarding").</li>
862<li>Pointers values (e.g. formal arguments) that must-alias simpler expressions
863(e.g. global variables or the null pointer) are replaced. Note that this
864implements transformations like "virtual method resolution", turning indirect
865calls into direct calls.</li>
866</ul>
867
868</div>
869
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]870<!-- ======================================================================= -->
871<div class="doc_subsection">
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]872 <a name="aliasanalysis-debug">Clients for debugging and evaluation of
873 implementations</a>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]874</div>
875
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]876<div class="doc_text">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]877
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]878<p>These passes are useful for evaluating the various alias analysis
879implementations. You can use them with commands like '<tt>opt -anders-aa -ds-aa
880-aa-eval foo.bc -disable-output -stats</tt>'.</p>
881
882</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]883
884<!-- _______________________________________________________________________ -->
885<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]886 <a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
887</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]888
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]889<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]890
891<p>The <tt>-print-alias-sets</tt> pass is exposed as part of the
Chris Lattner05d5c9d2006-01-03 06:04:48 +0000[diff] [blame]892<tt>opt</tt> tool to print out the Alias Sets formed by the <a
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]893href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
Chris Lattner05d5c9d2006-01-03 06:04:48 +0000[diff] [blame]894the <tt>AliasSetTracker</tt> class. To use it, use something like:</p>
895
896<div class="doc_code">
897<pre>
898% opt -ds-aa -print-alias-sets -disable-output
899</pre>
900</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]901
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]902</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]903
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]904
905<!-- _______________________________________________________________________ -->
906<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]907 <a name="count-aa">The <tt>-count-aa</tt> pass</a>
908</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]909
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]910<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]911
912<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]913pass is making and what responses are returned by the alias analysis. As an
914example,</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]915
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]916<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]917<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]918% opt -basicaa -count-aa -ds-aa -count-aa -licm
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]919</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]920</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]921
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]922<p>will print out how many queries (and what responses are returned) by the
923<tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are made
924of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be useful
925when debugging a transformation or an alias analysis implementation.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]926
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]927</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]928
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]929<!-- _______________________________________________________________________ -->
930<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]931 <a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
932</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]933
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]934<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]935
936<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]937function and asks an alias analysis whether or not the pointers alias. This
938gives an indication of the precision of the alias analysis. Statistics are
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]939printed indicating the percent of no/may/must aliases found (a more precise
940algorithm will have a lower number of may aliases).</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]941
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]942</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]943
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]944<!-- *********************************************************************** -->
945
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]946<hr>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]947<address>
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Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]952
953 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame^]954 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]955 Last modified: $Date$
956</address>
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