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4<head>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]5 <title>The 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">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]11 The LLVM Alias Analysis Infrastructure
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]12</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]13
14<ol>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]15 <li><a href="#introduction">Introduction</a></li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]16
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]17 <li><a href="#overview"><tt>AliasAnalysis</tt> Class Overview</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]18 <ul>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]19 <li><a href="#pointers">Representation of Pointers</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]20 <li><a href="#alias">The <tt>alias</tt> method</a></li>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]21 <li><a href="#ModRefInfo">The <tt>getModRefInfo</tt> methods</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]22 <li><a href="#OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a></li>
23 </ul>
24 </li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]25
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]26 <li><a href="#writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]27 <ul>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]28 <li><a href="#passsubclasses">Different Pass styles</a></li>
29 <li><a href="#requiredcalls">Required initialization calls</a></li>
30 <li><a href="#interfaces">Interfaces which may be specified</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]31 <li><a href="#chaining"><tt>AliasAnalysis</tt> chaining behavior</a></li>
32 <li><a href="#updating">Updating analysis results for transformations</a></li>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]33 <li><a href="#implefficiency">Efficiency Issues</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]34 </ul>
35 </li>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]36
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]37 <li><a href="#using">Using alias analysis results</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]38 <ul>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]39 <li><a href="#loadvn">Using the <tt>-load-vn</tt> Pass</a></li>
40 <li><a href="#ast">Using the <tt>AliasSetTracker</tt> class</a></li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]41 <li><a href="#direct">Using the <tt>AliasAnalysis</tt> interface directly</a></li>
42 </ul>
43 </li>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]44
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]45 <li><a href="#exist">Existing alias analysis implementations and clients</a>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]46 <ul>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]47 <li><a href="#impls">Available <tt>AliasAnalysis</tt> implementations</a></li>
48 <li><a href="#aliasanalysis-xforms">Alias analysis driven transformations</a></li>
49 <li><a href="#aliasanalysis-debug">Clients for debugging and evaluation of implementations</a></li>
50 </ul>
51 </li>
Misha Brukmanb2154252003-10-23 02:29:42 +0000[diff] [blame]52</ol>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]53
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]54<div class="doc_author">
55 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]56</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]57
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]58<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]59<div class="doc_section">
60 <a name="introduction">Introduction</a>
61</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]62<!-- *********************************************************************** -->
63
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]64<div class="doc_text">
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]65
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]66<p>Alias Analysis (aka Pointer Analysis) is a class of techniques which attempt
67to determine whether or not two pointers ever can point to the same object in
68memory. There are many different algorithms for alias analysis and many
69different ways of classifying them: flow-sensitive vs flow-insensitive,
70context-sensitive vs context-insensitive, field-sensitive vs field-insensitive,
71unification-based vs subset-based, etc. Traditionally, alias analyses respond
72to a query with a <a href="#MustNoMay">Must, May, or No</a> alias response,
73indicating that two pointers always point to the same object, might point to the
74same object, or are known to never point to the same object.</p>
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]75
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]76<p>The LLVM <a
77href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
78class is the primary interface used by clients and implementations of alias
79analyses in the LLVM system. This class is the common interface between clients
80of alias analysis information and the implementations providing it, and is
81designed to support a wide range of implementations and clients (but currently
82all clients are assumed to be flow-insensitive). In addition to simple alias
83analysis information, this class exposes Mod/Ref information from those
84implementations which can provide it, allowing for powerful analyses and
85transformations to work well together.</p>
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]86
87<p>This document contains information necessary to successfully implement this
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]88interface, use it, and to test both sides. It also explains some of the finer
89points about what exactly results mean. If you feel that something is unclear
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]90or should be added, please <a href="mailto:sabre@nondot.org">let me
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]91know</a>.</p>
92
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]93</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]94
95<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]96<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]97 <a name="overview"><tt>AliasAnalysis</tt> Class Overview</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]98</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]99<!-- *********************************************************************** -->
100
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]101<div class="doc_text">
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]102
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]103<p>The <a
104href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
105class defines the interface that the various alias analysis implementations
106should support. This class exports two important enums: <tt>AliasResult</tt>
107and <tt>ModRefResult</tt> which represent the result of an alias query or a
108mod/ref query, respectively.</p>
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]109
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]110<p>The <tt>AliasAnalysis</tt> interface exposes information about memory,
111represented in several different ways. In particular, memory objects are
112represented as a starting address and size, and function calls are represented
113as the actual <tt>call</tt> or <tt>invoke</tt> instructions that performs the
114call. The <tt>AliasAnalysis</tt> interface also exposes some helper methods
115which allow you to get mod/ref information for arbitrary instructions.</p>
Misha Brukman1f254d52003-11-21 22:30:25 +0000[diff] [blame]116
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]117</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]118
119<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]120<div class="doc_subsection">
121 <a name="pointers">Representation of Pointers</a>
122</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]123
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]124<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]125
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]126<p>Most importantly, the <tt>AliasAnalysis</tt> class provides several methods
127which are used to query whether or not two memory objects alias, whether
128function calls can modify or read a memory object, etc. For all of these
129queries, memory objects are represented as a pair of their starting address (a
130symbolic LLVM <tt>Value*</tt>) and a static size.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]131
132<p>Representing memory objects as a starting address and a size is critically
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]133important for correct Alias Analyses. For example, consider this (silly, but
134possible) C code:</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]135
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]136<pre>
137 int i;
138 char C[2];
139 char A[10];
140 /* ... */
141 for (i = 0; i != 10; ++i) {
142 C[0] = A[i]; /* One byte store */
143 C[1] = A[9-i]; /* One byte store */
144 }
145</pre>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]146
147<p>In this case, the <tt>basicaa</tt> pass will disambiguate the stores to
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]148<tt>C[0]</tt> and <tt>C[1]</tt> because they are accesses to two distinct
149locations one byte apart, and the accesses are each one byte. In this case, the
150LICM pass can use store motion to remove the stores from the loop. In
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]151constrast, the following code:</p>
152
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]153<pre>
154 int i;
155 char C[2];
156 char A[10];
157 /* ... */
158 for (i = 0; i != 10; ++i) {
159 ((short*)C)[0] = A[i]; /* Two byte store! */
160 C[1] = A[9-i]; /* One byte store */
161 }
162</pre>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]163
164<p>In this case, the two stores to C do alias each other, because the access to
165the <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]166available in the query, even the first case would have to conservatively assume
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]167that the accesses alias.</p>
168
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]169</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]170
171<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]172<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]173 <a name="alias">The <tt>alias</tt> method</a>
174</div>
175
176<div class="doc_text">
177The <tt>alias</tt> method is the primary interface used to determine whether or
178not two memory objects alias each other. It takes two memory objects as input
179and returns MustAlias, MayAlias, or NoAlias as appropriate.
180</div>
181
182<!-- _______________________________________________________________________ -->
183<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]184 <a name="MustMayNo">Must, May, and No Alias Responses</a>
185</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]186
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]187<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]188
189<p>An Alias Analysis implementation can return one of three responses:
190MustAlias, MayAlias, and NoAlias. The No and May alias results are obvious: if
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]191the two pointers can never equal each other, return NoAlias, if they might,
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]192return MayAlias.</p>
193
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]194<p>The MustAlias response is trickier though. In LLVM, the Must Alias response
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]195may only be returned if the two memory objects are guaranteed to always start at
196exactly the same location. If two memory objects overlap, but do not start at
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]197the same location, return MayAlias.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]198
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]199</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]200
201<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]202<div class="doc_subsection">
203 <a name="ModRefInfo">The <tt>getModRefInfo</tt> methods</a>
204</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]205
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]206<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]207
208<p>The <tt>getModRefInfo</tt> methods return information about whether the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]209execution of an instruction can read or modify a memory location. Mod/Ref
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]210information is always conservative: if an instruction <b>might</b> read or write
211a location, ModRef is returned.</p>
212
213<p>The <tt>AliasAnalysis</tt> class also provides a <tt>getModRefInfo</tt>
214method for testing dependencies between function calls. This method takes two
215call sites (CS1 &amp; CS2), returns NoModRef if the two calls refer to disjoint
216memory locations, Ref if CS1 reads memory written by CS2, Mod if CS1 writes to
217memory read or written by CS2, or ModRef if CS1 might read or write memory
218accessed by CS2. Note that this relation is not commutative. Clients that use
219this method should be predicated on the <tt>hasNoModRefInfoForCalls()</tt>
220method, which indicates whether or not an analysis can provide mod/ref
221information for function call pairs (most can not). If this predicate is false,
222the client shouldn't waste analysis time querying the <tt>getModRefInfo</tt>
223method many times.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]224
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]225</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]226
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]227
228<!-- ======================================================================= -->
229<div class="doc_subsection">
230 <a name="OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a>
231</div>
232
233<div class="doc_text">
234
235<p>
236Several other tidbits of information are often collected by various alias
237analysis implementations and can be put to good use by various clients.
238</p>
239
240</div>
241
242<!-- _______________________________________________________________________ -->
243<div class="doc_subsubsection">
244 The <tt>getMustAliases</tt> method
245</div>
246
247<div class="doc_text">
248
249<p>The <tt>getMustAliases</tt> method returns all values that are known to
250always must alias a pointer. This information can be provided in some cases for
251important objects like the null pointer and global values. Knowing that a
252pointer always points to a particular function allows indirect calls to be
253turned into direct calls, for example.</p>
254
255</div>
256
257<!-- _______________________________________________________________________ -->
258<div class="doc_subsubsection">
259 The <tt>pointsToConstantMemory</tt> method
260</div>
261
262<div class="doc_text">
263
264<p>The <tt>pointsToConstantMemory</tt> method returns true if and only if the
265analysis can prove that the pointer only points to unchanging memory locations
266(functions, constant global variables, and the null pointer). This information
267can be used to refine mod/ref information: it is impossible for an unchanging
268memory location to be modified.</p>
269
270</div>
271
272
273<!-- _______________________________________________________________________ -->
274<div class="doc_subsubsection">
275 <a name="simplemodref">The <tt>doesNotAccessMemory</tt> and
276 <tt>onlyReadsMemory</tt> methods</a>
277</div>
278
279<div class="doc_text">
280
281<p>These methods are used to provide very simple mod/ref information for
282function calls. The <tt>doesNotAccessMemory</tt> method returns true for a
283function if the analysis can prove that the function never reads or writes to
284memory, or if the function only reads from constant memory. Functions with this
285property are side-effect free and only depend on their input arguments, allowing
286them to be eliminated if they form common subexpressions or be hoisted out of
287loops. Many common functions behave this way (e.g., <tt>sin</tt> and
288<tt>cos</tt>) but many others do not (e.g., <tt>acos</tt>, which modifies the
289<tt>errno</tt> variable).</p>
290
291<p>The <tt>onlyReadsMemory</tt> method returns true for a function if analysis
292can prove that (at most) the function only reads from non-volatile memory.
293Functions with this property are side-effect free, only depending on their input
294arguments and the state of memory when they are called. This property allows
295calls to these functions to be eliminated and moved around, as long as there is
296no store instruction that changes the contents of memory. Note that all
297functions that satisfy the <tt>doesNotAccessMemory</tt> method also satisfies
298<tt>onlyReadsMemory</tt>.</p>
299
300</div>
301
302
303
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]304<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]305<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]306 <a name="writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]307</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]308<!-- *********************************************************************** -->
309
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]310<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]311
312<p>Writing a new alias analysis implementation for LLVM is quite
313straight-forward. There are already several implementations that you can use
314for examples, and the following information should help fill in any details.
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]315For a examples, take a look at the <a href="#impls">various alias analysis
316implementations</a> included with LLVM.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]317
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]318</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]319
320<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]321<div class="doc_subsection">
322 <a name="passsubclasses">Different Pass styles</a>
323</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]324
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]325<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]326
327<p>The first step to determining what type of <a
328href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias
329Analysis. As is the case with most other analyses and transformations, the
330answer should be fairly obvious from what type of problem you are trying to
331solve:</p>
332
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]333<ol>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]334 <li>If you require interprocedural analysis, it should be a
335 <tt>Pass</tt>.</li>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]336 <li>If you are a function-local analysis, subclass <tt>FunctionPass</tt>.</li>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]337 <li>If you don't need to look at the program at all, subclass
338 <tt>ImmutablePass</tt>.</li>
339</ol>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]340
341<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]342<tt>AliasAnalysis</tt> interface, of course, and use the
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]343<tt>RegisterAnalysisGroup</tt> template to register as an implementation of
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]344<tt>AliasAnalysis</tt>.</p>
345
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]346</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]347
348<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]349<div class="doc_subsection">
350 <a name="requiredcalls">Required initialization calls</a>
351</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]352
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]353<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]354
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]355<p>Your subclass of <tt>AliasAnalysis</tt> is required to invoke two methods on
356the <tt>AliasAnalysis</tt> base class: <tt>getAnalysisUsage</tt> and
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]357<tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
358<tt>getAnalysisUsage</tt> should explicitly call into the
359<tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
360declaring any pass dependencies your pass has. Thus you should have something
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]361like this:</p>
362
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]363<pre>
364 void getAnalysisUsage(AnalysisUsage &amp;AU) const {
365 AliasAnalysis::getAnalysisUsage(AU);
366 <i>// declare your dependencies here.</i>
367 }
368</pre>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]369
370<p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
371from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]372<tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, or <tt>InitializePass</tt>
373for an <tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]374
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]375<pre>
376 bool run(Module &amp;M) {
377 InitializeAliasAnalysis(this);
378 <i>// Perform analysis here...</i>
379 return false;
380 }
381</pre>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]382
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]383</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]384
385<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]386<div class="doc_subsection">
387 <a name="interfaces">Interfaces which may be specified</a>
388</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]389
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]390<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]391
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]392<p>All of the <a
393href="/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
394virtual methods default to providing <a href="#chaining">chaining</a> to another
395alias analysis implementation, which ends up returning conservatively correct
396information (returning "May" Alias and "Mod/Ref" for alias and mod/ref queries
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]397respectively). Depending on the capabilities of the analysis you are
398implementing, you just override the interfaces you can improve.</p>
399
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]400</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]401
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]402
403
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]404<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]405<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]406 <a name="chaining"><tt>AliasAnalysis</tt> chaining behavior</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]407</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]408
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]409<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]410
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]411<p>With only two special exceptions (the <tt><a
412href="#basic-aa">basicaa</a></tt> and <a href="#no-aa"><tt>no-aa</tt></a>
413passes) every alias analysis pass chains to another alias analysis
414implementation (for example, the user can specify "<tt>-basicaa -ds-aa
415-anders-aa -licm</tt>" to get the maximum benefit from the three alias
416analyses). The alias analysis class automatically takes care of most of this
417for methods that you don't override. For methods that you do override, in code
418paths that return a conservative MayAlias or Mod/Ref result, simply return
419whatever the superclass computes. For example:</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]420
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]421<pre>
422 AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
423 const Value *V2, unsigned V2Size) {
424 if (...)
425 return NoAlias;
426 ...
427
428 <i>// Couldn't determine a must or no-alias result.</i>
429 return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
430 }
431</pre>
432
433<p>In addition to analysis queries, you must make sure to unconditionally pass
434LLVM <a href="#updating">update notification</a> methods to the superclass as
435well if you override them, which allows all alias analyses in a change to be
436updated.</p>
437
438</div>
439
440
441<!-- ======================================================================= -->
442<div class="doc_subsection">
443 <a name="updating">Updating analysis results for transformations</a>
444</div>
445
446<div class="doc_text">
447<p>
448Alias analysis information is initially computed for a static snapshot of the
449program, but clients will use this information to make transformations to the
450code. All but the most trivial forms of alias analysis will need to have their
451analysis results updated to reflect the changes made by these transformations.
452</p>
453
454<p>
455The <tt>AliasAnalysis</tt> interface exposes two methods which are used to
456communicate program changes from the clients to the analysis implementations.
457Various alias analysis implementations should use these methods to ensure that
458their internal data structures are kept up-to-date as the program changes (for
459example, when an instruction is deleted), and clients of alias analysis must be
460sure to call these interfaces appropriately.
461</p>
462</div>
463
464<!-- _______________________________________________________________________ -->
465<div class="doc_subsubsection">The <tt>deleteValue</tt> method</div>
466
467<div class="doc_text">
468The <tt>deleteValue</tt> method is called by transformations when they remove an
469instruction or any other value from the program (including values that do not
470use pointers). Typically alias analyses keep data structures that have entries
471for each value in the program. When this method is called, they should remove
472any entries for the specified value, if they exist.
473</div>
474
475
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
487
488<!-- _______________________________________________________________________ -->
489<div class="doc_subsubsection">The <tt>replaceWithNewValue</tt> method</div>
490
491<div class="doc_text">
492This method is a simple helper method that is provided to make clients easier to
493use. It is implemented by copying the old analysis information to the new
494value, then deleting the old value. This method cannot be overridden by alias
495analysis implementations.
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]496</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]497
498<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]499<div class="doc_subsection">
500 <a name="implefficiency">Efficiency Issues</a>
501</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]502
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]503<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]504
505<p>From the LLVM perspective, the only thing you need to do to provide an
506efficient alias analysis is to make sure that alias analysis <b>queries</b> are
507serviced quickly. The actual calculation of the alias analysis results (the
508"run" method) is only performed once, but many (perhaps duplicate) queries may
509be performed. Because of this, try to move as much computation to the run
510method as possible (within reason).</p>
511
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]512</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]513
514<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]515<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]516 <a name="using">Using alias analysis results</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]517</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]518<!-- *********************************************************************** -->
519
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]520<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]521
522<p>There are several different ways to use alias analysis results. In order of
523preference, these are...</p>
524
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]525</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]526
527<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]528<div class="doc_subsection">
529 <a name="loadvn">Using the <tt>-load-vn</tt> Pass</a>
530</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]531
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]532<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]533
534<p>The <tt>load-vn</tt> pass uses alias analysis to provide value numbering
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]535information for <tt>load</tt> instructions and pointer values. If your analysis
536or transformation can be modeled in a form that uses value numbering
537information, you don't have to do anything special to handle load instructions:
538just use the <tt>load-vn</tt> pass, which uses alias analysis.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]539
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]540</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]541
542<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]543<div class="doc_subsection">
544 <a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
545</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]546
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]547<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]548
549<p>Many transformations need information about alias <b>sets</b> that are active
550in some scope, rather than information about pairwise aliasing. The <tt><a
Misha Brukmane12e5f32003-12-17 23:10:49 +0000[diff] [blame]551href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class is used
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]552to efficiently build these Alias Sets from the pairwise alias analysis
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]553information provided by the <tt>AliasAnalysis</tt> interface.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]554
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]555<p>First you initialize the AliasSetTracker by using the "<tt>add</tt>" methods
556to add information about various potentially aliasing instructions in the scope
557you are interested in. Once all of the alias sets are completed, your pass
558should simply iterate through the constructed alias sets, using the
559<tt>AliasSetTracker</tt> <tt>begin()</tt>/<tt>end()</tt> methods.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]560
561<p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]562to be disjoint, calculate mod/ref information and volatility for the set, and
563keep track of whether or not all of the pointers in the set are Must aliases.
564The AliasSetTracker also makes sure that sets are properly folded due to call
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]565instructions, and can provide a list of pointers in each set.</p>
566
567<p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]568Invariant Code Motion</a> pass uses <tt>AliasSetTracker</tt>s to calculate alias
569sets for each loop nest. If an <tt>AliasSet</tt> in a loop is not modified,
570then all load instructions from that set may be hoisted out of the loop. If any
571alias sets are stored to <b>and</b> are must alias sets, then the stores may be
572sunk to outside of the loop, promoting the memory location to a register for the
573duration of the loop nest. Both of these transformations only apply if the
574pointer argument is loop-invariant.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]575
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]576</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]577
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]578<!-- _______________________________________________________________________ -->
Chris Lattner539ca702003-12-19 08:43:07 +0000[diff] [blame]579<div class="doc_subsubsection">
580 The AliasSetTracker implementation
581</div>
582
583<div class="doc_text">
584
585<p>The AliasSetTracker class is implemented to be as efficient as possible. It
586uses the union-find algorithm to efficiently merge AliasSets when a pointer is
587inserted into the AliasSetTracker that aliases multiple sets. The primary data
588structure is a hash table mapping pointers to the AliasSet they are in.</p>
589
590<p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s
591that are in each AliasSet. Since the hash table already has entries for each
592LLVM Value* of interest, the AliasesSets thread the linked list through these
593hash-table nodes to avoid having to allocate memory unnecessarily, and to make
594merging alias sets extremely efficient (the linked list merge is constant time).
595</p>
596
597<p>You shouldn't need to understand these details if you are just a client of
598the AliasSetTracker, but if you look at the code, hopefully this brief
599description will help make sense of why things are designed the way they
600are.</p>
601
602</div>
603
604
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]605<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]606<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]607 <a name="direct">Using the <tt>AliasAnalysis</tt> interface directly</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]608</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]609
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]610<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]611
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]612<p>If neither of these utility class are what your pass needs, you should use
613the interfaces exposed by the <tt>AliasAnalysis</tt> class directly. Try to use
614the higher-level methods when possible (e.g., use mod/ref information instead of
615the <a href="#alias"><tt>alias</tt></a> method directly if possible) to get the
616best precision and efficiency.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]617
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]618</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]619
620<!-- *********************************************************************** -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]621<div class="doc_section">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]622 <a name="exist">Existing alias analysis implementations and clients</a>
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]623</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]624<!-- *********************************************************************** -->
625
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]626<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]627
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]628<p>If you're going to be working with the LLVM alias analysis infrastructure,
629you should know what clients and implementations of alias analysis are
630available. In particular, if you are implementing an alias analysis, you should
631be aware of the <a href="#aliasanalysis-debug">the clients</a> that are useful
632for monitoring and evaluating different implementations.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]633
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]634</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]635
636<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]637<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]638 <a name="impls">Available <tt>AliasAnalysis</tt> implementations</a>
639</div>
640
641<div class="doc_text">
642
643<p>This section lists the various implementations of the <tt>AliasAnalysis</tt>
644interface. With the exception of the <a href="#no-aa"><tt>-no-aa</tt></a> and
645<a href="#basic-aa"><tt>-basicaa</tt></a> implementations, all of these <a
Chris Lattnerfcd37252004-06-21 22:52:48 +0000[diff] [blame]646href="#chaining">chain</a> to other alias analysis implementations.</p>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]647
648</div>
649
650<!-- _______________________________________________________________________ -->
651<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]652 <a name="no-aa">The <tt>-no-aa</tt> pass</a>
653</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]654
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]655<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]656
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]657<p>The <tt>-no-aa</tt> pass is just like what it sounds: an alias analysis that
658never returns any useful information. This pass can be useful if you think that
659alias analysis is doing something wrong and are trying to narrow down a
660problem.</p>
661
662</div>
663
664
665<!-- _______________________________________________________________________ -->
666<div class="doc_subsubsection">
667 <a name="basic-aa">The <tt>-basicaa</tt> pass</a>
668</div>
669
670<div class="doc_text">
671
672<p>The <tt>-basicaa</tt> pass is the default LLVM alias analysis. It is an
673aggressive local analysis that "knows" many important facts:</p>
674
675<ul>
676<li>Distinct globals, stack allocations, and heap allocations can never
677 alias.</li>
678<li>Globals, stack allocations, and heap allocations never alias the null
679 pointer.</li>
680<li>Different fields of a structure do not alias.</li>
681<li>Indexes into arrays with statically differing subscripts cannot alias.</li>
682<li>Many common standard C library functions <a
683 href="#simplemodref">never access memory or only read memory</a>.</li>
684<li>Pointers that obviously point to constant globals
685 "<tt>pointToConstantMemory</tt>".</li>
686<li>Function calls can not modify or references stack allocations if they never
687 escape from the function that allocates them (a common case for automatic
688 arrays).</li>
689</ul>
690
691</div>
692
Chris Lattner100a4f82004-06-28 19:19:47 +0000[diff] [blame^]693<!-- _______________________________________________________________________ -->
694<div class="doc_subsubsection">
695 <a name="globalsmodref">The <tt>-globalsmodref-aa</tt> pass</a>
696</div>
697
698<div class="doc_text">
699
700<p>This pass implements a simple context-sensitive mod/ref and alias analysis
701for internal global variables that don't "have their address taken". If a
702global does not have its address taken, the pass knows that no pointers alias
703the global.
704</p>
705
706<p>The real power of this pass is that it provides context-sensitive mod/ref
707information for call instructions. This allows the optimizer to know that
708calls to a function do not clobber or read the value of the global, allowing
709loads and stores to be eliminated.</p>
710
711<p>Note that this pass is somewhat limited in its scope (only support
712non-address taken globals), but is very quick analysis.</p>
713</div>
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]714
715<!-- _______________________________________________________________________ -->
716<div class="doc_subsubsection">
717 <a name="anders-aa">The <tt>-anders-aa</tt> pass</a>
718</div>
719
720<div class="doc_text">
721
722<p>The <tt>-anders-aa</tt> pass implements the well-known "Andersen's algorithm"
723for interprocedural alias analysis. This algorithm is a subset-based,
724flow-insensitive, context-insensitive, and field-insensitive alias analysis that
725is widely believed to be fairly precise. Unfortunately, this algorithm is also
726O(N<sup>3</sup>). The LLVM implementation currently does not implement any of
727the refinements (such as "online cycle elimination" or "offline variable
728substitution") to improve its efficiency, so it can be quite slow in common
729cases.
730</p>
731
732</div>
733
734<!-- _______________________________________________________________________ -->
735<div class="doc_subsubsection">
736 <a name="steens-aa">The <tt>-steens-aa</tt> pass</a>
737</div>
738
739<div class="doc_text">
740
741<p>The <tt>-steens-aa</tt> pass implements a variation on the well-known
742"Steensgaard's algorithm" for interprocedural alias analysis. Steensgaard's
743algorithm is a unification-based, flow-insensitive, context-insensitive, and
744field-insensitive alias analysis that is also very scalable (effectively linear
745time).</p>
746
747<p>The LLVM <tt>-steens-aa</tt> pass implements a "speculatively
748field-<b>sensitive</b>" version of Steensgaard's algorithm using the Data
749Structure Analysis framework. This gives it substantially more precision than
750the standard algorithm while maintaining excellent analysis scalability.</p>
751
752</div>
753
754<!-- _______________________________________________________________________ -->
755<div class="doc_subsubsection">
756 <a name="ds-aa">The <tt>-ds-aa</tt> pass</a>
757</div>
758
759<div class="doc_text">
760
761<p>The <tt>-ds-aa</tt> pass implements the full Data Structure Analysis
762algorithm. Data Structure Analysis is a modular unification-based,
763flow-insensitive, context-<b>sensitive</b>, and speculatively
764field-<b>sensitive</b> alias analysis that is also quite scalable, usually at
765O(n*log(n)).</p>
766
767<p>This algorithm is capable of responding to a full variety of alias analysis
768queries, and can provide context-sensitive mod/ref information as well. The
769only major facility not implemented so far is support for must-alias
770information.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]771
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]772</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]773
774
775<!-- ======================================================================= -->
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]776<div class="doc_subsection">
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]777 <a name="aliasanalysis-xforms">Alias analysis driven transformations</a>
778</div>
779
780<div class="doc_text">
781LLVM includes several alias-analysis driven transformations which can be used
782with any of the implementations above.
783</div>
784
785<!-- _______________________________________________________________________ -->
786<div class="doc_subsubsection">
787 <a name="adce">The <tt>-adce</tt> pass</a>
788</div>
789
790<div class="doc_text">
791
792<p>The <tt>-adce</tt> pass, which implements Aggressive Dead Code Elimination
793uses the <tt>AliasAnalysis</tt> interface to delete calls to functions that do
794not have side-effects and are not used.</p>
795
796</div>
797
798
799<!-- _______________________________________________________________________ -->
800<div class="doc_subsubsection">
801 <a name="licm">The <tt>-licm</tt> pass</a>
802</div>
803
804<div class="doc_text">
805
806<p>The <tt>-licm</tt> pass implements various Loop Invariant Code Motion related
807transformations. It uses the <tt>AliasAnalysis</tt> interface for several
808different transformations:</p>
809
810<ul>
811<li>It uses mod/ref information to hoist or sink load instructions out of loops
812if there are no instructions in the loop that modifies the memory loaded.</li>
813
814<li>It uses mod/ref information to hoist function calls out of loops that do not
815write to memory and are loop-invariant.</li>
816
817<li>If uses alias information to promote memory objects that are loaded and
818stored to in loops to live in a register instead. It can do this if there are
819no may aliases to the loaded/stored memory location.</li>
820</ul>
821
822</div>
823
824<!-- _______________________________________________________________________ -->
825<div class="doc_subsubsection">
826 <a name="argpromotion">The <tt>-argpromotion</tt> pass</a>
827</div>
828
829<div class="doc_text">
830<p>
831The <tt>-argpromotion</tt> pass promotes by-reference arguments to be passed in
832by-value instead. In particular, if pointer arguments are only loaded from it
833passes in the value loaded instead of the address to the function. This pass
834uses alias information to make sure that the value loaded from the argument
835pointer is not modified between the entry of the function and any load of the
836pointer.</p>
837</div>
838
839<!-- _______________________________________________________________________ -->
840<div class="doc_subsubsection">
841 <a name="gcseloadvn">The <tt>-load-vn</tt> &amp; <tt>-gcse</tt> passes</a>
842</div>
843
844<div class="doc_text">
845<p>
846The <tt>-load-vn</tt> pass uses alias analysis to "<a href="#loadvn">value
847number</a>" loads and pointers values, which is used by the GCSE pass to
848eliminate instructions. The <tt>-load-vn</tt> pass relies on alias information
849and must-alias information. This combination of passes can make the following
850transformations:</p>
851
852<ul>
853<li>Redundant load instructions are eliminated.</li>
854<li>Load instructions that follow a store to the same location are replaced with
855the stored value ("store forwarding").</li>
856<li>Pointers values (e.g. formal arguments) that must-alias simpler expressions
857(e.g. global variables or the null pointer) are replaced. Note that this
858implements transformations like "virtual method resolution", turning indirect
859calls into direct calls.</li>
860</ul>
861
862</div>
863
864
865
866
867<!-- ======================================================================= -->
868<div class="doc_subsection">
869 <a name="aliasanalysis-debug">Clients for debugging and evaluation of implementations</a>
870</div>
871
872These passes are useful for evaluating the various alias analysis
873implementations. You can use them with commands like '<tt>opt -anders-aa -ds-aa
874-aa-eval foo.bc -disable-output -stats</tt>'.
875
876
877<!-- _______________________________________________________________________ -->
878<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]879 <a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
880</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]881
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]882<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]883
884<p>The <tt>-print-alias-sets</tt> pass is exposed as part of the
885<tt>analyze</tt> tool to print out the Alias Sets formed by the <a
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]886href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]887the <tt>AliasSetTracker</tt> class.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]888
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]889</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]890
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]891
892<!-- _______________________________________________________________________ -->
893<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]894 <a name="count-aa">The <tt>-count-aa</tt> pass</a>
895</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]896
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]897<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]898
899<p>The <tt>-count-aa</tt> pass is useful to see how many queries a particular
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]900pass is making and what responses are returned by the alias analysis. An
901example usage is:</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]902
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]903<pre>
904 $ opt -basicaa -count-aa -ds-aa -count-aa -licm
905</pre>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]906
907<p>Which will print out how many queries (and what responses are returned) by
908the <tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are
909made of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]910useful when debugging a transformation or an alias analysis implementation.</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]911
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]912</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]913
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]914<!-- _______________________________________________________________________ -->
915<div class="doc_subsubsection">
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]916 <a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
917</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]918
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]919<div class="doc_text">
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]920
921<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]922function and asks an alias analysis whether or not the pointers alias. This
923gives an indication of the precision of the alias analysis. Statistics are
Chris Lattnerccc80652004-05-23 21:04:01 +0000[diff] [blame]924printed indicating the percent of no/may/must aliases found (a more precise
925algorithm will have a lower number of may aliases).</p>
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]926
Misha Brukmancd11e452003-10-22 23:27:16 +0000[diff] [blame]927</div>
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]928
Chris Lattner9f648752003-03-04 19:37:49 +0000[diff] [blame]929<!-- *********************************************************************** -->
930
Misha Brukman3896be22003-10-24 18:06:11 +0000[diff] [blame]931<hr>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]932<address>
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938 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Misha Brukmanf6acb512004-05-12 18:02:00 +0000[diff] [blame]939 <a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
Misha Brukman915cab22003-11-22 01:26:21 +0000[diff] [blame]940 Last modified: $Date$
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