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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
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]137<pre>
138 int i;
139 char C[2];
140 char A[10];
141 /* ... */
142 for (i = 0; i != 10; ++i) {
143 C[0] = A[i]; /* One byte store */
144 C[1] = A[9-i]; /* One byte store */
145 }
146</pre>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]147
148<p>In this case, the <tt>basicaa</tt> pass will disambiguate the stores to
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]149<tt>C[0]</tt> and <tt>C[1]</tt> because they are accesses to two distinct
150locations one byte apart, and the accesses are each one byte. In this case, the
151LICM pass can use store motion to remove the stores from the loop. In
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]152constrast, the following code:</p>
153
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]154<pre>
155 int i;
156 char C[2];
157 char A[10];
158 /* ... */
159 for (i = 0; i != 10; ++i) {
160 ((short*)C)[0] = A[i]; /* Two byte store! */
161 C[1] = A[9-i]; /* One byte store */
162 }
163</pre>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]164
165<p>In this case, the two stores to C do alias each other, because the access to
166the <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]167available in the query, even the first case would have to conservatively assume
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]168that the accesses alias.</p>
169
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]170</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]171
172<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]173<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]174 <a name="alias">The <tt>alias</tt> method</a>
175</div>
176
177<div class="doc_text">
178The <tt>alias</tt> method is the primary interface used to determine whether or
179not two memory objects alias each other. It takes two memory objects as input
180and returns MustAlias, MayAlias, or NoAlias as appropriate.
181</div>
182
183<!-- _______________________________________________________________________ -->
184<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]185 <a name="MustMayNo">Must, May, and No Alias Responses</a>
186</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]187
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]188<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]189
190<p>An Alias Analysis implementation can return one of three responses:
191MustAlias, MayAlias, and NoAlias. The No and May alias results are obvious: if
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]192the two pointers can never equal each other, return NoAlias, if they might,
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]193return MayAlias.</p>
194
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]195<p>The MustAlias response is trickier though. In LLVM, the Must Alias response
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]196may only be returned if the two memory objects are guaranteed to always start at
197exactly the same location. If two memory objects overlap, but do not start at
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]198the same location, return MayAlias.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]199
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]200</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]201
202<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]203<div class="doc_subsection">
204 <a name="ModRefInfo">The <tt>getModRefInfo</tt> methods</a>
205</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]206
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]207<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]208
209<p>The <tt>getModRefInfo</tt> methods return information about whether the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]210execution of an instruction can read or modify a memory location. Mod/Ref
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]211information is always conservative: if an instruction <b>might</b> read or write
212a location, ModRef is returned.</p>
213
214<p>The <tt>AliasAnalysis</tt> class also provides a <tt>getModRefInfo</tt>
215method for testing dependencies between function calls. This method takes two
216call sites (CS1 &amp; CS2), returns NoModRef if the two calls refer to disjoint
217memory locations, Ref if CS1 reads memory written by CS2, Mod if CS1 writes to
218memory read or written by CS2, or ModRef if CS1 might read or write memory
219accessed by CS2. Note that this relation is not commutative. Clients that use
220this method should be predicated on the <tt>hasNoModRefInfoForCalls()</tt>
221method, which indicates whether or not an analysis can provide mod/ref
222information for function call pairs (most can not). If this predicate is false,
223the client shouldn't waste analysis time querying the <tt>getModRefInfo</tt>
224method many times.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]225
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]226</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]227
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]228
229<!-- ======================================================================= -->
230<div class="doc_subsection">
231 <a name="OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a>
232</div>
233
234<div class="doc_text">
235
236<p>
237Several other tidbits of information are often collected by various alias
238analysis implementations and can be put to good use by various clients.
239</p>
240
241</div>
242
243<!-- _______________________________________________________________________ -->
244<div class="doc_subsubsection">
245 The <tt>getMustAliases</tt> method
246</div>
247
248<div class="doc_text">
249
250<p>The <tt>getMustAliases</tt> method returns all values that are known to
251always must alias a pointer. This information can be provided in some cases for
252important objects like the null pointer and global values. Knowing that a
253pointer always points to a particular function allows indirect calls to be
254turned into direct calls, for example.</p>
255
256</div>
257
258<!-- _______________________________________________________________________ -->
259<div class="doc_subsubsection">
260 The <tt>pointsToConstantMemory</tt> method
261</div>
262
263<div class="doc_text">
264
265<p>The <tt>pointsToConstantMemory</tt> method returns true if and only if the
266analysis can prove that the pointer only points to unchanging memory locations
267(functions, constant global variables, and the null pointer). This information
268can be used to refine mod/ref information: it is impossible for an unchanging
269memory location to be modified.</p>
270
271</div>
272
273
274<!-- _______________________________________________________________________ -->
275<div class="doc_subsubsection">
276 <a name="simplemodref">The <tt>doesNotAccessMemory</tt> and
277 <tt>onlyReadsMemory</tt> methods</a>
278</div>
279
280<div class="doc_text">
281
282<p>These methods are used to provide very simple mod/ref information for
283function calls. The <tt>doesNotAccessMemory</tt> method returns true for a
284function if the analysis can prove that the function never reads or writes to
285memory, or if the function only reads from constant memory. Functions with this
286property are side-effect free and only depend on their input arguments, allowing
287them to be eliminated if they form common subexpressions or be hoisted out of
288loops. Many common functions behave this way (e.g., <tt>sin</tt> and
289<tt>cos</tt>) but many others do not (e.g., <tt>acos</tt>, which modifies the
290<tt>errno</tt> variable).</p>
291
292<p>The <tt>onlyReadsMemory</tt> method returns true for a function if analysis
293can prove that (at most) the function only reads from non-volatile memory.
294Functions with this property are side-effect free, only depending on their input
295arguments and the state of memory when they are called. This property allows
296calls to these functions to be eliminated and moved around, as long as there is
297no store instruction that changes the contents of memory. Note that all
298functions that satisfy the <tt>doesNotAccessMemory</tt> method also satisfies
299<tt>onlyReadsMemory</tt>.</p>
300
301</div>
302
303
304
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]305<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]306<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]307 <a name="writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]308</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]309<!-- *********************************************************************** -->
310
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]311<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]312
313<p>Writing a new alias analysis implementation for LLVM is quite
314straight-forward. There are already several implementations that you can use
315for examples, and the following information should help fill in any details.
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]316For a examples, take a look at the <a href="#impls">various alias analysis
317implementations</a> included with LLVM.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]318
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]319</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]320
321<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]322<div class="doc_subsection">
323 <a name="passsubclasses">Different Pass styles</a>
324</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]325
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]326<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]327
328<p>The first step to determining what type of <a
329href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias
330Analysis. As is the case with most other analyses and transformations, the
331answer should be fairly obvious from what type of problem you are trying to
332solve:</p>
333
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]334<ol>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]335 <li>If you require interprocedural analysis, it should be a
336 <tt>Pass</tt>.</li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]337 <li>If you are a function-local analysis, subclass <tt>FunctionPass</tt>.</li>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]338 <li>If you don't need to look at the program at all, subclass
339 <tt>ImmutablePass</tt>.</li>
340</ol>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]341
342<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]343<tt>AliasAnalysis</tt> interface, of course, and use the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]344<tt>RegisterAnalysisGroup</tt> template to register as an implementation of
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]345<tt>AliasAnalysis</tt>.</p>
346
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]347</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]348
349<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]350<div class="doc_subsection">
351 <a name="requiredcalls">Required initialization calls</a>
352</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]353
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]354<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]355
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]356<p>Your subclass of <tt>AliasAnalysis</tt> is required to invoke two methods on
357the <tt>AliasAnalysis</tt> base class: <tt>getAnalysisUsage</tt> and
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]358<tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
359<tt>getAnalysisUsage</tt> should explicitly call into the
360<tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
361declaring any pass dependencies your pass has. Thus you should have something
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]362like this:</p>
363
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]364<pre>
365 void getAnalysisUsage(AnalysisUsage &amp;AU) const {
366 AliasAnalysis::getAnalysisUsage(AU);
367 <i>// declare your dependencies here.</i>
368 }
369</pre>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]370
371<p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
372from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]373<tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, or <tt>InitializePass</tt>
374for an <tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]375
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]376<pre>
377 bool run(Module &amp;M) {
378 InitializeAliasAnalysis(this);
379 <i>// Perform analysis here...</i>
380 return false;
381 }
382</pre>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]383
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]384</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]385
386<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]387<div class="doc_subsection">
388 <a name="interfaces">Interfaces which may be specified</a>
389</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]390
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]391<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]392
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]393<p>All of the <a
394href="/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
395virtual methods default to providing <a href="#chaining">chaining</a> to another
396alias analysis implementation, which ends up returning conservatively correct
397information (returning "May" Alias and "Mod/Ref" for alias and mod/ref queries
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]398respectively). Depending on the capabilities of the analysis you are
399implementing, you just override the interfaces you can improve.</p>
400
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]401</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]402
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]403
404
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]405<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]406<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]407 <a name="chaining"><tt>AliasAnalysis</tt> chaining behavior</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]408</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]409
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]410<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]411
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]412<p>With only two special exceptions (the <tt><a
413href="#basic-aa">basicaa</a></tt> and <a href="#no-aa"><tt>no-aa</tt></a>
414passes) every alias analysis pass chains to another alias analysis
415implementation (for example, the user can specify "<tt>-basicaa -ds-aa
416-anders-aa -licm</tt>" to get the maximum benefit from the three alias
417analyses). The alias analysis class automatically takes care of most of this
418for methods that you don't override. For methods that you do override, in code
419paths that return a conservative MayAlias or Mod/Ref result, simply return
420whatever the superclass computes. For example:</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]421
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]422<pre>
423 AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
424 const Value *V2, unsigned V2Size) {
425 if (...)
426 return NoAlias;
427 ...
428
429 <i>// Couldn't determine a must or no-alias result.</i>
430 return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
431 }
432</pre>
433
434<p>In addition to analysis queries, you must make sure to unconditionally pass
435LLVM <a href="#updating">update notification</a> methods to the superclass as
436well if you override them, which allows all alias analyses in a change to be
437updated.</p>
438
439</div>
440
441
442<!-- ======================================================================= -->
443<div class="doc_subsection">
444 <a name="updating">Updating analysis results for transformations</a>
445</div>
446
447<div class="doc_text">
448<p>
449Alias analysis information is initially computed for a static snapshot of the
450program, but clients will use this information to make transformations to the
451code. All but the most trivial forms of alias analysis will need to have their
452analysis results updated to reflect the changes made by these transformations.
453</p>
454
455<p>
456The <tt>AliasAnalysis</tt> interface exposes two methods which are used to
457communicate program changes from the clients to the analysis implementations.
458Various alias analysis implementations should use these methods to ensure that
459their internal data structures are kept up-to-date as the program changes (for
460example, when an instruction is deleted), and clients of alias analysis must be
461sure to call these interfaces appropriately.
462</p>
463</div>
464
465<!-- _______________________________________________________________________ -->
466<div class="doc_subsubsection">The <tt>deleteValue</tt> method</div>
467
468<div class="doc_text">
469The <tt>deleteValue</tt> method is called by transformations when they remove an
470instruction or any other value from the program (including values that do not
471use pointers). Typically alias analyses keep data structures that have entries
472for each value in the program. When this method is called, they should remove
473any entries for the specified value, if they exist.
474</div>
475
476
477<!-- _______________________________________________________________________ -->
478<div class="doc_subsubsection">The <tt>copyValue</tt> method</div>
479
480<div class="doc_text">
481The <tt>copyValue</tt> method is used when a new value is introduced into the
482program. There is no way to introduce a value into the program that did not
483exist before (this doesn't make sense for a safe compiler transformation), so
484this is the only way to introduce a new value. This method indicates that the
485new value has exactly the same properties as the value being copied.
486</div>
487
488
489<!-- _______________________________________________________________________ -->
490<div class="doc_subsubsection">The <tt>replaceWithNewValue</tt> method</div>
491
492<div class="doc_text">
493This method is a simple helper method that is provided to make clients easier to
494use. It is implemented by copying the old analysis information to the new
495value, then deleting the old value. This method cannot be overridden by alias
496analysis implementations.
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]497</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]498
499<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]500<div class="doc_subsection">
501 <a name="implefficiency">Efficiency Issues</a>
502</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]503
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]504<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]505
506<p>From the LLVM perspective, the only thing you need to do to provide an
507efficient alias analysis is to make sure that alias analysis <b>queries</b> are
508serviced quickly. The actual calculation of the alias analysis results (the
509"run" method) is only performed once, but many (perhaps duplicate) queries may
510be performed. Because of this, try to move as much computation to the run
511method as possible (within reason).</p>
512
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]513</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]514
515<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]516<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]517 <a name="using">Using alias analysis results</a>
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_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]522
523<p>There are several different ways to use alias analysis results. In order of
524preference, these are...</p>
525
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_subsection">
530 <a name="loadvn">Using the <tt>-load-vn</tt> Pass</a>
531</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]532
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]533<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]534
535<p>The <tt>load-vn</tt> pass uses alias analysis to provide value numbering
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]536information for <tt>load</tt> instructions and pointer values. If your analysis
537or transformation can be modeled in a form that uses value numbering
538information, you don't have to do anything special to handle load instructions:
539just use the <tt>load-vn</tt> pass, which uses alias analysis.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]540
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]541</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]542
543<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]544<div class="doc_subsection">
545 <a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
546</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]547
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]548<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]549
550<p>Many transformations need information about alias <b>sets</b> that are active
551in some scope, rather than information about pairwise aliasing. The <tt><a
Misha Brukmane12e5f32003-12-17 23:10:49 +0000[diff] [blame]552href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class is used
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]553to efficiently build these Alias Sets from the pairwise alias analysis
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]554information provided by the <tt>AliasAnalysis</tt> interface.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]555
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]556<p>First you initialize the AliasSetTracker by using the "<tt>add</tt>" methods
557to add information about various potentially aliasing instructions in the scope
558you are interested in. Once all of the alias sets are completed, your pass
559should simply iterate through the constructed alias sets, using the
560<tt>AliasSetTracker</tt> <tt>begin()</tt>/<tt>end()</tt> methods.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]561
562<p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]563to be disjoint, calculate mod/ref information and volatility for the set, and
564keep track of whether or not all of the pointers in the set are Must aliases.
565The AliasSetTracker also makes sure that sets are properly folded due to call
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]566instructions, and can provide a list of pointers in each set.</p>
567
568<p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]569Invariant Code Motion</a> pass uses <tt>AliasSetTracker</tt>s to calculate alias
570sets for each loop nest. If an <tt>AliasSet</tt> in a loop is not modified,
571then all load instructions from that set may be hoisted out of the loop. If any
572alias sets are stored to <b>and</b> are must alias sets, then the stores may be
573sunk to outside of the loop, promoting the memory location to a register for the
574duration of the loop nest. Both of these transformations only apply if the
575pointer argument is loop-invariant.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]576
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]577</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]578
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]579<!-- _______________________________________________________________________ -->
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]580<div class="doc_subsubsection">
581 The AliasSetTracker implementation
582</div>
583
584<div class="doc_text">
585
586<p>The AliasSetTracker class is implemented to be as efficient as possible. It
587uses the union-find algorithm to efficiently merge AliasSets when a pointer is
588inserted into the AliasSetTracker that aliases multiple sets. The primary data
589structure is a hash table mapping pointers to the AliasSet they are in.</p>
590
591<p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s
592that are in each AliasSet. Since the hash table already has entries for each
593LLVM Value* of interest, the AliasesSets thread the linked list through these
594hash-table nodes to avoid having to allocate memory unnecessarily, and to make
595merging alias sets extremely efficient (the linked list merge is constant time).
596</p>
597
598<p>You shouldn't need to understand these details if you are just a client of
599the AliasSetTracker, but if you look at the code, hopefully this brief
600description will help make sense of why things are designed the way they
601are.</p>
602
603</div>
604
605
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]606<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]607<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]608 <a name="direct">Using the <tt>AliasAnalysis</tt> interface directly</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]609</div>
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_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]612
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]613<p>If neither of these utility class are what your pass needs, you should use
614the interfaces exposed by the <tt>AliasAnalysis</tt> class directly. Try to use
615the higher-level methods when possible (e.g., use mod/ref information instead of
616the <a href="#alias"><tt>alias</tt></a> method directly if possible) to get the
617best precision and efficiency.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]618
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]619</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]620
621<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]622<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]623 <a name="exist">Existing alias analysis implementations and clients</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]624</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]625<!-- *********************************************************************** -->
626
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]627<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]628
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]629<p>If you're going to be working with the LLVM alias analysis infrastructure,
630you should know what clients and implementations of alias analysis are
631available. In particular, if you are implementing an alias analysis, you should
632be aware of the <a href="#aliasanalysis-debug">the clients</a> that are useful
633for monitoring and evaluating different implementations.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]634
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]635</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]636
637<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]638<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]639 <a name="impls">Available <tt>AliasAnalysis</tt> implementations</a>
640</div>
641
642<div class="doc_text">
643
644<p>This section lists the various implementations of the <tt>AliasAnalysis</tt>
645interface. With the exception of the <a href="#no-aa"><tt>-no-aa</tt></a> and
646<a href="#basic-aa"><tt>-basicaa</tt></a> implementations, all of these <a
Chris Lattnerfcd37252004-06-21 22:52:48 +0000[diff] [blame]647href="#chaining">chain</a> to other alias analysis implementations.</p>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]648
649</div>
650
651<!-- _______________________________________________________________________ -->
652<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]653 <a name="no-aa">The <tt>-no-aa</tt> pass</a>
654</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]655
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]656<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]657
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]658<p>The <tt>-no-aa</tt> pass is just like what it sounds: an alias analysis that
659never returns any useful information. This pass can be useful if you think that
660alias analysis is doing something wrong and are trying to narrow down a
661problem.</p>
662
663</div>
664
665
666<!-- _______________________________________________________________________ -->
667<div class="doc_subsubsection">
668 <a name="basic-aa">The <tt>-basicaa</tt> pass</a>
669</div>
670
671<div class="doc_text">
672
673<p>The <tt>-basicaa</tt> pass is the default LLVM alias analysis. It is an
674aggressive local analysis that "knows" many important facts:</p>
675
676<ul>
677<li>Distinct globals, stack allocations, and heap allocations can never
678 alias.</li>
679<li>Globals, stack allocations, and heap allocations never alias the null
680 pointer.</li>
681<li>Different fields of a structure do not alias.</li>
682<li>Indexes into arrays with statically differing subscripts cannot alias.</li>
683<li>Many common standard C library functions <a
684 href="#simplemodref">never access memory or only read memory</a>.</li>
685<li>Pointers that obviously point to constant globals
686 "<tt>pointToConstantMemory</tt>".</li>
687<li>Function calls can not modify or references stack allocations if they never
688 escape from the function that allocates them (a common case for automatic
689 arrays).</li>
690</ul>
691
692</div>
693
Chris Lattner100a4f82004-06-28 19:19:47 +0000[diff] [blame]694<!-- _______________________________________________________________________ -->
695<div class="doc_subsubsection">
696 <a name="globalsmodref">The <tt>-globalsmodref-aa</tt> pass</a>
697</div>
698
699<div class="doc_text">
700
701<p>This pass implements a simple context-sensitive mod/ref and alias analysis
Chris Lattnerccb354b2004-07-27 07:50:07 +0000[diff] [blame]702for internal global variables that don't "have their address taken". If a
703global does not have its address taken, the pass knows that no pointers alias
704the global. This pass also keeps track of functions that it knows never access
705memory or never read memory. This allows certain optimizations (e.g. GCSE) to
706eliminate call instructions entirely.
Chris Lattner100a4f82004-06-28 19:19:47 +0000[diff] [blame]707</p>
708
709<p>The real power of this pass is that it provides context-sensitive mod/ref
710information for call instructions. This allows the optimizer to know that
711calls to a function do not clobber or read the value of the global, allowing
712loads and stores to be eliminated.</p>
713
714<p>Note that this pass is somewhat limited in its scope (only support
715non-address taken globals), but is very quick analysis.</p>
716</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]717
718<!-- _______________________________________________________________________ -->
719<div class="doc_subsubsection">
720 <a name="anders-aa">The <tt>-anders-aa</tt> pass</a>
721</div>
722
723<div class="doc_text">
724
725<p>The <tt>-anders-aa</tt> pass implements the well-known "Andersen's algorithm"
726for interprocedural alias analysis. This algorithm is a subset-based,
727flow-insensitive, context-insensitive, and field-insensitive alias analysis that
728is widely believed to be fairly precise. Unfortunately, this algorithm is also
729O(N<sup>3</sup>). The LLVM implementation currently does not implement any of
730the refinements (such as "online cycle elimination" or "offline variable
731substitution") to improve its efficiency, so it can be quite slow in common
732cases.
733</p>
734
735</div>
736
737<!-- _______________________________________________________________________ -->
738<div class="doc_subsubsection">
739 <a name="steens-aa">The <tt>-steens-aa</tt> pass</a>
740</div>
741
742<div class="doc_text">
743
744<p>The <tt>-steens-aa</tt> pass implements a variation on the well-known
745"Steensgaard's algorithm" for interprocedural alias analysis. Steensgaard's
746algorithm is a unification-based, flow-insensitive, context-insensitive, and
747field-insensitive alias analysis that is also very scalable (effectively linear
748time).</p>
749
750<p>The LLVM <tt>-steens-aa</tt> pass implements a "speculatively
751field-<b>sensitive</b>" version of Steensgaard's algorithm using the Data
752Structure Analysis framework. This gives it substantially more precision than
753the standard algorithm while maintaining excellent analysis scalability.</p>
754
755</div>
756
757<!-- _______________________________________________________________________ -->
758<div class="doc_subsubsection">
759 <a name="ds-aa">The <tt>-ds-aa</tt> pass</a>
760</div>
761
762<div class="doc_text">
763
764<p>The <tt>-ds-aa</tt> pass implements the full Data Structure Analysis
765algorithm. Data Structure Analysis is a modular unification-based,
766flow-insensitive, context-<b>sensitive</b>, and speculatively
767field-<b>sensitive</b> alias analysis that is also quite scalable, usually at
768O(n*log(n)).</p>
769
770<p>This algorithm is capable of responding to a full variety of alias analysis
771queries, and can provide context-sensitive mod/ref information as well. The
772only major facility not implemented so far is support for must-alias
773information.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]774
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]775</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]776
777
778<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]779<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]780 <a name="aliasanalysis-xforms">Alias analysis driven transformations</a>
781</div>
782
783<div class="doc_text">
784LLVM includes several alias-analysis driven transformations which can be used
785with any of the implementations above.
786</div>
787
788<!-- _______________________________________________________________________ -->
789<div class="doc_subsubsection">
790 <a name="adce">The <tt>-adce</tt> pass</a>
791</div>
792
793<div class="doc_text">
794
795<p>The <tt>-adce</tt> pass, which implements Aggressive Dead Code Elimination
796uses the <tt>AliasAnalysis</tt> interface to delete calls to functions that do
797not have side-effects and are not used.</p>
798
799</div>
800
801
802<!-- _______________________________________________________________________ -->
803<div class="doc_subsubsection">
804 <a name="licm">The <tt>-licm</tt> pass</a>
805</div>
806
807<div class="doc_text">
808
809<p>The <tt>-licm</tt> pass implements various Loop Invariant Code Motion related
810transformations. It uses the <tt>AliasAnalysis</tt> interface for several
811different transformations:</p>
812
813<ul>
814<li>It uses mod/ref information to hoist or sink load instructions out of loops
815if there are no instructions in the loop that modifies the memory loaded.</li>
816
817<li>It uses mod/ref information to hoist function calls out of loops that do not
818write to memory and are loop-invariant.</li>
819
820<li>If uses alias information to promote memory objects that are loaded and
821stored to in loops to live in a register instead. It can do this if there are
822no may aliases to the loaded/stored memory location.</li>
823</ul>
824
825</div>
826
827<!-- _______________________________________________________________________ -->
828<div class="doc_subsubsection">
829 <a name="argpromotion">The <tt>-argpromotion</tt> pass</a>
830</div>
831
832<div class="doc_text">
833<p>
834The <tt>-argpromotion</tt> pass promotes by-reference arguments to be passed in
835by-value instead. In particular, if pointer arguments are only loaded from it
836passes in the value loaded instead of the address to the function. This pass
837uses alias information to make sure that the value loaded from the argument
838pointer is not modified between the entry of the function and any load of the
839pointer.</p>
840</div>
841
842<!-- _______________________________________________________________________ -->
843<div class="doc_subsubsection">
844 <a name="gcseloadvn">The <tt>-load-vn</tt> &amp; <tt>-gcse</tt> passes</a>
845</div>
846
847<div class="doc_text">
848<p>
849The <tt>-load-vn</tt> pass uses alias analysis to "<a href="#loadvn">value
850number</a>" loads and pointers values, which is used by the GCSE pass to
851eliminate instructions. The <tt>-load-vn</tt> pass relies on alias information
852and must-alias information. This combination of passes can make the following
853transformations:</p>
854
855<ul>
856<li>Redundant load instructions are eliminated.</li>
857<li>Load instructions that follow a store to the same location are replaced with
858the stored value ("store forwarding").</li>
859<li>Pointers values (e.g. formal arguments) that must-alias simpler expressions
860(e.g. global variables or the null pointer) are replaced. Note that this
861implements transformations like "virtual method resolution", turning indirect
862calls into direct calls.</li>
863</ul>
864
865</div>
866
867
868
869
870<!-- ======================================================================= -->
871<div class="doc_subsection">
872 <a name="aliasanalysis-debug">Clients for debugging and evaluation of implementations</a>
873</div>
874
875These passes are useful for evaluating the various alias analysis
876implementations. You can use them with commands like '<tt>opt -anders-aa -ds-aa
877-aa-eval foo.bc -disable-output -stats</tt>'.
878
879
880<!-- _______________________________________________________________________ -->
881<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]882 <a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
883</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]884
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]885<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]886
887<p>The <tt>-print-alias-sets</tt> pass is exposed as part of the
888<tt>analyze</tt> tool to print out the Alias Sets formed by the <a
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]889href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]890the <tt>AliasSetTracker</tt> class.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]891
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]892</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]893
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]894
895<!-- _______________________________________________________________________ -->
896<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]897 <a name="count-aa">The <tt>-count-aa</tt> pass</a>
898</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]899
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]900<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]901
902<p>The <tt>-count-aa</tt> pass is useful to see how many queries a particular
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]903pass is making and what responses are returned by the alias analysis. An
904example usage is:</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]905
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]906<pre>
907 $ opt -basicaa -count-aa -ds-aa -count-aa -licm
908</pre>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]909
910<p>Which will print out how many queries (and what responses are returned) by
911the <tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are
912made of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]913useful when debugging a transformation or an alias analysis implementation.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]914
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]915</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]916
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]917<!-- _______________________________________________________________________ -->
918<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]919 <a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
920</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]921
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]922<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]923
924<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]925function and asks an alias analysis whether or not the pointers alias. This
926gives an indication of the precision of the alias analysis. Statistics are
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]927printed indicating the percent of no/may/must aliases found (a more precise
928algorithm will have a lower number of may aliases).</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]929
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]930</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]931
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]932<!-- *********************************************************************** -->
933
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]934<hr>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]935<address>
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941 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Misha Brukmanf6acb512004-05-12 18:02:00 +0000[diff] [blame]942 <a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]943 Last modified: $Date$
944</address>
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