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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>
Dan Gohman3f43dc32010-06-24 19:34:03 +0000[diff] [blame]34 <li><a href="#passmanager">Pass Manager Issues</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]35 </ul>
36 </li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]37
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]38 <li><a href="#using">Using alias analysis results</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]39 <ul>
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]40 <li><a href="#memdep">Using the <tt>MemoryDependenceAnalysis</tt> Pass</a></li>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]41 <li><a href="#ast">Using the <tt>AliasSetTracker</tt> class</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]42 <li><a href="#direct">Using the <tt>AliasAnalysis</tt> interface directly</a></li>
43 </ul>
44 </li>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]45
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]46 <li><a href="#exist">Existing alias analysis implementations and clients</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]47 <ul>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]48 <li><a href="#impls">Available <tt>AliasAnalysis</tt> implementations</a></li>
49 <li><a href="#aliasanalysis-xforms">Alias analysis driven transformations</a></li>
Misha Brukmana97e6cb2004-07-01 15:33:24 +0000[diff] [blame]50 <li><a href="#aliasanalysis-debug">Clients for debugging and evaluation of
51 implementations</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]52 </ul>
53 </li>
Owen Andersond8b47282007-10-02 00:44:20 +0000[diff] [blame]54 <li><a href="#memdep">Memory Dependence Analysis</a></li>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]55</ol>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]56
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]57<div class="doc_author">
58 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]59</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]60
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]61<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]62<div class="doc_section">
63 <a name="introduction">Introduction</a>
64</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]65<!-- *********************************************************************** -->
66
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]67<div class="doc_text">
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]68
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]69<p>Alias Analysis (aka Pointer Analysis) is a class of techniques which attempt
70to determine whether or not two pointers ever can point to the same object in
71memory. There are many different algorithms for alias analysis and many
72different ways of classifying them: flow-sensitive vs flow-insensitive,
73context-sensitive vs context-insensitive, field-sensitive vs field-insensitive,
74unification-based vs subset-based, etc. Traditionally, alias analyses respond
Duncan Sands8036ca42007-03-30 12:22:09 +0000[diff] [blame]75to a query with a <a href="#MustMayNo">Must, May, or No</a> alias response,
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]76indicating that two pointers always point to the same object, might point to the
77same object, or are known to never point to the same object.</p>
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]78
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]79<p>The LLVM <a
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]80href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]81class is the primary interface used by clients and implementations of alias
82analyses in the LLVM system. This class is the common interface between clients
83of alias analysis information and the implementations providing it, and is
84designed to support a wide range of implementations and clients (but currently
85all clients are assumed to be flow-insensitive). In addition to simple alias
86analysis information, this class exposes Mod/Ref information from those
87implementations which can provide it, allowing for powerful analyses and
88transformations to work well together.</p>
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]89
90<p>This document contains information necessary to successfully implement this
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]91interface, use it, and to test both sides. It also explains some of the finer
92points about what exactly results mean. If you feel that something is unclear
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]93or should be added, please <a href="mailto:sabre@nondot.org">let me
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]94know</a>.</p>
95
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]96</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]97
98<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]99<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]100 <a name="overview"><tt>AliasAnalysis</tt> Class Overview</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]101</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]102<!-- *********************************************************************** -->
103
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]104<div class="doc_text">
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]105
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]106<p>The <a
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]107href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]108class defines the interface that the various alias analysis implementations
109should support. This class exports two important enums: <tt>AliasResult</tt>
110and <tt>ModRefResult</tt> which represent the result of an alias query or a
111mod/ref query, respectively.</p>
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]112
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]113<p>The <tt>AliasAnalysis</tt> interface exposes information about memory,
114represented in several different ways. In particular, memory objects are
115represented as a starting address and size, and function calls are represented
116as the actual <tt>call</tt> or <tt>invoke</tt> instructions that performs the
117call. The <tt>AliasAnalysis</tt> interface also exposes some helper methods
118which allow you to get mod/ref information for arbitrary instructions.</p>
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]119
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]120</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]121
122<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]123<div class="doc_subsection">
124 <a name="pointers">Representation of Pointers</a>
125</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]126
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]127<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]128
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]129<p>Most importantly, the <tt>AliasAnalysis</tt> class provides several methods
130which are used to query whether or not two memory objects alias, whether
131function calls can modify or read a memory object, etc. For all of these
132queries, memory objects are represented as a pair of their starting address (a
133symbolic LLVM <tt>Value*</tt>) and a static size.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]134
135<p>Representing memory objects as a starting address and a size is critically
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]136important for correct Alias Analyses. For example, consider this (silly, but
137possible) C code:</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]138
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]139<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]140<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]141int i;
142char C[2];
143char A[10];
144/* ... */
145for (i = 0; i != 10; ++i) {
146 C[0] = A[i]; /* One byte store */
147 C[1] = A[9-i]; /* One byte store */
148}
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]149</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]150</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]151
152<p>In this case, the <tt>basicaa</tt> pass will disambiguate the stores to
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]153<tt>C[0]</tt> and <tt>C[1]</tt> because they are accesses to two distinct
154locations one byte apart, and the accesses are each one byte. In this case, the
155LICM pass can use store motion to remove the stores from the loop. In
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]156constrast, the following code:</p>
157
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]158<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]159<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]160int i;
161char C[2];
162char A[10];
163/* ... */
164for (i = 0; i != 10; ++i) {
165 ((short*)C)[0] = A[i]; /* Two byte store! */
166 C[1] = A[9-i]; /* One byte store */
167}
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]168</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]169</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]170
171<p>In this case, the two stores to C do alias each other, because the access to
172the <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]173available in the query, even the first case would have to conservatively assume
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]174that the accesses alias.</p>
175
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]176</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]177
178<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]179<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]180 <a name="alias">The <tt>alias</tt> method</a>
181</div>
182
183<div class="doc_text">
184The <tt>alias</tt> method is the primary interface used to determine whether or
185not two memory objects alias each other. It takes two memory objects as input
186and returns MustAlias, MayAlias, or NoAlias as appropriate.
187</div>
188
189<!-- _______________________________________________________________________ -->
190<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]191 <a name="MustMayNo">Must, May, and No Alias Responses</a>
192</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]193
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]194<div class="doc_text">
Nick Lewycky01557ce2008-12-14 21:08:48 +0000[diff] [blame]195<p>The NoAlias response is used when the two pointers refer to distinct objects,
Nick Lewycky78eb8042008-12-18 02:15:05 +0000[diff] [blame]196regardless of whether the pointers compare equal. For example, freed pointers
197don't alias any pointers that were allocated afterwards. As a degenerate case,
198pointers returned by malloc(0) have no bytes for an object, and are considered
199NoAlias even when malloc returns the same pointer. The same rule applies to
200NULL pointers.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]201
Nick Lewycky01557ce2008-12-14 21:08:48 +0000[diff] [blame]202<p>The MayAlias response is used whenever the two pointers might refer to the
203same object. If the two memory objects overlap, but do not start at the same
204location, return MayAlias.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]205
Nick Lewycky01557ce2008-12-14 21:08:48 +0000[diff] [blame]206<p>The MustAlias response may only be returned if the two memory objects are
207guaranteed to always start at exactly the same location. A MustAlias response
208implies that the pointers compare equal.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]209
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]210</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]211
212<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]213<div class="doc_subsection">
214 <a name="ModRefInfo">The <tt>getModRefInfo</tt> methods</a>
215</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]216
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]217<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]218
219<p>The <tt>getModRefInfo</tt> methods return information about whether the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]220execution of an instruction can read or modify a memory location. Mod/Ref
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]221information is always conservative: if an instruction <b>might</b> read or write
222a location, ModRef is returned.</p>
223
224<p>The <tt>AliasAnalysis</tt> class also provides a <tt>getModRefInfo</tt>
225method for testing dependencies between function calls. This method takes two
226call sites (CS1 &amp; CS2), returns NoModRef if the two calls refer to disjoint
227memory locations, Ref if CS1 reads memory written by CS2, Mod if CS1 writes to
228memory read or written by CS2, or ModRef if CS1 might read or write memory
Chris Lattner66e08cf2009-11-22 16:01:44 +0000[diff] [blame]229accessed by CS2. Note that this relation is not commutative.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]230
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]231</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]232
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]233
234<!-- ======================================================================= -->
235<div class="doc_subsection">
236 <a name="OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a>
237</div>
238
239<div class="doc_text">
240
241<p>
242Several other tidbits of information are often collected by various alias
243analysis implementations and can be put to good use by various clients.
244</p>
245
246</div>
247
248<!-- _______________________________________________________________________ -->
249<div class="doc_subsubsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]250 The <tt>pointsToConstantMemory</tt> method
251</div>
252
253<div class="doc_text">
254
255<p>The <tt>pointsToConstantMemory</tt> method returns true if and only if the
256analysis can prove that the pointer only points to unchanging memory locations
257(functions, constant global variables, and the null pointer). This information
258can be used to refine mod/ref information: it is impossible for an unchanging
259memory location to be modified.</p>
260
261</div>
262
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]263<!-- _______________________________________________________________________ -->
264<div class="doc_subsubsection">
265 <a name="simplemodref">The <tt>doesNotAccessMemory</tt> and
266 <tt>onlyReadsMemory</tt> methods</a>
267</div>
268
269<div class="doc_text">
270
271<p>These methods are used to provide very simple mod/ref information for
272function calls. The <tt>doesNotAccessMemory</tt> method returns true for a
273function if the analysis can prove that the function never reads or writes to
274memory, or if the function only reads from constant memory. Functions with this
275property are side-effect free and only depend on their input arguments, allowing
276them to be eliminated if they form common subexpressions or be hoisted out of
277loops. Many common functions behave this way (e.g., <tt>sin</tt> and
278<tt>cos</tt>) but many others do not (e.g., <tt>acos</tt>, which modifies the
279<tt>errno</tt> variable).</p>
280
281<p>The <tt>onlyReadsMemory</tt> method returns true for a function if analysis
282can prove that (at most) the function only reads from non-volatile memory.
283Functions with this property are side-effect free, only depending on their input
284arguments and the state of memory when they are called. This property allows
285calls to these functions to be eliminated and moved around, as long as there is
286no store instruction that changes the contents of memory. Note that all
287functions that satisfy the <tt>doesNotAccessMemory</tt> method also satisfies
288<tt>onlyReadsMemory</tt>.</p>
289
290</div>
291
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]292<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]293<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]294 <a name="writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]295</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]296<!-- *********************************************************************** -->
297
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]298<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]299
300<p>Writing a new alias analysis implementation for LLVM is quite
301straight-forward. There are already several implementations that you can use
302for examples, and the following information should help fill in any details.
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]303For a examples, take a look at the <a href="#impls">various alias analysis
304implementations</a> included with LLVM.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]305
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]306</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]307
308<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]309<div class="doc_subsection">
310 <a name="passsubclasses">Different Pass styles</a>
311</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]312
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]313<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]314
315<p>The first step to determining what type of <a
316href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias
317Analysis. As is the case with most other analyses and transformations, the
318answer should be fairly obvious from what type of problem you are trying to
319solve:</p>
320
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]321<ol>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]322 <li>If you require interprocedural analysis, it should be a
323 <tt>Pass</tt>.</li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]324 <li>If you are a function-local analysis, subclass <tt>FunctionPass</tt>.</li>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]325 <li>If you don't need to look at the program at all, subclass
326 <tt>ImmutablePass</tt>.</li>
327</ol>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]328
329<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]330<tt>AliasAnalysis</tt> interface, of course, and use the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]331<tt>RegisterAnalysisGroup</tt> template to register as an implementation of
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]332<tt>AliasAnalysis</tt>.</p>
333
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]334</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]335
336<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]337<div class="doc_subsection">
338 <a name="requiredcalls">Required initialization calls</a>
339</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]340
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]341<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]342
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]343<p>Your subclass of <tt>AliasAnalysis</tt> is required to invoke two methods on
344the <tt>AliasAnalysis</tt> base class: <tt>getAnalysisUsage</tt> and
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]345<tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
346<tt>getAnalysisUsage</tt> should explicitly call into the
347<tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
348declaring any pass dependencies your pass has. Thus you should have something
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]349like this:</p>
350
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]351<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]352<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]353void getAnalysisUsage(AnalysisUsage &amp;AU) const {
354 AliasAnalysis::getAnalysisUsage(AU);
355 <i>// declare your dependencies here.</i>
356}
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]357</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]358</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]359
360<p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
361from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]362<tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, or <tt>InitializePass</tt>
363for an <tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]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]367bool run(Module &amp;M) {
368 InitializeAliasAnalysis(this);
369 <i>// Perform analysis here...</i>
370 return false;
371}
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]372</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]373</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]374
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]375</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]376
377<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]378<div class="doc_subsection">
379 <a name="interfaces">Interfaces which may be specified</a>
380</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]381
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]382<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]383
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]384<p>All of the <a
385href="/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
386virtual methods default to providing <a href="#chaining">chaining</a> to another
387alias analysis implementation, which ends up returning conservatively correct
388information (returning "May" Alias and "Mod/Ref" for alias and mod/ref queries
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]389respectively). Depending on the capabilities of the analysis you are
390implementing, you just override the interfaces you can improve.</p>
391
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]392</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]393
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]394
395
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]396<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]397<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]398 <a name="chaining"><tt>AliasAnalysis</tt> chaining behavior</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]399</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]400
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]401<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]402
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]403<p>With only two special exceptions (the <tt><a
404href="#basic-aa">basicaa</a></tt> and <a href="#no-aa"><tt>no-aa</tt></a>
405passes) every alias analysis pass chains to another alias analysis
406implementation (for example, the user can specify "<tt>-basicaa -ds-aa
Chris Lattnera9cf1962010-03-01 19:24:17 +0000[diff] [blame]407-licm</tt>" to get the maximum benefit from both alias
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]408analyses). The alias analysis class automatically takes care of most of this
409for methods that you don't override. For methods that you do override, in code
410paths that return a conservative MayAlias or Mod/Ref result, simply return
411whatever the superclass computes. For example:</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]412
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]413<div class="doc_code">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]414<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]415AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
416 const Value *V2, unsigned V2Size) {
417 if (...)
418 return NoAlias;
419 ...
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]420
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]421 <i>// Couldn't determine a must or no-alias result.</i>
422 return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
423}
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]424</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]425</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]426
427<p>In addition to analysis queries, you must make sure to unconditionally pass
428LLVM <a href="#updating">update notification</a> methods to the superclass as
429well if you override them, which allows all alias analyses in a change to be
430updated.</p>
431
432</div>
433
434
435<!-- ======================================================================= -->
436<div class="doc_subsection">
437 <a name="updating">Updating analysis results for transformations</a>
438</div>
439
440<div class="doc_text">
441<p>
442Alias analysis information is initially computed for a static snapshot of the
443program, but clients will use this information to make transformations to the
444code. All but the most trivial forms of alias analysis will need to have their
445analysis results updated to reflect the changes made by these transformations.
446</p>
447
448<p>
449The <tt>AliasAnalysis</tt> interface exposes two methods which are used to
450communicate program changes from the clients to the analysis implementations.
451Various alias analysis implementations should use these methods to ensure that
452their internal data structures are kept up-to-date as the program changes (for
453example, when an instruction is deleted), and clients of alias analysis must be
454sure to call these interfaces appropriately.
455</p>
456</div>
457
458<!-- _______________________________________________________________________ -->
459<div class="doc_subsubsection">The <tt>deleteValue</tt> method</div>
460
461<div class="doc_text">
462The <tt>deleteValue</tt> method is called by transformations when they remove an
463instruction or any other value from the program (including values that do not
464use pointers). Typically alias analyses keep data structures that have entries
465for each value in the program. When this method is called, they should remove
466any entries for the specified value, if they exist.
467</div>
468
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]469<!-- _______________________________________________________________________ -->
470<div class="doc_subsubsection">The <tt>copyValue</tt> method</div>
471
472<div class="doc_text">
473The <tt>copyValue</tt> method is used when a new value is introduced into the
474program. There is no way to introduce a value into the program that did not
475exist before (this doesn't make sense for a safe compiler transformation), so
476this is the only way to introduce a new value. This method indicates that the
477new value has exactly the same properties as the value being copied.
478</div>
479
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]480<!-- _______________________________________________________________________ -->
481<div class="doc_subsubsection">The <tt>replaceWithNewValue</tt> method</div>
482
483<div class="doc_text">
484This method is a simple helper method that is provided to make clients easier to
485use. It is implemented by copying the old analysis information to the new
486value, then deleting the old value. This method cannot be overridden by alias
487analysis implementations.
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]488</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]489
490<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]491<div class="doc_subsection">
492 <a name="implefficiency">Efficiency Issues</a>
493</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]494
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]495<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]496
497<p>From the LLVM perspective, the only thing you need to do to provide an
498efficient alias analysis is to make sure that alias analysis <b>queries</b> are
499serviced quickly. The actual calculation of the alias analysis results (the
500"run" method) is only performed once, but many (perhaps duplicate) queries may
501be performed. Because of this, try to move as much computation to the run
502method as possible (within reason).</p>
503
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]504</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]505
Dan Gohman3f43dc32010-06-24 19:34:03 +0000[diff] [blame]506<!-- ======================================================================= -->
507<div class="doc_subsection">
508 <a name="passmanager">Pass Manager Issues</a>
509</div>
510
511<div class="doc_text">
512
513<p>PassManager support for alternative AliasAnalysis implementation
514has some issues.</p>
515
516<p>There is no way to override the default alias analysis. It would
517be very useful to be able to do something like "opt -my-aa -O2" and
518have it use -my-aa for all passes which need AliasAnalysis, but there
519is currently no support for that, short of changing the source code
520and recompiling. Similarly, there is also no way of setting a chain
521of analyses as the default.</p>
522
523<p>There is no way for transform passes to declare that they preserve
524<tt>AliasAnalysis</tt> implementations. The <tt>AliasAnalysis</tt>
525interface includes <tt>deleteValue</tt> and <tt>copyValue</tt> methods
526which are intended to allow a pass to keep an AliasAnalysis consistent,
527however there's no way for a pass to declare in its
528<tt>getAnalysisUsage</tt> that it does so. Some passes attempt to use
529<tt>AU.addPreserved&lt;AliasAnalysis&gt;</tt>, however this doesn't
530actually have any effect.</tt>
531
532<p><tt>AliasAnalysisCounter</tt> (<tt>-count-aa</tt>) and <tt>AliasDebugger</tt>
533(<tt>-debug-aa</tt>) are implemented as <tt>ModulePass</tt> classes, so if your
534alias analysis uses <tt>FunctionPass</tt>, it won't be able to use
535these utilities. If you try to use them, the pass manager will
536silently route alias analysis queries directly to
537<tt>BasicAliasAnalysis</tt> instead.</p>
538
539<p>Similarly, the <tt>opt -p</tt> option introduces <tt>ModulePass</tt>
540passes between each pass, which prevents the use of <tt>FunctionPass</tt>
541alias analysis passes.</p>
542
543</div>
544
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]545<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]546<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]547 <a name="using">Using alias analysis results</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]548</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]549<!-- *********************************************************************** -->
550
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]551<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]552
553<p>There are several different ways to use alias analysis results. In order of
554preference, these are...</p>
555
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]556</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]557
558<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]559<div class="doc_subsection">
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]560 <a name="memdep">Using the <tt>MemoryDependenceAnalysis</tt> Pass</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]561</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]562
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]563<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]564
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]565<p>The <tt>memdep</tt> pass uses alias analysis to provide high-level dependence
566information about memory-using instructions. This will tell you which store
567feeds into a load, for example. It uses caching and other techniques to be
568efficient, and is used by Dead Store Elimination, GVN, and memcpy optimizations.
569</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]570
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]571</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]572
573<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]574<div class="doc_subsection">
575 <a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
576</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]577
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]578<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]579
580<p>Many transformations need information about alias <b>sets</b> that are active
581in some scope, rather than information about pairwise aliasing. The <tt><a
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]582href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class
583is used to efficiently build these Alias Sets from the pairwise alias analysis
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]584information provided by the <tt>AliasAnalysis</tt> interface.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]585
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]586<p>First you initialize the AliasSetTracker by using the "<tt>add</tt>" methods
587to add information about various potentially aliasing instructions in the scope
588you are interested in. Once all of the alias sets are completed, your pass
589should simply iterate through the constructed alias sets, using the
590<tt>AliasSetTracker</tt> <tt>begin()</tt>/<tt>end()</tt> methods.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]591
592<p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]593to be disjoint, calculate mod/ref information and volatility for the set, and
594keep track of whether or not all of the pointers in the set are Must aliases.
595The AliasSetTracker also makes sure that sets are properly folded due to call
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]596instructions, and can provide a list of pointers in each set.</p>
597
598<p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]599Invariant Code Motion</a> pass uses <tt>AliasSetTracker</tt>s to calculate alias
600sets for each loop nest. If an <tt>AliasSet</tt> in a loop is not modified,
601then all load instructions from that set may be hoisted out of the loop. If any
602alias sets are stored to <b>and</b> are must alias sets, then the stores may be
603sunk to outside of the loop, promoting the memory location to a register for the
604duration of the loop nest. Both of these transformations only apply if the
605pointer argument is loop-invariant.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]606
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]607</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]608
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]609<!-- _______________________________________________________________________ -->
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]610<div class="doc_subsubsection">
611 The AliasSetTracker implementation
612</div>
613
614<div class="doc_text">
615
616<p>The AliasSetTracker class is implemented to be as efficient as possible. It
617uses the union-find algorithm to efficiently merge AliasSets when a pointer is
618inserted into the AliasSetTracker that aliases multiple sets. The primary data
619structure is a hash table mapping pointers to the AliasSet they are in.</p>
620
621<p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s
622that are in each AliasSet. Since the hash table already has entries for each
623LLVM Value* of interest, the AliasesSets thread the linked list through these
624hash-table nodes to avoid having to allocate memory unnecessarily, and to make
625merging alias sets extremely efficient (the linked list merge is constant time).
626</p>
627
628<p>You shouldn't need to understand these details if you are just a client of
629the AliasSetTracker, but if you look at the code, hopefully this brief
630description will help make sense of why things are designed the way they
631are.</p>
632
633</div>
634
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]635<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]636<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]637 <a name="direct">Using the <tt>AliasAnalysis</tt> interface directly</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]638</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]639
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]640<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]641
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]642<p>If neither of these utility class are what your pass needs, you should use
643the interfaces exposed by the <tt>AliasAnalysis</tt> class directly. Try to use
644the higher-level methods when possible (e.g., use mod/ref information instead of
645the <a href="#alias"><tt>alias</tt></a> method directly if possible) to get the
646best precision and efficiency.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]647
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]648</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]649
650<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]651<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]652 <a name="exist">Existing alias analysis implementations and clients</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]653</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]654<!-- *********************************************************************** -->
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>If you're going to be working with the LLVM alias analysis infrastructure,
659you should know what clients and implementations of alias analysis are
660available. In particular, if you are implementing an alias analysis, you should
661be aware of the <a href="#aliasanalysis-debug">the clients</a> that are useful
662for monitoring and evaluating different implementations.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]663
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]664</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]665
666<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]667<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]668 <a name="impls">Available <tt>AliasAnalysis</tt> implementations</a>
669</div>
670
671<div class="doc_text">
672
673<p>This section lists the various implementations of the <tt>AliasAnalysis</tt>
674interface. With the exception of the <a href="#no-aa"><tt>-no-aa</tt></a> and
675<a href="#basic-aa"><tt>-basicaa</tt></a> implementations, all of these <a
Chris Lattnerfcd37252004-06-21 22:52:48 +0000[diff] [blame]676href="#chaining">chain</a> to other alias analysis implementations.</p>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]677
678</div>
679
680<!-- _______________________________________________________________________ -->
681<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]682 <a name="no-aa">The <tt>-no-aa</tt> pass</a>
683</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]684
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]685<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]686
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]687<p>The <tt>-no-aa</tt> pass is just like what it sounds: an alias analysis that
688never returns any useful information. This pass can be useful if you think that
689alias analysis is doing something wrong and are trying to narrow down a
690problem.</p>
691
692</div>
693
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]694<!-- _______________________________________________________________________ -->
695<div class="doc_subsubsection">
696 <a name="basic-aa">The <tt>-basicaa</tt> pass</a>
697</div>
698
699<div class="doc_text">
700
701<p>The <tt>-basicaa</tt> pass is the default LLVM alias analysis. It is an
702aggressive local analysis that "knows" many important facts:</p>
703
704<ul>
705<li>Distinct globals, stack allocations, and heap allocations can never
706 alias.</li>
707<li>Globals, stack allocations, and heap allocations never alias the null
708 pointer.</li>
709<li>Different fields of a structure do not alias.</li>
710<li>Indexes into arrays with statically differing subscripts cannot alias.</li>
711<li>Many common standard C library functions <a
712 href="#simplemodref">never access memory or only read memory</a>.</li>
713<li>Pointers that obviously point to constant globals
714 "<tt>pointToConstantMemory</tt>".</li>
715<li>Function calls can not modify or references stack allocations if they never
716 escape from the function that allocates them (a common case for automatic
717 arrays).</li>
718</ul>
719
720</div>
721
Chris Lattner100a4f82004-06-28 19:19:47 +0000[diff] [blame]722<!-- _______________________________________________________________________ -->
723<div class="doc_subsubsection">
724 <a name="globalsmodref">The <tt>-globalsmodref-aa</tt> pass</a>
725</div>
726
727<div class="doc_text">
728
729<p>This pass implements a simple context-sensitive mod/ref and alias analysis
Chris Lattnerccb354b2004-07-27 07:50:07 +0000[diff] [blame]730for internal global variables that don't "have their address taken". If a
731global does not have its address taken, the pass knows that no pointers alias
732the global. This pass also keeps track of functions that it knows never access
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]733memory or never read memory. This allows certain optimizations (e.g. GVN) to
Chris Lattnerccb354b2004-07-27 07:50:07 +0000[diff] [blame]734eliminate call instructions entirely.
Chris Lattner100a4f82004-06-28 19:19:47 +0000[diff] [blame]735</p>
736
737<p>The real power of this pass is that it provides context-sensitive mod/ref
738information for call instructions. This allows the optimizer to know that
739calls to a function do not clobber or read the value of the global, allowing
740loads and stores to be eliminated.</p>
741
742<p>Note that this pass is somewhat limited in its scope (only support
743non-address taken globals), but is very quick analysis.</p>
744</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]745
746<!-- _______________________________________________________________________ -->
747<div class="doc_subsubsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]748 <a name="steens-aa">The <tt>-steens-aa</tt> pass</a>
749</div>
750
751<div class="doc_text">
752
753<p>The <tt>-steens-aa</tt> pass implements a variation on the well-known
754"Steensgaard's algorithm" for interprocedural alias analysis. Steensgaard's
755algorithm is a unification-based, flow-insensitive, context-insensitive, and
756field-insensitive alias analysis that is also very scalable (effectively linear
757time).</p>
758
759<p>The LLVM <tt>-steens-aa</tt> pass implements a "speculatively
760field-<b>sensitive</b>" version of Steensgaard's algorithm using the Data
761Structure Analysis framework. This gives it substantially more precision than
762the standard algorithm while maintaining excellent analysis scalability.</p>
763
Chris Lattnerd6e0dd12007-07-03 04:41:50 +0000[diff] [blame]764<p>Note that <tt>-steens-aa</tt> is available in the optional "poolalloc"
765module, it is not part of the LLVM core.</p>
766
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]767</div>
768
769<!-- _______________________________________________________________________ -->
770<div class="doc_subsubsection">
771 <a name="ds-aa">The <tt>-ds-aa</tt> pass</a>
772</div>
773
774<div class="doc_text">
775
776<p>The <tt>-ds-aa</tt> pass implements the full Data Structure Analysis
777algorithm. Data Structure Analysis is a modular unification-based,
778flow-insensitive, context-<b>sensitive</b>, and speculatively
779field-<b>sensitive</b> alias analysis that is also quite scalable, usually at
780O(n*log(n)).</p>
781
782<p>This algorithm is capable of responding to a full variety of alias analysis
783queries, and can provide context-sensitive mod/ref information as well. The
784only major facility not implemented so far is support for must-alias
785information.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]786
Chris Lattnerd6e0dd12007-07-03 04:41:50 +0000[diff] [blame]787<p>Note that <tt>-ds-aa</tt> is available in the optional "poolalloc"
788module, it is not part of the LLVM core.</p>
789
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]790</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]791
Dan Gohman85dfca62010-06-28 22:09:52 +0000[diff] [blame^]792<!-- _______________________________________________________________________ -->
793<div class="doc_subsubsection">
794 <a name="scev-aa">The <tt>-scev-aa</tt> pass</a>
795</div>
796
797<div class="doc_text">
798
799<p>The <tt>-scev-aa</tt> pass implements AliasAnalysis queries by
800translating them into ScalarEvolution queries. This gives it a
801more complete understanding of <tt>getelementptr</tt> instructions
802and loop induction variables than other alias analyses have.</p>
803
804</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]805
806<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]807<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]808 <a name="aliasanalysis-xforms">Alias analysis driven transformations</a>
809</div>
810
811<div class="doc_text">
812LLVM includes several alias-analysis driven transformations which can be used
813with any of the implementations above.
814</div>
815
816<!-- _______________________________________________________________________ -->
817<div class="doc_subsubsection">
818 <a name="adce">The <tt>-adce</tt> pass</a>
819</div>
820
821<div class="doc_text">
822
823<p>The <tt>-adce</tt> pass, which implements Aggressive Dead Code Elimination
824uses the <tt>AliasAnalysis</tt> interface to delete calls to functions that do
825not have side-effects and are not used.</p>
826
827</div>
828
829
830<!-- _______________________________________________________________________ -->
831<div class="doc_subsubsection">
832 <a name="licm">The <tt>-licm</tt> pass</a>
833</div>
834
835<div class="doc_text">
836
837<p>The <tt>-licm</tt> pass implements various Loop Invariant Code Motion related
838transformations. It uses the <tt>AliasAnalysis</tt> interface for several
839different transformations:</p>
840
841<ul>
842<li>It uses mod/ref information to hoist or sink load instructions out of loops
843if there are no instructions in the loop that modifies the memory loaded.</li>
844
845<li>It uses mod/ref information to hoist function calls out of loops that do not
846write to memory and are loop-invariant.</li>
847
848<li>If uses alias information to promote memory objects that are loaded and
849stored to in loops to live in a register instead. It can do this if there are
850no may aliases to the loaded/stored memory location.</li>
851</ul>
852
853</div>
854
855<!-- _______________________________________________________________________ -->
856<div class="doc_subsubsection">
857 <a name="argpromotion">The <tt>-argpromotion</tt> pass</a>
858</div>
859
860<div class="doc_text">
861<p>
862The <tt>-argpromotion</tt> pass promotes by-reference arguments to be passed in
863by-value instead. In particular, if pointer arguments are only loaded from it
864passes in the value loaded instead of the address to the function. This pass
865uses alias information to make sure that the value loaded from the argument
866pointer is not modified between the entry of the function and any load of the
867pointer.</p>
868</div>
869
870<!-- _______________________________________________________________________ -->
871<div class="doc_subsubsection">
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]872 <a name="gvn">The <tt>-gvn</tt>, <tt>-memcpyopt</tt>, and <tt>-dse</tt>
873 passes</a>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]874</div>
875
876<div class="doc_text">
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]877
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]878<p>These passes use AliasAnalysis information to reason about loads and stores.
879</p>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]880
881</div>
882
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]883<!-- ======================================================================= -->
884<div class="doc_subsection">
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]885 <a name="aliasanalysis-debug">Clients for debugging and evaluation of
886 implementations</a>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]887</div>
888
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]889<div class="doc_text">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]890
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]891<p>These passes are useful for evaluating the various alias analysis
Chris Lattnera9cf1962010-03-01 19:24:17 +0000[diff] [blame]892implementations. You can use them with commands like '<tt>opt -ds-aa
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]893-aa-eval foo.bc -disable-output -stats</tt>'.</p>
894
895</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]896
897<!-- _______________________________________________________________________ -->
898<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]899 <a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
900</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]901
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]902<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]903
904<p>The <tt>-print-alias-sets</tt> pass is exposed as part of the
Chris Lattner05d5c9d2006-01-03 06:04:48 +0000[diff] [blame]905<tt>opt</tt> tool to print out the Alias Sets formed by the <a
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]906href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
Chris Lattner05d5c9d2006-01-03 06:04:48 +0000[diff] [blame]907the <tt>AliasSetTracker</tt> class. To use it, use something like:</p>
908
909<div class="doc_code">
910<pre>
911% opt -ds-aa -print-alias-sets -disable-output
912</pre>
913</div>
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<!-- _______________________________________________________________________ -->
919<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]920 <a name="count-aa">The <tt>-count-aa</tt> pass</a>
921</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]922
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]923<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]924
925<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]926pass is making and what responses are returned by the alias analysis. As an
927example,</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]928
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]929<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]930<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]931% opt -basicaa -count-aa -ds-aa -count-aa -licm
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]932</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]933</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]934
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]935<p>will print out how many queries (and what responses are returned) by the
936<tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are made
937of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be useful
938when debugging a transformation or an alias analysis implementation.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]939
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]940</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]941
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]942<!-- _______________________________________________________________________ -->
943<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]944 <a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
945</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]946
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]947<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]948
949<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]950function and asks an alias analysis whether or not the pointers alias. This
951gives an indication of the precision of the alias analysis. Statistics are
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]952printed indicating the percent of no/may/must aliases found (a more precise
953algorithm will have a lower number of may aliases).</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]954
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]955</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]956
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]957<!-- *********************************************************************** -->
Owen Anderson5a726b82007-10-02 00:43:25 +0000[diff] [blame]958<div class="doc_section">
959 <a name="memdep">Memory Dependence Analysis</a>
960</div>
961<!-- *********************************************************************** -->
962
963<div class="doc_text">
964
965<p>If you're just looking to be a client of alias analysis information, consider
966using the Memory Dependence Analysis interface instead. MemDep is a lazy,
967caching layer on top of alias analysis that is able to answer the question of
968what preceding memory operations a given instruction depends on, either at an
969intra- or inter-block level. Because of its laziness and caching
970policy, using MemDep can be a significant performance win over accessing alias
971analysis directly.</p>
972
973</div>
974
975<!-- *********************************************************************** -->
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]976
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]977<hr>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]978<address>
Misha Brukman38847d52003-12-21 22:53:21 +0000[diff] [blame]979 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]980 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]981 <a href="http://validator.w3.org/check/referer"><img
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Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]983
984 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]985 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]986 Last modified: $Date$
987</address>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]988
989</body>
990</html>