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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">
Dan Gohman1e109622010-07-02 18:41:32 +0000[diff] [blame]195<p>The NoAlias response may be used when there is never an immediate dependence
196between any memory reference <i>based</i> on one pointer and any memory
197reference <i>based</i> the other. The most obvious example is when the two
198pointers point to non-overlapping memory ranges. Another is when the two
199pointers are only ever used for reading memory. Another is when the memory is
200freed and reallocated between accesses through one pointer and accesses through
201the other -- in this case, there is a dependence, but it's mediated by the free
202and reallocation.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]203
Dan Gohmanefca7f92010-07-02 23:46:54 +0000[diff] [blame^]204<p>As an exception to this is with the
205<a href="LangRef.html#noalias"><tt>noalias</tt></a> keyword. AliasAnalysis
206implementations may choose to ignore the "irrelevant" dependencies, provided
207their clients do not need to be aware of these dependencies for correctness.</p>
208
Nick Lewycky01557ce2008-12-14 21:08:48 +0000[diff] [blame]209<p>The MayAlias response is used whenever the two pointers might refer to the
210same object. If the two memory objects overlap, but do not start at the same
211location, return MayAlias.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]212
Nick Lewycky01557ce2008-12-14 21:08:48 +0000[diff] [blame]213<p>The MustAlias response may only be returned if the two memory objects are
214guaranteed to always start at exactly the same location. A MustAlias response
215implies that the pointers compare equal.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]216
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]217</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]218
219<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]220<div class="doc_subsection">
221 <a name="ModRefInfo">The <tt>getModRefInfo</tt> methods</a>
222</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]223
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]224<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]225
226<p>The <tt>getModRefInfo</tt> methods return information about whether the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]227execution of an instruction can read or modify a memory location. Mod/Ref
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]228information is always conservative: if an instruction <b>might</b> read or write
229a location, ModRef is returned.</p>
230
231<p>The <tt>AliasAnalysis</tt> class also provides a <tt>getModRefInfo</tt>
232method for testing dependencies between function calls. This method takes two
233call sites (CS1 &amp; CS2), returns NoModRef if the two calls refer to disjoint
234memory locations, Ref if CS1 reads memory written by CS2, Mod if CS1 writes to
235memory read or written by CS2, or ModRef if CS1 might read or write memory
Chris Lattner66e08cf2009-11-22 16:01:44 +0000[diff] [blame]236accessed by CS2. Note that this relation is not commutative.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]237
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]238</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]239
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]240
241<!-- ======================================================================= -->
242<div class="doc_subsection">
243 <a name="OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a>
244</div>
245
246<div class="doc_text">
247
248<p>
249Several other tidbits of information are often collected by various alias
250analysis implementations and can be put to good use by various clients.
251</p>
252
253</div>
254
255<!-- _______________________________________________________________________ -->
256<div class="doc_subsubsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]257 The <tt>pointsToConstantMemory</tt> method
258</div>
259
260<div class="doc_text">
261
262<p>The <tt>pointsToConstantMemory</tt> method returns true if and only if the
263analysis can prove that the pointer only points to unchanging memory locations
264(functions, constant global variables, and the null pointer). This information
265can be used to refine mod/ref information: it is impossible for an unchanging
266memory location to be modified.</p>
267
268</div>
269
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]270<!-- _______________________________________________________________________ -->
271<div class="doc_subsubsection">
272 <a name="simplemodref">The <tt>doesNotAccessMemory</tt> and
273 <tt>onlyReadsMemory</tt> methods</a>
274</div>
275
276<div class="doc_text">
277
278<p>These methods are used to provide very simple mod/ref information for
279function calls. The <tt>doesNotAccessMemory</tt> method returns true for a
280function if the analysis can prove that the function never reads or writes to
281memory, or if the function only reads from constant memory. Functions with this
282property are side-effect free and only depend on their input arguments, allowing
283them to be eliminated if they form common subexpressions or be hoisted out of
284loops. Many common functions behave this way (e.g., <tt>sin</tt> and
285<tt>cos</tt>) but many others do not (e.g., <tt>acos</tt>, which modifies the
286<tt>errno</tt> variable).</p>
287
288<p>The <tt>onlyReadsMemory</tt> method returns true for a function if analysis
289can prove that (at most) the function only reads from non-volatile memory.
290Functions with this property are side-effect free, only depending on their input
291arguments and the state of memory when they are called. This property allows
292calls to these functions to be eliminated and moved around, as long as there is
293no store instruction that changes the contents of memory. Note that all
294functions that satisfy the <tt>doesNotAccessMemory</tt> method also satisfies
295<tt>onlyReadsMemory</tt>.</p>
296
297</div>
298
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]299<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]300<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]301 <a name="writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]302</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]303<!-- *********************************************************************** -->
304
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]305<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]306
307<p>Writing a new alias analysis implementation for LLVM is quite
308straight-forward. There are already several implementations that you can use
309for examples, and the following information should help fill in any details.
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]310For a examples, take a look at the <a href="#impls">various alias analysis
311implementations</a> included with LLVM.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]312
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]313</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]314
315<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]316<div class="doc_subsection">
317 <a name="passsubclasses">Different Pass styles</a>
318</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]319
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]320<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]321
322<p>The first step to determining what type of <a
323href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias
324Analysis. As is the case with most other analyses and transformations, the
325answer should be fairly obvious from what type of problem you are trying to
326solve:</p>
327
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]328<ol>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]329 <li>If you require interprocedural analysis, it should be a
330 <tt>Pass</tt>.</li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]331 <li>If you are a function-local analysis, subclass <tt>FunctionPass</tt>.</li>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]332 <li>If you don't need to look at the program at all, subclass
333 <tt>ImmutablePass</tt>.</li>
334</ol>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]335
336<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]337<tt>AliasAnalysis</tt> interface, of course, and use the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]338<tt>RegisterAnalysisGroup</tt> template to register as an implementation of
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]339<tt>AliasAnalysis</tt>.</p>
340
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]341</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]342
343<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]344<div class="doc_subsection">
345 <a name="requiredcalls">Required initialization calls</a>
346</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]347
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]348<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]349
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]350<p>Your subclass of <tt>AliasAnalysis</tt> is required to invoke two methods on
351the <tt>AliasAnalysis</tt> base class: <tt>getAnalysisUsage</tt> and
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]352<tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
353<tt>getAnalysisUsage</tt> should explicitly call into the
354<tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
355declaring any pass dependencies your pass has. Thus you should have something
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]356like this:</p>
357
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]358<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]359<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]360void getAnalysisUsage(AnalysisUsage &amp;AU) const {
361 AliasAnalysis::getAnalysisUsage(AU);
362 <i>// declare your dependencies here.</i>
363}
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]364</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]365</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]366
367<p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
368from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]369<tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, or <tt>InitializePass</tt>
370for an <tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]371
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]372<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]373<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]374bool run(Module &amp;M) {
375 InitializeAliasAnalysis(this);
376 <i>// Perform analysis here...</i>
377 return false;
378}
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]379</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]380</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]381
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]382</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]383
384<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]385<div class="doc_subsection">
386 <a name="interfaces">Interfaces which may be specified</a>
387</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]388
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]389<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]390
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]391<p>All of the <a
392href="/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
393virtual methods default to providing <a href="#chaining">chaining</a> to another
394alias analysis implementation, which ends up returning conservatively correct
395information (returning "May" Alias and "Mod/Ref" for alias and mod/ref queries
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]396respectively). Depending on the capabilities of the analysis you are
397implementing, you just override the interfaces you can improve.</p>
398
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]399</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]400
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]401
402
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]403<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]404<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]405 <a name="chaining"><tt>AliasAnalysis</tt> chaining behavior</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]406</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]407
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]408<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]409
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]410<p>With only two special exceptions (the <tt><a
411href="#basic-aa">basicaa</a></tt> and <a href="#no-aa"><tt>no-aa</tt></a>
412passes) every alias analysis pass chains to another alias analysis
413implementation (for example, the user can specify "<tt>-basicaa -ds-aa
Chris Lattnera9cf1962010-03-01 19:24:17 +0000[diff] [blame]414-licm</tt>" to get the maximum benefit from both alias
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]415analyses). The alias analysis class automatically takes care of most of this
416for methods that you don't override. For methods that you do override, in code
417paths that return a conservative MayAlias or Mod/Ref result, simply return
418whatever the superclass computes. For example:</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]419
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]420<div class="doc_code">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]421<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]422AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
423 const Value *V2, unsigned V2Size) {
424 if (...)
425 return NoAlias;
426 ...
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]427
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]428 <i>// Couldn't determine a must or no-alias result.</i>
429 return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
430}
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]431</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]432</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]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
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]476<!-- _______________________________________________________________________ -->
477<div class="doc_subsubsection">The <tt>copyValue</tt> method</div>
478
479<div class="doc_text">
480The <tt>copyValue</tt> method is used when a new value is introduced into the
481program. There is no way to introduce a value into the program that did not
482exist before (this doesn't make sense for a safe compiler transformation), so
483this is the only way to introduce a new value. This method indicates that the
484new value has exactly the same properties as the value being copied.
485</div>
486
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]487<!-- _______________________________________________________________________ -->
488<div class="doc_subsubsection">The <tt>replaceWithNewValue</tt> method</div>
489
490<div class="doc_text">
491This method is a simple helper method that is provided to make clients easier to
492use. It is implemented by copying the old analysis information to the new
493value, then deleting the old value. This method cannot be overridden by alias
494analysis implementations.
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]495</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]496
497<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]498<div class="doc_subsection">
499 <a name="implefficiency">Efficiency Issues</a>
500</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]501
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]502<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]503
504<p>From the LLVM perspective, the only thing you need to do to provide an
505efficient alias analysis is to make sure that alias analysis <b>queries</b> are
506serviced quickly. The actual calculation of the alias analysis results (the
507"run" method) is only performed once, but many (perhaps duplicate) queries may
508be performed. Because of this, try to move as much computation to the run
509method as possible (within reason).</p>
510
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]511</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]512
Dan Gohman3f43dc32010-06-24 19:34:03 +0000[diff] [blame]513<!-- ======================================================================= -->
514<div class="doc_subsection">
515 <a name="passmanager">Pass Manager Issues</a>
516</div>
517
518<div class="doc_text">
519
520<p>PassManager support for alternative AliasAnalysis implementation
521has some issues.</p>
522
523<p>There is no way to override the default alias analysis. It would
524be very useful to be able to do something like "opt -my-aa -O2" and
525have it use -my-aa for all passes which need AliasAnalysis, but there
526is currently no support for that, short of changing the source code
527and recompiling. Similarly, there is also no way of setting a chain
528of analyses as the default.</p>
529
530<p>There is no way for transform passes to declare that they preserve
531<tt>AliasAnalysis</tt> implementations. The <tt>AliasAnalysis</tt>
532interface includes <tt>deleteValue</tt> and <tt>copyValue</tt> methods
533which are intended to allow a pass to keep an AliasAnalysis consistent,
534however there's no way for a pass to declare in its
535<tt>getAnalysisUsage</tt> that it does so. Some passes attempt to use
536<tt>AU.addPreserved&lt;AliasAnalysis&gt;</tt>, however this doesn't
537actually have any effect.</tt>
538
539<p><tt>AliasAnalysisCounter</tt> (<tt>-count-aa</tt>) and <tt>AliasDebugger</tt>
540(<tt>-debug-aa</tt>) are implemented as <tt>ModulePass</tt> classes, so if your
541alias analysis uses <tt>FunctionPass</tt>, it won't be able to use
542these utilities. If you try to use them, the pass manager will
543silently route alias analysis queries directly to
544<tt>BasicAliasAnalysis</tt> instead.</p>
545
546<p>Similarly, the <tt>opt -p</tt> option introduces <tt>ModulePass</tt>
547passes between each pass, which prevents the use of <tt>FunctionPass</tt>
548alias analysis passes.</p>
549
550</div>
551
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]552<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]553<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]554 <a name="using">Using alias analysis results</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]555</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]556<!-- *********************************************************************** -->
557
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]558<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]559
560<p>There are several different ways to use alias analysis results. In order of
561preference, these are...</p>
562
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]563</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]564
565<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]566<div class="doc_subsection">
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]567 <a name="memdep">Using the <tt>MemoryDependenceAnalysis</tt> Pass</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]568</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]569
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]570<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]571
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]572<p>The <tt>memdep</tt> pass uses alias analysis to provide high-level dependence
573information about memory-using instructions. This will tell you which store
574feeds into a load, for example. It uses caching and other techniques to be
575efficient, and is used by Dead Store Elimination, GVN, and memcpy optimizations.
576</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]577
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]578</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]579
580<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]581<div class="doc_subsection">
582 <a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
583</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]584
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]585<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]586
587<p>Many transformations need information about alias <b>sets</b> that are active
588in some scope, rather than information about pairwise aliasing. The <tt><a
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]589href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class
590is used to efficiently build these Alias Sets from the pairwise alias analysis
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]591information provided by the <tt>AliasAnalysis</tt> interface.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]592
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]593<p>First you initialize the AliasSetTracker by using the "<tt>add</tt>" methods
594to add information about various potentially aliasing instructions in the scope
595you are interested in. Once all of the alias sets are completed, your pass
596should simply iterate through the constructed alias sets, using the
597<tt>AliasSetTracker</tt> <tt>begin()</tt>/<tt>end()</tt> methods.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]598
599<p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]600to be disjoint, calculate mod/ref information and volatility for the set, and
601keep track of whether or not all of the pointers in the set are Must aliases.
602The AliasSetTracker also makes sure that sets are properly folded due to call
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]603instructions, and can provide a list of pointers in each set.</p>
604
605<p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]606Invariant Code Motion</a> pass uses <tt>AliasSetTracker</tt>s to calculate alias
607sets for each loop nest. If an <tt>AliasSet</tt> in a loop is not modified,
608then all load instructions from that set may be hoisted out of the loop. If any
609alias sets are stored to <b>and</b> are must alias sets, then the stores may be
610sunk to outside of the loop, promoting the memory location to a register for the
611duration of the loop nest. Both of these transformations only apply if the
612pointer argument is loop-invariant.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]613
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]614</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]615
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]616<!-- _______________________________________________________________________ -->
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]617<div class="doc_subsubsection">
618 The AliasSetTracker implementation
619</div>
620
621<div class="doc_text">
622
623<p>The AliasSetTracker class is implemented to be as efficient as possible. It
624uses the union-find algorithm to efficiently merge AliasSets when a pointer is
625inserted into the AliasSetTracker that aliases multiple sets. The primary data
626structure is a hash table mapping pointers to the AliasSet they are in.</p>
627
628<p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s
629that are in each AliasSet. Since the hash table already has entries for each
630LLVM Value* of interest, the AliasesSets thread the linked list through these
631hash-table nodes to avoid having to allocate memory unnecessarily, and to make
632merging alias sets extremely efficient (the linked list merge is constant time).
633</p>
634
635<p>You shouldn't need to understand these details if you are just a client of
636the AliasSetTracker, but if you look at the code, hopefully this brief
637description will help make sense of why things are designed the way they
638are.</p>
639
640</div>
641
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]642<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]643<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]644 <a name="direct">Using the <tt>AliasAnalysis</tt> interface directly</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]645</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]646
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]647<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]648
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]649<p>If neither of these utility class are what your pass needs, you should use
650the interfaces exposed by the <tt>AliasAnalysis</tt> class directly. Try to use
651the higher-level methods when possible (e.g., use mod/ref information instead of
652the <a href="#alias"><tt>alias</tt></a> method directly if possible) to get the
653best precision and efficiency.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]654
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]655</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]656
657<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]658<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]659 <a name="exist">Existing alias analysis implementations and clients</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]660</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]661<!-- *********************************************************************** -->
662
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]663<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]664
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]665<p>If you're going to be working with the LLVM alias analysis infrastructure,
666you should know what clients and implementations of alias analysis are
667available. In particular, if you are implementing an alias analysis, you should
668be aware of the <a href="#aliasanalysis-debug">the clients</a> that are useful
669for monitoring and evaluating different implementations.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]670
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]671</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]672
673<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]674<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]675 <a name="impls">Available <tt>AliasAnalysis</tt> implementations</a>
676</div>
677
678<div class="doc_text">
679
680<p>This section lists the various implementations of the <tt>AliasAnalysis</tt>
681interface. With the exception of the <a href="#no-aa"><tt>-no-aa</tt></a> and
682<a href="#basic-aa"><tt>-basicaa</tt></a> implementations, all of these <a
Chris Lattnerfcd37252004-06-21 22:52:48 +0000[diff] [blame]683href="#chaining">chain</a> to other alias analysis implementations.</p>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]684
685</div>
686
687<!-- _______________________________________________________________________ -->
688<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]689 <a name="no-aa">The <tt>-no-aa</tt> pass</a>
690</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]691
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]692<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]693
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]694<p>The <tt>-no-aa</tt> pass is just like what it sounds: an alias analysis that
695never returns any useful information. This pass can be useful if you think that
696alias analysis is doing something wrong and are trying to narrow down a
697problem.</p>
698
699</div>
700
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]701<!-- _______________________________________________________________________ -->
702<div class="doc_subsubsection">
703 <a name="basic-aa">The <tt>-basicaa</tt> pass</a>
704</div>
705
706<div class="doc_text">
707
708<p>The <tt>-basicaa</tt> pass is the default LLVM alias analysis. It is an
709aggressive local analysis that "knows" many important facts:</p>
710
711<ul>
712<li>Distinct globals, stack allocations, and heap allocations can never
713 alias.</li>
714<li>Globals, stack allocations, and heap allocations never alias the null
715 pointer.</li>
716<li>Different fields of a structure do not alias.</li>
717<li>Indexes into arrays with statically differing subscripts cannot alias.</li>
718<li>Many common standard C library functions <a
719 href="#simplemodref">never access memory or only read memory</a>.</li>
720<li>Pointers that obviously point to constant globals
721 "<tt>pointToConstantMemory</tt>".</li>
722<li>Function calls can not modify or references stack allocations if they never
723 escape from the function that allocates them (a common case for automatic
724 arrays).</li>
725</ul>
726
727</div>
728
Chris Lattner100a4f82004-06-28 19:19:47 +0000[diff] [blame]729<!-- _______________________________________________________________________ -->
730<div class="doc_subsubsection">
731 <a name="globalsmodref">The <tt>-globalsmodref-aa</tt> pass</a>
732</div>
733
734<div class="doc_text">
735
736<p>This pass implements a simple context-sensitive mod/ref and alias analysis
Chris Lattnerccb354b2004-07-27 07:50:07 +0000[diff] [blame]737for internal global variables that don't "have their address taken". If a
738global does not have its address taken, the pass knows that no pointers alias
739the global. This pass also keeps track of functions that it knows never access
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]740memory or never read memory. This allows certain optimizations (e.g. GVN) to
Chris Lattnerccb354b2004-07-27 07:50:07 +0000[diff] [blame]741eliminate call instructions entirely.
Chris Lattner100a4f82004-06-28 19:19:47 +0000[diff] [blame]742</p>
743
744<p>The real power of this pass is that it provides context-sensitive mod/ref
745information for call instructions. This allows the optimizer to know that
746calls to a function do not clobber or read the value of the global, allowing
747loads and stores to be eliminated.</p>
748
749<p>Note that this pass is somewhat limited in its scope (only support
750non-address taken globals), but is very quick analysis.</p>
751</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]752
753<!-- _______________________________________________________________________ -->
754<div class="doc_subsubsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]755 <a name="steens-aa">The <tt>-steens-aa</tt> pass</a>
756</div>
757
758<div class="doc_text">
759
760<p>The <tt>-steens-aa</tt> pass implements a variation on the well-known
761"Steensgaard's algorithm" for interprocedural alias analysis. Steensgaard's
762algorithm is a unification-based, flow-insensitive, context-insensitive, and
763field-insensitive alias analysis that is also very scalable (effectively linear
764time).</p>
765
766<p>The LLVM <tt>-steens-aa</tt> pass implements a "speculatively
767field-<b>sensitive</b>" version of Steensgaard's algorithm using the Data
768Structure Analysis framework. This gives it substantially more precision than
769the standard algorithm while maintaining excellent analysis scalability.</p>
770
Chris Lattnerd6e0dd12007-07-03 04:41:50 +0000[diff] [blame]771<p>Note that <tt>-steens-aa</tt> is available in the optional "poolalloc"
772module, it is not part of the LLVM core.</p>
773
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]774</div>
775
776<!-- _______________________________________________________________________ -->
777<div class="doc_subsubsection">
778 <a name="ds-aa">The <tt>-ds-aa</tt> pass</a>
779</div>
780
781<div class="doc_text">
782
783<p>The <tt>-ds-aa</tt> pass implements the full Data Structure Analysis
784algorithm. Data Structure Analysis is a modular unification-based,
785flow-insensitive, context-<b>sensitive</b>, and speculatively
786field-<b>sensitive</b> alias analysis that is also quite scalable, usually at
787O(n*log(n)).</p>
788
789<p>This algorithm is capable of responding to a full variety of alias analysis
790queries, and can provide context-sensitive mod/ref information as well. The
791only major facility not implemented so far is support for must-alias
792information.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]793
Chris Lattnerd6e0dd12007-07-03 04:41:50 +0000[diff] [blame]794<p>Note that <tt>-ds-aa</tt> is available in the optional "poolalloc"
795module, it is not part of the LLVM core.</p>
796
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]797</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]798
Dan Gohman85dfca62010-06-28 22:09:52 +0000[diff] [blame]799<!-- _______________________________________________________________________ -->
800<div class="doc_subsubsection">
801 <a name="scev-aa">The <tt>-scev-aa</tt> pass</a>
802</div>
803
804<div class="doc_text">
805
806<p>The <tt>-scev-aa</tt> pass implements AliasAnalysis queries by
807translating them into ScalarEvolution queries. This gives it a
808more complete understanding of <tt>getelementptr</tt> instructions
809and loop induction variables than other alias analyses have.</p>
810
811</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]812
813<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]814<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]815 <a name="aliasanalysis-xforms">Alias analysis driven transformations</a>
816</div>
817
818<div class="doc_text">
819LLVM includes several alias-analysis driven transformations which can be used
820with any of the implementations above.
821</div>
822
823<!-- _______________________________________________________________________ -->
824<div class="doc_subsubsection">
825 <a name="adce">The <tt>-adce</tt> pass</a>
826</div>
827
828<div class="doc_text">
829
830<p>The <tt>-adce</tt> pass, which implements Aggressive Dead Code Elimination
831uses the <tt>AliasAnalysis</tt> interface to delete calls to functions that do
832not have side-effects and are not used.</p>
833
834</div>
835
836
837<!-- _______________________________________________________________________ -->
838<div class="doc_subsubsection">
839 <a name="licm">The <tt>-licm</tt> pass</a>
840</div>
841
842<div class="doc_text">
843
844<p>The <tt>-licm</tt> pass implements various Loop Invariant Code Motion related
845transformations. It uses the <tt>AliasAnalysis</tt> interface for several
846different transformations:</p>
847
848<ul>
849<li>It uses mod/ref information to hoist or sink load instructions out of loops
850if there are no instructions in the loop that modifies the memory loaded.</li>
851
852<li>It uses mod/ref information to hoist function calls out of loops that do not
853write to memory and are loop-invariant.</li>
854
855<li>If uses alias information to promote memory objects that are loaded and
856stored to in loops to live in a register instead. It can do this if there are
857no may aliases to the loaded/stored memory location.</li>
858</ul>
859
860</div>
861
862<!-- _______________________________________________________________________ -->
863<div class="doc_subsubsection">
864 <a name="argpromotion">The <tt>-argpromotion</tt> pass</a>
865</div>
866
867<div class="doc_text">
868<p>
869The <tt>-argpromotion</tt> pass promotes by-reference arguments to be passed in
870by-value instead. In particular, if pointer arguments are only loaded from it
871passes in the value loaded instead of the address to the function. This pass
872uses alias information to make sure that the value loaded from the argument
873pointer is not modified between the entry of the function and any load of the
874pointer.</p>
875</div>
876
877<!-- _______________________________________________________________________ -->
878<div class="doc_subsubsection">
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]879 <a name="gvn">The <tt>-gvn</tt>, <tt>-memcpyopt</tt>, and <tt>-dse</tt>
880 passes</a>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]881</div>
882
883<div class="doc_text">
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]884
Chris Lattner4f4365e2009-04-25 21:11:37 +0000[diff] [blame]885<p>These passes use AliasAnalysis information to reason about loads and stores.
886</p>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]887
888</div>
889
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]890<!-- ======================================================================= -->
891<div class="doc_subsection">
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]892 <a name="aliasanalysis-debug">Clients for debugging and evaluation of
893 implementations</a>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]894</div>
895
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]896<div class="doc_text">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]897
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]898<p>These passes are useful for evaluating the various alias analysis
Chris Lattnera9cf1962010-03-01 19:24:17 +0000[diff] [blame]899implementations. You can use them with commands like '<tt>opt -ds-aa
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]900-aa-eval foo.bc -disable-output -stats</tt>'.</p>
901
902</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]903
904<!-- _______________________________________________________________________ -->
905<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]906 <a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
907</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]908
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]909<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]910
911<p>The <tt>-print-alias-sets</tt> pass is exposed as part of the
Chris Lattner05d5c9d2006-01-03 06:04:48 +0000[diff] [blame]912<tt>opt</tt> tool to print out the Alias Sets formed by the <a
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]913href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
Chris Lattner05d5c9d2006-01-03 06:04:48 +0000[diff] [blame]914the <tt>AliasSetTracker</tt> class. To use it, use something like:</p>
915
916<div class="doc_code">
917<pre>
918% opt -ds-aa -print-alias-sets -disable-output
919</pre>
920</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]921
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]922</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]923
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]924
925<!-- _______________________________________________________________________ -->
926<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]927 <a name="count-aa">The <tt>-count-aa</tt> pass</a>
928</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]929
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]930<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]931
932<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]933pass is making and what responses are returned by the alias analysis. As an
934example,</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]935
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]936<div class="doc_code">
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]937<pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]938% opt -basicaa -count-aa -ds-aa -count-aa -licm
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]939</pre>
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]940</div>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]941
Misha Brukman294611a2004-07-28 22:18:33 +0000[diff] [blame]942<p>will print out how many queries (and what responses are returned) by the
943<tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are made
944of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be useful
945when debugging a transformation or an alias analysis implementation.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]946
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]947</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]948
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]949<!-- _______________________________________________________________________ -->
950<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]951 <a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
952</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]953
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]954<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]955
956<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]957function and asks an alias analysis whether or not the pointers alias. This
958gives an indication of the precision of the alias analysis. Statistics are
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]959printed indicating the percent of no/may/must aliases found (a more precise
960algorithm will have a lower number of may aliases).</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]961
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]962</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]963
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]964<!-- *********************************************************************** -->
Owen Anderson5a726b82007-10-02 00:43:25 +0000[diff] [blame]965<div class="doc_section">
966 <a name="memdep">Memory Dependence Analysis</a>
967</div>
968<!-- *********************************************************************** -->
969
970<div class="doc_text">
971
972<p>If you're just looking to be a client of alias analysis information, consider
973using the Memory Dependence Analysis interface instead. MemDep is a lazy,
974caching layer on top of alias analysis that is able to answer the question of
975what preceding memory operations a given instruction depends on, either at an
976intra- or inter-block level. Because of its laziness and caching
977policy, using MemDep can be a significant performance win over accessing alias
978analysis directly.</p>
979
980</div>
981
982<!-- *********************************************************************** -->
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]983
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]984<hr>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]985<address>
Misha Brukman38847d52003-12-21 22:53:21 +0000[diff] [blame]986 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]990
991 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]992 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]993 Last modified: $Date$
994</address>
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996</body>
997</html>