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521 lines
19 KiB
HTML
<!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>
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<head>
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<title>Alias Analysis Infrastructure in LLVM</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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</head>
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<body>
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<div class="doc_title">
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Alias Analysis Infrastructure in LLVM
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</div>
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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>
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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></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></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></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></li>
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</ol>
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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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<a name="introduction">Introduction</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>Alias Analysis (or Pointer Analysis) is a technique which attempts to
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determine whether or not two pointers ever can point to the same object in
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memory. Traditionally, Alias Analyses respond to a query with either a <a
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href="#MustNoMay">Must, May, or No</a> alias response, indicating that two
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pointers do point to the same object, might point to the same object, or are
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known not to point to the same object.</p>
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<p>The <a href="/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a> class is the
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centerpiece of the LLVM Alias Analysis related infrastructure. This class is
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the common interface between clients of alias analysis information and the
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implementations providing it. In addition to simple alias analysis information,
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this class exposes Mod/Ref information from those implementations which can
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provide it, allowing for powerful analyses and transformations to work well
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together.</p>
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<p>This document contains information necessary to successfully implement this
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interface, use it, and to test both sides. It also explains some of the finer
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points about what exactly results mean. If you feel that something is unclear
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or should be added, please <a href="mailto:sabre@nondot.org">let me
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know</a>.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="overview">AliasAnalysis Overview</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>The <a href="/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a> class
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defines the interface that Alias Analysis implementations should support. This
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class exports two important enums: <tt>AliasResult</tt> and
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<tt>ModRefResult</tt> which represent the result of an alias query or a mod/ref
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query, respectively.</p>
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<p>The AliasAnalysis interface exposes information about memory, represented in
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several different ways. In particular, memory objects are represented as a
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starting address and size, and function calls are represented as the actual
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<tt>call</tt> or <tt>invoke</tt> instructions that performs the call. The
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AliasAnalysis interface also exposes some helper methods which allow you to get
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mod/ref information for arbitrary instructions.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="pointers">Representation of Pointers</a>
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</div>
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<div class="doc_text">
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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.</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:</p>
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<pre>
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int i;
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char C[2];
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char A[10];
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/* ... */
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for (i = 0; i != 10; ++i) {
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C[0] = A[i]; /* One byte store */
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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>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:</p>
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<pre>
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int i;
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char C[2];
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char A[10];
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/* ... */
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for (i = 0; i != 10; ++i) {
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((short*)C)[0] = A[i]; /* Two byte store! */
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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>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.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="MustMayNo">Must, May, and No Alias Responses</a>
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</div>
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<div class="doc_text">
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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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<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.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="ModRefInfo">The <tt>getModRefInfo</tt> methods</a>
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</div>
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<div class="doc_text">
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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.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="writingnew">Writing a new AliasAnalysis Implementation</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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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
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for examples, and the following information should help fill in any details.
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For a minimal example, take a look at the <a
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href="/doxygen/structllvm_1_1NoAA.html"><tt>no-aa</tt></a> implementation.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="passsubclasses">Different Pass styles</a>
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</div>
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<div class="doc_text">
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<p>The first step to determining what type of <a
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href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias
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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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<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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<li>If you are a global analysis, subclass <tt>FunctionPass</tt>.</li>
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<li>If you are a local pass, subclass <tt>BasicBlockPass</tt>.</li>
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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>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>.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="requiredcalls">Required initialization calls</a>
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</div>
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<div class="doc_text">
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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
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<tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
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<tt>getAnalysisUsage</tt> should explicitly call into the
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<tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
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declaring any pass dependencies your pass has. Thus you should have something
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like this:</p>
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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>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
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from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
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<tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, <tt>runOnBasicBlock</tt> for
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a <tt>BasicBlockPass</tt>, or <tt>InitializeAliasAnalysis</tt> for an
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<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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<i>// Perform analysis here...</i>
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return false;
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}
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</pre>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="interfaces">Interfaces which may be specified</a>
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</div>
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<div class="doc_text">
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<p>All of the <a href="/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>
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virtual methods default to providing conservatively correct information
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(returning "May" Alias and "Mod/Ref" for alias and mod/ref queries
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respectively). Depending on the capabilities of the analysis you are
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implementing, you just override the interfaces you can improve.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="chaining">The AliasAnalysis chaining behavior</a>
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</div>
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<div class="doc_text">
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<p>With only two special exceptions (the <tt>basicaa</tt> and <a
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href="#no-aa"><tt>no-aa</tt></a> passes) every alias analysis pass should chain
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to another alias analysis implementation (for example, you could specify
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"<tt>-basic-aa -ds-aa -andersens-aa -licm</tt>" to get the maximum benefit from
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the three alias analyses). To do this, simply "Require" AliasAnalysis in your
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<tt>getAnalysisUsage</tt> method, and if you need to return a conservative
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MayAlias or Mod/Ref result, simply chain to a lower analysis.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="implefficiency">Efficiency Issues</a>
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</div>
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<div class="doc_text">
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<p>From the LLVM perspective, the only thing you need to do to provide an
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efficient alias analysis is to make sure that alias analysis <b>queries</b> are
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serviced quickly. The actual calculation of the alias analysis results (the
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"run" method) is only performed once, but many (perhaps duplicate) queries may
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be performed. Because of this, try to move as much computation to the run
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method as possible (within reason).</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="using">Using AliasAnalysis results</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>There are several different ways to use alias analysis results. In order of
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preference, these are...</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="loadvn">Using the <tt>-load-vn</tt> Pass</a>
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</div>
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<div class="doc_text">
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<p>The <tt>load-vn</tt> pass uses alias analysis to provide value numbering
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information for <tt>load</tt> instructions. If your analysis or transformation
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can be modelled in a form that uses value numbering information, you don't have
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to do anything special to handle load instructions: just use the
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<tt>load-vn</tt> pass, which uses alias analysis.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
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</div>
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<div class="doc_text">
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<p>Many transformations need information about alias <b>sets</b> that are active
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in some scope, rather than information about pairwise aliasing. The <tt><a
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href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class is used
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to efficiently build these Alias Sets from the pairwise alias analysis
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information provided by the AliasAnalysis interface.</p>
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<p>First you initialize the AliasSetTracker by use the "<tt>add</tt>" methods to
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add information about various potentially aliasing instructions in the scope you
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are interested in. Once all of the alias sets are completed, your pass should
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simply iterate through the constructed alias sets, using the AliasSetTracker
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<tt>begin()</tt>/<tt>end()</tt> methods.</p>
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<p>The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed
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to be disjoint, calculate mod/ref information and volatility for the set, and
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keep track of whether or not all of the pointers in the set are Must aliases.
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The AliasSetTracker also makes sure that sets are properly folded due to call
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instructions, and can provide a list of pointers in each set.</p>
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<p>As an example user of this, the <a href="/doxygen/structLICM.html">Loop
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Invariant Code Motion</a> pass uses AliasSetTrackers to build alias information
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about each loop nest. If an AliasSet in a loop is not modified, then all load
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instructions from that set may be hoisted out of the loop. If any alias sets
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are stored <b>and</b> are must alias sets, then the stores may be sunk to
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outside of the loop, promoting the memory location to a register for the
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duration of the loop nest. Both of these transformations obviously only apply
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if the pointer argument is loop-invariant.</p>
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</div>
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<div class="doc_subsubsection">
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The AliasSetTracker implementation
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</div>
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<div class="doc_text">
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<p>The AliasSetTracker class is implemented to be as efficient as possible. It
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uses the union-find algorithm to efficiently merge AliasSets when a pointer is
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inserted into the AliasSetTracker that aliases multiple sets. The primary data
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structure is a hash table mapping pointers to the AliasSet they are in.</p>
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<p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s
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that are in each AliasSet. Since the hash table already has entries for each
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LLVM Value* of interest, the AliasesSets thread the linked list through these
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hash-table nodes to avoid having to allocate memory unnecessarily, and to make
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merging alias sets extremely efficient (the linked list merge is constant time).
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</p>
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<p>You shouldn't need to understand these details if you are just a client of
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the AliasSetTracker, but if you look at the code, hopefully this brief
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description will help make sense of why things are designed the way they
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are.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="direct">Using the AliasAnalysis interface directly</a>
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</div>
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<div class="doc_text">
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<p>As a last resort, your pass could use the AliasAnalysis interface directly to
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service your pass. If you find the need to do this, please <a
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href="mailto:sabre@nondot.org">let me know</a> so I can see if something new
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needs to be added to LLVM.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="tools">Helpful alias-analysis-related tools</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>If you're going to be working with the AliasAnalysis infrastructure, there
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are several nice tools that may be useful for you and are worth knowing
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about...</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="no-aa">The <tt>-no-aa</tt> pass</a>
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</div>
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<div class="doc_text">
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<p>The <tt>-no-aa</tt> analysis is just like what it sounds: an alias analysis
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that never returns any useful information. This pass can be useful if you think
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that alias analysis is doing something wrong and are trying to narrow down a
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problem. If you don't specify an alias analysis, the default will be to use the
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<tt>basicaa</tt> pass which does quite a bit of disambiguation on its own.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
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</div>
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<div class="doc_text">
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<p>The <tt>-print-alias-sets</tt> pass is exposed as part of the
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<tt>analyze</tt> tool to print out the Alias Sets formed by the <a
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href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
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the <tt>AliasSetTracker</tt>.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="count-aa">The <tt>-count-aa</tt> pass</a>
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</div>
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<div class="doc_text">
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<p>The <tt>-count-aa</tt> pass is useful to see how many queries a particular
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pass is making and what kinds of responses are returned by the alias analysis.
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An example usage is:</p>
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<pre>
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$ opt -basicaa -count-aa -ds-aa -count-aa -licm
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</pre>
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<p>Which will print out how many queries (and what responses are returned) by
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the <tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are
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made of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be
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useful when evaluating an alias analysis for precision.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
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</div>
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<div class="doc_text">
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<p>The <tt>-aa-eval</tt> pass simply iterates through all pairs of pointers in a
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function and asks an alias analysis whether or not the pointers alias. This
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gives an indication of the precision of the alias analysis. Statistics are
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printed.</p>
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</div>
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<!-- *********************************************************************** -->
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<hr>
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<address>
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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
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<a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a><br>
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Last modified: $Date$
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</address>
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