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8bdae4db80
callee will not introduce any new aliases of that pointer. The attributes had all bits allocated already, so I decided to collapse alignment. Alignment was previously stored as a 16-bit integer from bits 16 to 32 of the attribute, but it was required to be a power of 2. Now it's stored in log2 encoded form in five bits from 16 to 21. That gives us 11 more bits of space. You may have already noticed that you only need four bits to encode a 16-bit power of two, so why five bits? Because the AsmParser accepted 32-bit alignments, even though we couldn't store them (they were silently discarded). Now we can store them in memory, but not in the bitcode. The bitcode format was already storing these as 64-bit VBR integers. So, the bitcode format stays the same, keeping the alignment values stored as 16 bit raw values. There's some hideous code in the reader and writer that deals with this, waiting to be ripped out the moment we run out of bits again and have to replace the parameter attributes table encoding. llvm-svn: 61019
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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>
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<head>
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<title>LLVM Alias Analysis Infrastructure</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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LLVM Alias Analysis Infrastructure
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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"><tt>AliasAnalysis</tt> Class 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="#alias">The <tt>alias</tt> method</a></li>
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<li><a href="#ModRefInfo">The <tt>getModRefInfo</tt> methods</a></li>
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<li><a href="#OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a></li>
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</ul>
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</li>
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<li><a href="#writingnew">Writing a new <tt>AliasAnalysis</tt> 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"><tt>AliasAnalysis</tt> chaining behavior</a></li>
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<li><a href="#updating">Updating analysis results for transformations</a></li>
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<li><a href="#implefficiency">Efficiency Issues</a></li>
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</ul>
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</li>
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<li><a href="#using">Using alias analysis 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 <tt>AliasAnalysis</tt> interface directly</a></li>
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</ul>
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</li>
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<li><a href="#exist">Existing alias analysis implementations and clients</a>
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<ul>
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<li><a href="#impls">Available <tt>AliasAnalysis</tt> implementations</a></li>
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<li><a href="#aliasanalysis-xforms">Alias analysis driven transformations</a></li>
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<li><a href="#aliasanalysis-debug">Clients for debugging and evaluation of
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implementations</a></li>
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</ul>
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</li>
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<li><a href="#memdep">Memory Dependence Analysis</a></li>
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</ol>
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<div class="doc_author">
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<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</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="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 (aka Pointer Analysis) is a class of techniques which attempt
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to determine whether or not two pointers ever can point to the same object in
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memory. There are many different algorithms for alias analysis and many
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different ways of classifying them: flow-sensitive vs flow-insensitive,
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context-sensitive vs context-insensitive, field-sensitive vs field-insensitive,
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unification-based vs subset-based, etc. Traditionally, alias analyses respond
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to a query with a <a href="#MustMayNo">Must, May, or No</a> alias response,
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indicating that two pointers always point to the same object, might point to the
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same object, or are known to never point to the same object.</p>
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<p>The LLVM <a
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href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
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class is the primary interface used by clients and implementations of alias
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analyses in the LLVM system. This class is the common interface between clients
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of alias analysis information and the implementations providing it, and is
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designed to support a wide range of implementations and clients (but currently
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all clients are assumed to be flow-insensitive). In addition to simple alias
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analysis information, this class exposes Mod/Ref information from those
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implementations which can provide it, allowing for powerful analyses and
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transformations to work well 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"><tt>AliasAnalysis</tt> Class 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
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href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
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class defines the interface that the various alias analysis implementations
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should support. This class exports two important enums: <tt>AliasResult</tt>
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and <tt>ModRefResult</tt> which represent the result of an alias query or a
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mod/ref query, respectively.</p>
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<p>The <tt>AliasAnalysis</tt> interface exposes information about memory,
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represented in several different ways. In particular, memory objects are
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represented as a starting address and size, and function calls are represented
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as the actual <tt>call</tt> or <tt>invoke</tt> instructions that performs the
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call. The <tt>AliasAnalysis</tt> interface also exposes some helper methods
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which allow you to get 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 <tt>AliasAnalysis</tt> class provides several methods
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which are used to query whether or not two memory objects alias, whether
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function calls can modify or read a memory object, etc. For all of these
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queries, memory objects are represented as a pair of their starting address (a
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symbolic LLVM <tt>Value*</tt>) and a static size.</p>
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<p>Representing memory objects as a starting address and a size is critically
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important for correct Alias Analyses. For example, consider this (silly, but
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possible) C code:</p>
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<div class="doc_code">
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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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</div>
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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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<div class="doc_code">
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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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</div>
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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="alias">The <tt>alias</tt> method</a>
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</div>
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<div class="doc_text">
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The <tt>alias</tt> method is the primary interface used to determine whether or
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not two memory objects alias each other. It takes two memory objects as input
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and returns MustAlias, MayAlias, or NoAlias as appropriate.
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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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>The NoAlias response is used when the two pointers refer to distinct objects,
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regardless of whether the pointers compare equal. For example, freed pointers
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don't alias any pointers that were allocated afterwards. As a degenerate case,
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pointers returned by malloc(0) have no bytes for an object, and are considered
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NoAlias even when malloc returns the same pointer. The same rule applies to
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NULL pointers.</p>
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<p>The MayAlias response is used whenever the two pointers might refer to the
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same object. If the two memory objects overlap, but do not start at the same
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location, return MayAlias.</p>
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<p>The MustAlias response may only be returned if the two memory objects are
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guaranteed to always start at exactly the same location. A MustAlias response
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implies that the pointers compare equal.</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 instruction <b>might</b> read or write
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a location, ModRef is returned.</p>
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<p>The <tt>AliasAnalysis</tt> class also provides a <tt>getModRefInfo</tt>
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method for testing dependencies between function calls. This method takes two
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call sites (CS1 & CS2), returns NoModRef if the two calls refer to disjoint
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memory locations, Ref if CS1 reads memory written by CS2, Mod if CS1 writes to
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memory read or written by CS2, or ModRef if CS1 might read or write memory
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accessed by CS2. Note that this relation is not commutative. Clients that use
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this method should be predicated on the <tt>hasNoModRefInfoForCalls()</tt>
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method, which indicates whether or not an analysis can provide mod/ref
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information for function call pairs (most can not). If this predicate is false,
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the client shouldn't waste analysis time querying the <tt>getModRefInfo</tt>
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method many times.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a>
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</div>
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<div class="doc_text">
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<p>
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Several other tidbits of information are often collected by various alias
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analysis implementations and can be put to good use by various clients.
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</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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The <tt>getMustAliases</tt> method
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</div>
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<div class="doc_text">
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<p>The <tt>getMustAliases</tt> method returns all values that are known to
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always must alias a pointer. This information can be provided in some cases for
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important objects like the null pointer and global values. Knowing that a
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pointer always points to a particular function allows indirect calls to be
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turned into direct calls, for example.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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The <tt>pointsToConstantMemory</tt> method
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</div>
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<div class="doc_text">
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<p>The <tt>pointsToConstantMemory</tt> method returns true if and only if the
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analysis can prove that the pointer only points to unchanging memory locations
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(functions, constant global variables, and the null pointer). This information
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can be used to refine mod/ref information: it is impossible for an unchanging
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memory location to be modified.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="simplemodref">The <tt>doesNotAccessMemory</tt> and
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<tt>onlyReadsMemory</tt> methods</a>
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</div>
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<div class="doc_text">
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<p>These methods are used to provide very simple mod/ref information for
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function calls. The <tt>doesNotAccessMemory</tt> method returns true for a
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function if the analysis can prove that the function never reads or writes to
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memory, or if the function only reads from constant memory. Functions with this
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property are side-effect free and only depend on their input arguments, allowing
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them to be eliminated if they form common subexpressions or be hoisted out of
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loops. Many common functions behave this way (e.g., <tt>sin</tt> and
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<tt>cos</tt>) but many others do not (e.g., <tt>acos</tt>, which modifies the
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<tt>errno</tt> variable).</p>
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<p>The <tt>onlyReadsMemory</tt> method returns true for a function if analysis
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can prove that (at most) the function only reads from non-volatile memory.
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Functions with this property are side-effect free, only depending on their input
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arguments and the state of memory when they are called. This property allows
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calls to these functions to be eliminated and moved around, as long as there is
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no store instruction that changes the contents of memory. Note that all
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functions that satisfy the <tt>doesNotAccessMemory</tt> method also satisfies
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<tt>onlyReadsMemory</tt>.</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 <tt>AliasAnalysis</tt> 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 examples, take a look at the <a href="#impls">various alias analysis
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implementations</a> included with LLVM.</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 function-local analysis, subclass <tt>FunctionPass</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 <tt>AliasAnalysis</tt> is required to invoke two methods on
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the <tt>AliasAnalysis</tt> 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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<div class="doc_code">
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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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</div>
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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>, or <tt>InitializePass</tt>
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for an <tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
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<div class="doc_code">
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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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</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
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href="/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
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virtual methods default to providing <a href="#chaining">chaining</a> to another
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alias analysis implementation, which ends up returning conservatively correct
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information (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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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="chaining"><tt>AliasAnalysis</tt> 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><a
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href="#basic-aa">basicaa</a></tt> and <a href="#no-aa"><tt>no-aa</tt></a>
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passes) every alias analysis pass chains to another alias analysis
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implementation (for example, the user can specify "<tt>-basicaa -ds-aa
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-anders-aa -licm</tt>" to get the maximum benefit from the three alias
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analyses). The alias analysis class automatically takes care of most of this
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for methods that you don't override. For methods that you do override, in code
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paths that return a conservative MayAlias or Mod/Ref result, simply return
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whatever the superclass computes. For example:</p>
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<div class="doc_code">
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<pre>
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AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
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const Value *V2, unsigned V2Size) {
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if (...)
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return NoAlias;
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...
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<i>// Couldn't determine a must or no-alias result.</i>
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return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
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}
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</pre>
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</div>
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<p>In addition to analysis queries, you must make sure to unconditionally pass
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LLVM <a href="#updating">update notification</a> methods to the superclass as
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well if you override them, which allows all alias analyses in a change to be
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updated.</p>
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</div>
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|
|
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<!-- ======================================================================= -->
|
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<div class="doc_subsection">
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<a name="updating">Updating analysis results for transformations</a>
|
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</div>
|
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|
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<div class="doc_text">
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<p>
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|
Alias analysis information is initially computed for a static snapshot of the
|
|
program, but clients will use this information to make transformations to the
|
|
code. All but the most trivial forms of alias analysis will need to have their
|
|
analysis results updated to reflect the changes made by these transformations.
|
|
</p>
|
|
|
|
<p>
|
|
The <tt>AliasAnalysis</tt> interface exposes two methods which are used to
|
|
communicate program changes from the clients to the analysis implementations.
|
|
Various alias analysis implementations should use these methods to ensure that
|
|
their internal data structures are kept up-to-date as the program changes (for
|
|
example, when an instruction is deleted), and clients of alias analysis must be
|
|
sure to call these interfaces appropriately.
|
|
</p>
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">The <tt>deleteValue</tt> method</div>
|
|
|
|
<div class="doc_text">
|
|
The <tt>deleteValue</tt> method is called by transformations when they remove an
|
|
instruction or any other value from the program (including values that do not
|
|
use pointers). Typically alias analyses keep data structures that have entries
|
|
for each value in the program. When this method is called, they should remove
|
|
any entries for the specified value, if they exist.
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">The <tt>copyValue</tt> method</div>
|
|
|
|
<div class="doc_text">
|
|
The <tt>copyValue</tt> method is used when a new value is introduced into the
|
|
program. There is no way to introduce a value into the program that did not
|
|
exist before (this doesn't make sense for a safe compiler transformation), so
|
|
this is the only way to introduce a new value. This method indicates that the
|
|
new value has exactly the same properties as the value being copied.
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">The <tt>replaceWithNewValue</tt> method</div>
|
|
|
|
<div class="doc_text">
|
|
This method is a simple helper method that is provided to make clients easier to
|
|
use. It is implemented by copying the old analysis information to the new
|
|
value, then deleting the old value. This method cannot be overridden by alias
|
|
analysis implementations.
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="implefficiency">Efficiency Issues</a>
|
|
</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>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="using">Using alias analysis results</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>There are several different ways to use alias analysis results. In order of
|
|
preference, these are...</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="loadvn">Using the <tt>-load-vn</tt> Pass</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>load-vn</tt> pass uses alias analysis to provide value numbering
|
|
information for <tt>load</tt> instructions and pointer values. If your analysis
|
|
or transformation can be modeled 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>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
|
|
</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
|
|
href="/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a></tt> class
|
|
is used to efficiently build these Alias Sets from the pairwise alias analysis
|
|
information provided by the <tt>AliasAnalysis</tt> interface.</p>
|
|
|
|
<p>First you initialize the AliasSetTracker by using 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
|
|
<tt>AliasSetTracker</tt> <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 and volatility 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
|
|
Invariant Code Motion</a> pass uses <tt>AliasSetTracker</tt>s to calculate alias
|
|
sets for each loop nest. If an <tt>AliasSet</tt> 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 to <b>and</b> are must alias sets, then the stores may be
|
|
sunk to outside of the loop, promoting the memory location to a register for the
|
|
duration of the loop nest. Both of these transformations only apply if the
|
|
pointer argument is loop-invariant.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
The AliasSetTracker implementation
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The AliasSetTracker class is implemented to be as efficient as possible. It
|
|
uses the union-find algorithm to efficiently merge AliasSets when a pointer is
|
|
inserted into the AliasSetTracker that aliases multiple sets. The primary data
|
|
structure is a hash table mapping pointers to the AliasSet they are in.</p>
|
|
|
|
<p>The AliasSetTracker class must maintain a list of all of the LLVM Value*'s
|
|
that are in each AliasSet. Since the hash table already has entries for each
|
|
LLVM Value* of interest, the AliasesSets thread the linked list through these
|
|
hash-table nodes to avoid having to allocate memory unnecessarily, and to make
|
|
merging alias sets extremely efficient (the linked list merge is constant time).
|
|
</p>
|
|
|
|
<p>You shouldn't need to understand these details if you are just a client of
|
|
the AliasSetTracker, but if you look at the code, hopefully this brief
|
|
description will help make sense of why things are designed the way they
|
|
are.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="direct">Using the <tt>AliasAnalysis</tt> interface directly</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>If neither of these utility class are what your pass needs, you should use
|
|
the interfaces exposed by the <tt>AliasAnalysis</tt> class directly. Try to use
|
|
the higher-level methods when possible (e.g., use mod/ref information instead of
|
|
the <a href="#alias"><tt>alias</tt></a> method directly if possible) to get the
|
|
best precision and efficiency.</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section">
|
|
<a name="exist">Existing alias analysis implementations and clients</a>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>If you're going to be working with the LLVM alias analysis infrastructure,
|
|
you should know what clients and implementations of alias analysis are
|
|
available. In particular, if you are implementing an alias analysis, you should
|
|
be aware of the <a href="#aliasanalysis-debug">the clients</a> that are useful
|
|
for monitoring and evaluating different implementations.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="impls">Available <tt>AliasAnalysis</tt> implementations</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>This section lists the various implementations of the <tt>AliasAnalysis</tt>
|
|
interface. With the exception of the <a href="#no-aa"><tt>-no-aa</tt></a> and
|
|
<a href="#basic-aa"><tt>-basicaa</tt></a> implementations, all of these <a
|
|
href="#chaining">chain</a> to other alias analysis implementations.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="no-aa">The <tt>-no-aa</tt> pass</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>-no-aa</tt> pass 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.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="basic-aa">The <tt>-basicaa</tt> pass</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>-basicaa</tt> pass is the default LLVM alias analysis. It is an
|
|
aggressive local analysis that "knows" many important facts:</p>
|
|
|
|
<ul>
|
|
<li>Distinct globals, stack allocations, and heap allocations can never
|
|
alias.</li>
|
|
<li>Globals, stack allocations, and heap allocations never alias the null
|
|
pointer.</li>
|
|
<li>Different fields of a structure do not alias.</li>
|
|
<li>Indexes into arrays with statically differing subscripts cannot alias.</li>
|
|
<li>Many common standard C library functions <a
|
|
href="#simplemodref">never access memory or only read memory</a>.</li>
|
|
<li>Pointers that obviously point to constant globals
|
|
"<tt>pointToConstantMemory</tt>".</li>
|
|
<li>Function calls can not modify or references stack allocations if they never
|
|
escape from the function that allocates them (a common case for automatic
|
|
arrays).</li>
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="globalsmodref">The <tt>-globalsmodref-aa</tt> pass</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>This pass implements a simple context-sensitive mod/ref and alias analysis
|
|
for internal global variables that don't "have their address taken". If a
|
|
global does not have its address taken, the pass knows that no pointers alias
|
|
the global. This pass also keeps track of functions that it knows never access
|
|
memory or never read memory. This allows certain optimizations (e.g. GCSE) to
|
|
eliminate call instructions entirely.
|
|
</p>
|
|
|
|
<p>The real power of this pass is that it provides context-sensitive mod/ref
|
|
information for call instructions. This allows the optimizer to know that
|
|
calls to a function do not clobber or read the value of the global, allowing
|
|
loads and stores to be eliminated.</p>
|
|
|
|
<p>Note that this pass is somewhat limited in its scope (only support
|
|
non-address taken globals), but is very quick analysis.</p>
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="anders-aa">The <tt>-anders-aa</tt> pass</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>-anders-aa</tt> pass implements the well-known "Andersen's algorithm"
|
|
for interprocedural alias analysis. This algorithm is a subset-based,
|
|
flow-insensitive, context-insensitive, and field-insensitive alias analysis that
|
|
is widely believed to be fairly precise. Unfortunately, this algorithm is also
|
|
O(N<sup>3</sup>). The LLVM implementation currently does not implement any of
|
|
the refinements (such as "online cycle elimination" or "offline variable
|
|
substitution") to improve its efficiency, so it can be quite slow in common
|
|
cases.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="steens-aa">The <tt>-steens-aa</tt> pass</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>-steens-aa</tt> pass implements a variation on the well-known
|
|
"Steensgaard's algorithm" for interprocedural alias analysis. Steensgaard's
|
|
algorithm is a unification-based, flow-insensitive, context-insensitive, and
|
|
field-insensitive alias analysis that is also very scalable (effectively linear
|
|
time).</p>
|
|
|
|
<p>The LLVM <tt>-steens-aa</tt> pass implements a "speculatively
|
|
field-<b>sensitive</b>" version of Steensgaard's algorithm using the Data
|
|
Structure Analysis framework. This gives it substantially more precision than
|
|
the standard algorithm while maintaining excellent analysis scalability.</p>
|
|
|
|
<p>Note that <tt>-steens-aa</tt> is available in the optional "poolalloc"
|
|
module, it is not part of the LLVM core.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="ds-aa">The <tt>-ds-aa</tt> pass</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>-ds-aa</tt> pass implements the full Data Structure Analysis
|
|
algorithm. Data Structure Analysis is a modular unification-based,
|
|
flow-insensitive, context-<b>sensitive</b>, and speculatively
|
|
field-<b>sensitive</b> alias analysis that is also quite scalable, usually at
|
|
O(n*log(n)).</p>
|
|
|
|
<p>This algorithm is capable of responding to a full variety of alias analysis
|
|
queries, and can provide context-sensitive mod/ref information as well. The
|
|
only major facility not implemented so far is support for must-alias
|
|
information.</p>
|
|
|
|
<p>Note that <tt>-ds-aa</tt> is available in the optional "poolalloc"
|
|
module, it is not part of the LLVM core.</p>
|
|
|
|
</div>
|
|
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="aliasanalysis-xforms">Alias analysis driven transformations</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
LLVM includes several alias-analysis driven transformations which can be used
|
|
with any of the implementations above.
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="adce">The <tt>-adce</tt> pass</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>-adce</tt> pass, which implements Aggressive Dead Code Elimination
|
|
uses the <tt>AliasAnalysis</tt> interface to delete calls to functions that do
|
|
not have side-effects and are not used.</p>
|
|
|
|
</div>
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="licm">The <tt>-licm</tt> pass</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>-licm</tt> pass implements various Loop Invariant Code Motion related
|
|
transformations. It uses the <tt>AliasAnalysis</tt> interface for several
|
|
different transformations:</p>
|
|
|
|
<ul>
|
|
<li>It uses mod/ref information to hoist or sink load instructions out of loops
|
|
if there are no instructions in the loop that modifies the memory loaded.</li>
|
|
|
|
<li>It uses mod/ref information to hoist function calls out of loops that do not
|
|
write to memory and are loop-invariant.</li>
|
|
|
|
<li>If uses alias information to promote memory objects that are loaded and
|
|
stored to in loops to live in a register instead. It can do this if there are
|
|
no may aliases to the loaded/stored memory location.</li>
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="argpromotion">The <tt>-argpromotion</tt> pass</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
<p>
|
|
The <tt>-argpromotion</tt> pass promotes by-reference arguments to be passed in
|
|
by-value instead. In particular, if pointer arguments are only loaded from it
|
|
passes in the value loaded instead of the address to the function. This pass
|
|
uses alias information to make sure that the value loaded from the argument
|
|
pointer is not modified between the entry of the function and any load of the
|
|
pointer.</p>
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="gcseloadvn">The <tt>-load-vn</tt> & <tt>-gcse</tt> passes</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>-load-vn</tt> pass uses alias analysis to "<a href="#loadvn">value
|
|
number</a>" loads and pointers values, which is used by the GCSE pass to
|
|
eliminate instructions. The <tt>-load-vn</tt> pass relies on alias information
|
|
and must-alias information. This combination of passes can make the following
|
|
transformations:</p>
|
|
|
|
<ul>
|
|
<li>Redundant load instructions are eliminated.</li>
|
|
<li>Load instructions that follow a store to the same location are replaced with
|
|
the stored value ("store forwarding").</li>
|
|
<li>Pointers values (e.g. formal arguments) that must-alias simpler expressions
|
|
(e.g. global variables or the null pointer) are replaced. Note that this
|
|
implements transformations like "virtual method resolution", turning indirect
|
|
calls into direct calls.</li>
|
|
</ul>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsection">
|
|
<a name="aliasanalysis-debug">Clients for debugging and evaluation of
|
|
implementations</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>These passes are useful for evaluating the various alias analysis
|
|
implementations. You can use them with commands like '<tt>opt -anders-aa -ds-aa
|
|
-aa-eval foo.bc -disable-output -stats</tt>'.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>The <tt>-print-alias-sets</tt> pass is exposed as part of the
|
|
<tt>opt</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> class. To use it, use something like:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
% opt -ds-aa -print-alias-sets -disable-output
|
|
</pre>
|
|
</div>
|
|
|
|
</div>
|
|
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="count-aa">The <tt>-count-aa</tt> pass</a>
|
|
</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 responses are returned by the alias analysis. As an
|
|
example,</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
% opt -basicaa -count-aa -ds-aa -count-aa -licm
|
|
</pre>
|
|
</div>
|
|
|
|
<p>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 debugging a transformation or an alias analysis implementation.</p>
|
|
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsubsection">
|
|
<a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
|
|
</div>
|
|
|
|
<div class="doc_text">
|
|
|
|
<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 indicating the percent of no/may/must aliases found (a more precise
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algorithm will have a lower number of may aliases).</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="memdep">Memory Dependence Analysis</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 just looking to be a client of alias analysis information, consider
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|
using the Memory Dependence Analysis interface instead. MemDep is a lazy,
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|
caching layer on top of alias analysis that is able to answer the question of
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|
what preceding memory operations a given instruction depends on, either at an
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|
intra- or inter-block level. Because of its laziness and caching
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|
policy, using MemDep can be a significant performance win over accessing alias
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|
analysis directly.</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>
|
|
<a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
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Last modified: $Date$
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</address>
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