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@ -1,8 +1,8 @@
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
|
||||
<head>
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||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
|
||||
<link rel="stylesheet" href="llvm.css" type="text/css" media="screen" />
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<link rel="stylesheet" href="llvm.css" type="text/css">
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<title>Alias Analysis Infrastructure in LLVM</title>
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</head>
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@ -15,40 +15,41 @@
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<ol>
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<li><a href="#introduction">Introduction</a></li>
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<li><a href="#overview">AliasAnalysis Overview</a></li>
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<li><a href="#overview">AliasAnalysis Overview</a>
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<ul>
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<li><a href="#pointers">Representation of Pointers</a></li>
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<li><a href="#MustMayNo">Must, May, and No Alias Responses</a></li>
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<li><a href="#ModRefInfo">The <tt>getModRefInfo</tt> methods</a></li>
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</ul>
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</ul></li>
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<li><a href="#writingnew">Writing a new AliasAnalysis Implementation</a></li>
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<li><a href="#writingnew">Writing a new AliasAnalysis Implementation</a>
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<ul>
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<li><a href="#passsubclasses">Different Pass styles</a></li>
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<li><a href="#requiredcalls">Required initialization calls</a></li>
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<li><a href="#interfaces">Interfaces which may be specified</a></li>
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<li><a href="#chaining">The AliasAnalysis chaining behavior</a></li>
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<li><a href="#implefficiency">Efficiency Issues</a></li>
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</ul>
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||||
</ul></li>
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<li><a href="#using">Using AliasAnalysis results</a></li>
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<li><a href="#using">Using AliasAnalysis results</a>
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<ul>
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<li><a href="#loadvn">Using the <tt>-load-vn</tt> Pass</a></li>
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<li><a href="#ast">Using the <tt>AliasSetTracker</tt> class</a></li>
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<li><a href="#direct">Using the AliasAnalysis interface directly</a></li>
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</ul>
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<li><a href="#tools">Helpful alias analysis related tools</a></li>
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</ul></li>
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<li><a href="#tools">Helpful alias analysis related tools</a>
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<ul>
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<li><a href="#no-aa">The <tt>-no-aa</tt> pass</a></li>
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<li><a href="#print-alias-sets">The <tt>-print-alias-sets</tt> pass</a></li>
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<li><a href="#count-aa">The <tt>-count-aa</tt> pass</a></li>
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<li><a href="#aa-eval">The <tt>-aa-eval</tt> pass</a></li>
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</ul>
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</ul>
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<p><b>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></b></p>
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</ul></li>
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</ol>
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||||
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<div class="doc_text">
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<p><b>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></b></p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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@ -113,17 +114,15 @@ mod/ref information for arbitrary instructions.
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</div>
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<div class="doc_text">
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<p>
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Most importantly, the AliasAnalysis class provides several methods which are
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<p>Most importantly, the AliasAnalysis class provides several methods which are
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used to query whether or not pointers alias, whether function calls can modify
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or read memory, etc.
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</p>
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<p>
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Representing memory objects as a starting address and a size is critically
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or read memory, etc.</p>
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<p>Representing memory objects as a starting address and a size is critically
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important for precise Alias Analyses. For example, consider this (silly) C
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code:
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</p>
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<p>
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code:</p>
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<pre>
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int i;
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char C[2];
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@ -134,15 +133,13 @@ code:
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C[1] = A[9-i]; /* One byte store */
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}
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</pre>
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</p>
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<p>
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In this case, the <tt>basicaa</tt> pass will disambiguate the stores to
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<p>In this case, the <tt>basicaa</tt> pass will disambiguate the stores to
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<tt>C[0]</tt> and <tt>C[1]</tt> because they are accesses to two distinct
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locations one byte apart, and the accesses are each one byte. In this case, the
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LICM pass can use store motion to remove the stores from the loop. In
|
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constrast, the following code:
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||||
</p>
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||||
<p>
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||||
constrast, the following code:</p>
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|
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<pre>
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int i;
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char C[2];
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@ -153,13 +150,12 @@ constrast, the following code:
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C[1] = A[9-i]; /* One byte store */
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}
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</pre>
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</p>
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<p>
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In this case, the two stores to C do alias each other, because the access to the
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<tt>&C[0]</tt> element is a two byte access. If size information wasn't
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<p>In this case, the two stores to C do alias each other, because the access to
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the <tt>&C[0]</tt> element is a two byte access. If size information wasn't
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available in the query, even the first case would have to conservatively assume
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that the accesses alias.
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</p>
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that the accesses alias.</p>
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</div>
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<!-- ======================================================================= -->
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@ -168,18 +164,17 @@ that the accesses alias.
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</div>
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<div class="doc_text">
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<p>
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An Alias Analysis implementation can return one of three responses: MustAlias,
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MayAlias, and NoAlias. The No and May alias results are obvious: if the two
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pointers may never equal each other, return NoAlias, if they might, return
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MayAlias.
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</p>
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<p>
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The Must Alias response is trickier though. In LLVM, the Must Alias response
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<p>An Alias Analysis implementation can return one of three responses:
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MustAlias, MayAlias, and NoAlias. The No and May alias results are obvious: if
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the two pointers may never equal each other, return NoAlias, if they might,
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return MayAlias.</p>
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|
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<p>The Must Alias response is trickier though. In LLVM, the Must Alias response
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may only be returned if the two memory objects are guaranteed to always start at
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exactly the same location. If two memory objects overlap, but do not start at
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the same location, MayAlias must be returned.
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</p>
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the same location, MayAlias must be returned.</p>
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</div>
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<!-- ======================================================================= -->
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@ -188,12 +183,12 @@ the same location, MayAlias must be returned.
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</div>
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<div class="doc_text">
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<p>
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The <tt>getModRefInfo</tt> methods return information about whether the
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<p>The <tt>getModRefInfo</tt> methods return information about whether the
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execution of an instruction can read or modify a memory location. Mod/Ref
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information is always conservative: if an action <b>may</b> read a location, Ref
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is returned.
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</p>
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is returned.</p>
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||||
</div>
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<!-- *********************************************************************** -->
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@ -203,13 +198,13 @@ is returned.
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>
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Writing a new alias analysis implementation for LLVM is quite straight-forward.
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There are already several implementations that you can use for examples, and the
|
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following information should help fill in any details. For a minimal example,
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take a look at the <a href="/doxygen/structNoAA.html"><tt>no-aa</tt></a>
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implementation.
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||||
</p>
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||||
|
||||
<p>Writing a new alias analysis implementation for LLVM is quite
|
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straight-forward. There are already several implementations that you can use
|
||||
for examples, and the following information should help fill in any details.
|
||||
For a minimal example, take a look at the <a
|
||||
href="/doxygen/structNoAA.html"><tt>no-aa</tt></a> implementation.</p>
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||||
|
||||
</div>
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||||
|
||||
<!-- ======================================================================= -->
|
||||
@ -218,13 +213,13 @@ implementation.
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||||
</div>
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||||
|
||||
<div class="doc_text">
|
||||
<p>
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||||
The first step to determining what type of <a href="WritingAnLLVMPass.html">LLVM
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||||
pass</a> you need to use for your Alias Analysis. As is the case with most
|
||||
other analyses and transformations, the answer should be fairly obvious from
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what type of problem you are trying to solve:
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||||
</p>
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||||
<p>
|
||||
|
||||
<p>The first step to determining what type of <a
|
||||
href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias
|
||||
Analysis. As is the case with most other analyses and transformations, the
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||||
answer should be fairly obvious from what type of problem you are trying to
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||||
solve:</p>
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||||
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||||
<ol>
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||||
<li>If you require interprocedural analysis, it should be a
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<tt>Pass</tt>.</li>
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||||
@ -233,13 +228,12 @@ what type of problem you are trying to solve:
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||||
<li>If you don't need to look at the program at all, subclass
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<tt>ImmutablePass</tt>.</li>
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||||
</ol>
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||||
</p>
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||||
<p>
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||||
In addition to the pass that you subclass, you should also inherit from the
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||||
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||||
<p>In addition to the pass that you subclass, you should also inherit from the
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<tt>AliasAnalysis</tt> interface, of course, and use the
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<tt>RegisterAnalysisGroup</tt> template to register as an implementation of
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<tt>AliasAnalysis</tt>.
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||||
</p>
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||||
<tt>AliasAnalysis</tt>.</p>
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||||
</div>
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||||
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||||
<!-- ======================================================================= -->
|
||||
@ -248,31 +242,28 @@ In addition to the pass that you subclass, you should also inherit from the
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||||
</div>
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||||
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||||
<div class="doc_text">
|
||||
<p>
|
||||
Your subclass of AliasAnalysis is required to invoke two methods on the
|
||||
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||||
<p>Your subclass of AliasAnalysis is required to invoke two methods on the
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||||
AliasAnalysis base class: <tt>getAnalysisUsage</tt> and
|
||||
<tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
|
||||
<tt>getAnalysisUsage</tt> should explicitly call into the
|
||||
<tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
|
||||
declaring any pass dependencies your pass has. Thus you should have something
|
||||
like this:
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||||
</p>
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||||
<p>
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||||
like this:</p>
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||||
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||||
<pre>
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||||
void getAnalysisUsage(AnalysisUsage &AU) const {
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AliasAnalysis::getAnalysisUsage(AU);
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<i>// declare your dependencies here.</i>
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||||
}
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||||
</pre>
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||||
</p>
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||||
<p>
|
||||
Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method from
|
||||
your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
|
||||
|
||||
<p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
|
||||
from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
|
||||
<tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, <tt>runOnBasicBlock</tt> for
|
||||
a <tt>BasicBlockPass</tt>, or <tt>InitializeAliasAnalysis</tt> for an
|
||||
<tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):
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||||
</p>
|
||||
<p>
|
||||
<tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
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||||
|
||||
<pre>
|
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bool run(Module &M) {
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InitializeAliasAnalysis(this);
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@ -280,7 +271,7 @@ a <tt>BasicBlockPass</tt>, or <tt>InitializeAliasAnalysis</tt> for an
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return false;
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}
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</pre>
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||||
</p>
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||||
|
||||
</div>
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||||
<!-- ======================================================================= -->
|
||||
@ -289,13 +280,13 @@ a <tt>BasicBlockPass</tt>, or <tt>InitializeAliasAnalysis</tt> for an
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||||
</div>
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||||
|
||||
<div class="doc_text">
|
||||
<p>
|
||||
All of the <a href="/doxygen/classAliasAnalysis.html">AliasAnalysis</a> virtual
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methods default to providing conservatively correct information (returning "May"
|
||||
Alias and "Mod/Ref" for alias and mod/ref queries respectively). Depending on
|
||||
the capabilities of the analysis you are implementing, you just override the
|
||||
interfaces you can improve.
|
||||
</p>
|
||||
|
||||
<p>All of the <a href="/doxygen/classAliasAnalysis.html">AliasAnalysis</a>
|
||||
virtual methods default to providing conservatively correct information
|
||||
(returning "May" Alias and "Mod/Ref" for alias and mod/ref queries
|
||||
respectively). Depending on the capabilities of the analysis you are
|
||||
implementing, you just override the interfaces you can improve.</p>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
@ -304,15 +295,15 @@ interfaces you can improve.
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
<p>
|
||||
With only two special exceptions (the <tt>basicaa</tt> and <a
|
||||
|
||||
<p>With only two special exceptions (the <tt>basicaa</tt> and <a
|
||||
href="#no-aa"><tt>no-aa</tt></a> passes) every alias analysis pass should chain
|
||||
to another alias analysis implementation (for example, you could specify
|
||||
"<tt>-basic-aa -ds-aa -andersens-aa -licm</tt>" to get the maximum benefit from
|
||||
the three alias analyses). To do this, simply "Require" AliasAnalysis in your
|
||||
<tt>getAnalysisUsage</tt> method, and if you need to return a conservative
|
||||
MayAlias or Mod/Ref result, simply chain to a lower analysis.
|
||||
</p>
|
||||
MayAlias or Mod/Ref result, simply chain to a lower analysis.</p>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
@ -321,14 +312,14 @@ MayAlias or Mod/Ref result, simply chain to a lower analysis.
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
<p>
|
||||
From the LLVM perspective, the only thing you need to do to provide an efficient
|
||||
alias analysis is to make sure that alias analysis <b>queries</b> are serviced
|
||||
quickly. The actual calculation of the alias analysis results (the "run"
|
||||
method) is only performed once, but many (perhaps duplicate) queries may be
|
||||
performed. Because of this, try to move as much computation to the run method
|
||||
as possible (within reason).
|
||||
</p>
|
||||
|
||||
<p>From the LLVM perspective, the only thing you need to do to provide an
|
||||
efficient alias analysis is to make sure that alias analysis <b>queries</b> are
|
||||
serviced quickly. The actual calculation of the alias analysis results (the
|
||||
"run" method) is only performed once, but many (perhaps duplicate) queries may
|
||||
be performed. Because of this, try to move as much computation to the run
|
||||
method as possible (within reason).</p>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- *********************************************************************** -->
|
||||
@ -338,10 +329,10 @@ as possible (within reason).
|
||||
<!-- *********************************************************************** -->
|
||||
|
||||
<div class="doc_text">
|
||||
<p>
|
||||
There are several different ways to use alias analysis results. In order of
|
||||
preference, these are...
|
||||
</p>
|
||||
|
||||
<p>There are several different ways to use alias analysis results. In order of
|
||||
preference, these are...</p>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
@ -350,13 +341,13 @@ preference, these are...
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
<p>
|
||||
The <tt>load-vn</tt> pass uses alias analysis to provide value numbering
|
||||
|
||||
<p>The <tt>load-vn</tt> pass uses alias analysis to provide value numbering
|
||||
information for <tt>load</tt> instructions. If your analysis or transformation
|
||||
can be modelled in a form that uses value numbering information, you don't have
|
||||
to do anything special to handle load instructions: just use the
|
||||
<tt>load-vn</tt> pass, which uses alias analysis.
|
||||
</p>
|
||||
<tt>load-vn</tt> pass, which uses alias analysis.</p>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
@ -365,36 +356,33 @@ to do anything special to handle load instructions: just use the
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
<p>
|
||||
Many transformations need information about alias <b>sets</b> that are active in
|
||||
some scope, rather than information about pairwise aliasing. The <tt><a
|
||||
|
||||
<p>Many transformations need information about alias <b>sets</b> that are active
|
||||
in some scope, rather than information about pairwise aliasing. The <tt><a
|
||||
href="/doxygen/classAliasSetTracker.html">AliasSetTracker</a></tt> class is used
|
||||
to efficiently build these Alias Sets from the pairwise alias analysis
|
||||
information provided by the AliasAnalysis interface.
|
||||
</p>
|
||||
<p>
|
||||
First you initialize the AliasSetTracker by use the "<tt>add</tt>" methods to
|
||||
information provided by the AliasAnalysis interface.</p>
|
||||
|
||||
<p>First you initialize the AliasSetTracker by use the "<tt>add</tt>" methods to
|
||||
add information about various potentially aliasing instructions in the scope you
|
||||
are interested in. Once all of the alias sets are completed, your pass should
|
||||
simply iterate through the constructed alias sets, using the AliasSetTracker
|
||||
<tt>begin()</tt>/<tt>end()</tt> methods.
|
||||
</p>
|
||||
<p>
|
||||
The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed to
|
||||
be disjoint, calculate mod/ref information for the set, and keep track of
|
||||
<tt>begin()</tt>/<tt>end()</tt> methods.</p>
|
||||
|
||||
<p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed
|
||||
to be disjoint, calculate mod/ref information for the set, and keep track of
|
||||
whether or not all of the pointers in the set are Must aliases. The
|
||||
AliasSetTracker also makes sure that sets are properly folded due to call
|
||||
instructions, and can provide a list of pointers in each set.
|
||||
</p>
|
||||
<p>
|
||||
As an example user of this, the <a href="/doxygen/structLICM.html">Loop
|
||||
instructions, and can provide a list of pointers in each set.</p>
|
||||
|
||||
<p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop
|
||||
Invariant Code Motion</a> pass uses AliasSetTrackers to build alias information
|
||||
about each loop nest. If an AliasSet in a loop is not modified, then all load
|
||||
instructions from that set may be hoisted out of the loop. If any alias sets
|
||||
are stored <b>and</b> are must alias sets, then the stores may be sunk to
|
||||
outside of the loop. Both of these transformations obviously only apply if the
|
||||
pointer argument is loop-invariant.
|
||||
</p>
|
||||
pointer argument is loop-invariant.</p>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
@ -403,12 +391,12 @@ pointer argument is loop-invariant.
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
<p>
|
||||
As a last resort, your pass could use the AliasAnalysis interface directly to
|
||||
|
||||
<p>As a last resort, your pass could use the AliasAnalysis interface directly to
|
||||
service your pass. If you find the need to do this, please <a
|
||||
href="mailto:sabre@nondot.org">let me know</a> so I can see if something new
|
||||
needs to be added to LLVM.
|
||||
</p>
|
||||
needs to be added to LLVM.</p>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- *********************************************************************** -->
|
||||
@ -418,10 +406,11 @@ needs to be added to LLVM.
|
||||
<!-- *********************************************************************** -->
|
||||
|
||||
<div class="doc_text">
|
||||
<p>
|
||||
If you're going to be working with the AliasAnalysis infrastructure, there are
|
||||
several nice tools that may be useful for you and are worth knowing about...
|
||||
</p>
|
||||
|
||||
<p>If you're going to be working with the AliasAnalysis infrastructure, there
|
||||
are several nice tools that may be useful for you and are worth knowing
|
||||
about...</p>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
@ -430,13 +419,13 @@ several nice tools that may be useful for you and are worth knowing about...
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
<p>
|
||||
The <tt>-no-aa</tt> analysis is just like what it sounds: an alias analysis that
|
||||
never returns any useful information. This pass can be useful if you think that
|
||||
alias analysis is doing something wrong and are trying to narrow down a problem.
|
||||
If you don't specify an alias analysis, the default will be to use the
|
||||
<tt>basicaa</tt> pass which does quite a bit of disambiguation on its own.
|
||||
</p>
|
||||
|
||||
<p>The <tt>-no-aa</tt> analysis is just like what it sounds: an alias analysis
|
||||
that never returns any useful information. This pass can be useful if you think
|
||||
that alias analysis is doing something wrong and are trying to narrow down a
|
||||
problem. If you don't specify an alias analysis, the default will be to use the
|
||||
<tt>basicaa</tt> pass which does quite a bit of disambiguation on its own.</p>
|
||||
|
||||
</div>
|
||||
|
||||
|
||||
@ -446,12 +435,12 @@ If you don't specify an alias analysis, the default will be to use the
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
<p>
|
||||
The <tt>-print-alias-sets</tt> pass is exposed as part of the <tt>analyze</tt>
|
||||
tool to print out the Alias Sets formed by the <a
|
||||
|
||||
<p>The <tt>-print-alias-sets</tt> pass is exposed as part of the
|
||||
<tt>analyze</tt> tool to print out the Alias Sets formed by the <a
|
||||
href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
|
||||
the <tt>AliasSetTracker</tt>.
|
||||
</p>
|
||||
the <tt>AliasSetTracker</tt>.</p>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
@ -460,22 +449,20 @@ the <tt>AliasSetTracker</tt>.
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
<p>
|
||||
The <tt>-count-aa</tt> pass is useful to see how many queries a particular pass
|
||||
is making and what kinds of responses are returned by the alias analysis. An
|
||||
example usage is:
|
||||
</p>
|
||||
<p>
|
||||
|
||||
<p>The <tt>-count-aa</tt> pass is useful to see how many queries a particular
|
||||
pass is making and what kinds of responses are returned by the alias analysis.
|
||||
An example usage is:</p>
|
||||
|
||||
<pre>
|
||||
$ opt -basicaa -count-aa -ds-aa -count-aa -licm
|
||||
</pre>
|
||||
</p>
|
||||
<p>
|
||||
Which will print out how many queries (and what responses are returned) by the
|
||||
<tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are made
|
||||
of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be useful
|
||||
when evaluating an alias analysis for precision.
|
||||
</p>
|
||||
|
||||
<p>Which will print out how many queries (and what responses are returned) by
|
||||
the <tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are
|
||||
made of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be
|
||||
useful when evaluating an alias analysis for precision.</p>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- ======================================================================= -->
|
||||
@ -484,20 +471,22 @@ when evaluating an alias analysis for precision.
|
||||
</div>
|
||||
|
||||
<div class="doc_text">
|
||||
<p>
|
||||
The <tt>-aa-eval</tt> pass simply iterates through all pairs of pointers in a
|
||||
|
||||
<p>The <tt>-aa-eval</tt> pass simply iterates through all pairs of pointers in a
|
||||
function and asks an alias analysis whether or not the pointers alias. This
|
||||
gives an indication of the precision of the alias analysis. Statistics are
|
||||
printed.
|
||||
</p>
|
||||
|
||||
</div>
|
||||
|
||||
<!-- *********************************************************************** -->
|
||||
|
||||
<hr><font size="-1">
|
||||
<address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
|
||||
Last modified: $Date$
|
||||
</font>
|
||||
<hr>
|
||||
<div class="doc_footer">
|
||||
<address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
|
||||
Last modified: $Date$
|
||||
</div>
|
||||
|
||||
</body>
|
||||
</html>
|
||||
|
||||
Loading…
Reference in New Issue
Block a user