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|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>LLVM Alias Analysis Infrastructure</title>
<link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<body>
<div class="doc_title">
LLVM Alias Analysis Infrastructure
</div>
<ol>
<li><a href="#introduction">Introduction</a></li>
<li><a href="#overview"><tt>AliasAnalysis</tt> Class Overview</a>
<ul>
<li><a href="#pointers">Representation of Pointers</a></li>
<li><a href="#alias">The <tt>alias</tt> method</a></li>
<li><a href="#ModRefInfo">The <tt>getModRefInfo</tt> methods</a></li>
<li><a href="#OtherItfs">Other useful <tt>AliasAnalysis</tt> methods</a></li>
</ul>
</li>
<li><a href="#writingnew">Writing a new <tt>AliasAnalysis</tt> Implementation</a>
<ul>
<li><a href="#passsubclasses">Different Pass styles</a></li>
<li><a href="#requiredcalls">Required initialization calls</a></li>
<li><a href="#interfaces">Interfaces which may be specified</a></li>
<li><a href="#chaining"><tt>AliasAnalysis</tt> chaining behavior</a></li>
<li><a href="#updating">Updating analysis results for transformations</a></li>
<li><a href="#implefficiency">Efficiency Issues</a></li>
</ul>
</li>
<li><a href="#using">Using alias analysis results</a>
<ul>
<li><a href="#memdep">Using the <tt>MemoryDependenceAnalysis</tt> Pass</a></li>
<li><a href="#ast">Using the <tt>AliasSetTracker</tt> class</a></li>
<li><a href="#direct">Using the <tt>AliasAnalysis</tt> interface directly</a></li>
</ul>
</li>
<li><a href="#exist">Existing alias analysis implementations and clients</a>
<ul>
<li><a href="#impls">Available <tt>AliasAnalysis</tt> implementations</a></li>
<li><a href="#aliasanalysis-xforms">Alias analysis driven transformations</a></li>
<li><a href="#aliasanalysis-debug">Clients for debugging and evaluation of
implementations</a></li>
</ul>
</li>
<li><a href="#memdep">Memory Dependence Analysis</a></li>
</ol>
<div class="doc_author">
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="introduction">Introduction</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>Alias Analysis (aka Pointer Analysis) is a class of techniques which attempt
to determine whether or not two pointers ever can point to the same object in
memory. There are many different algorithms for alias analysis and many
different ways of classifying them: flow-sensitive vs flow-insensitive,
context-sensitive vs context-insensitive, field-sensitive vs field-insensitive,
unification-based vs subset-based, etc. Traditionally, alias analyses respond
to a query with a <a href="#MustMayNo">Must, May, or No</a> alias response,
indicating that two pointers always point to the same object, might point to the
same object, or are known to never point to the same object.</p>
<p>The LLVM <a
href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
class is the primary interface used by clients and implementations of alias
analyses in the LLVM system. This class is the common interface between clients
of alias analysis information and the implementations providing it, and is
designed to support a wide range of implementations and clients (but currently
all clients are assumed to be flow-insensitive). In addition to simple alias
analysis information, this class exposes Mod/Ref information from those
implementations which can provide it, allowing for powerful analyses and
transformations to work well together.</p>
<p>This document contains information necessary to successfully implement this
interface, use it, and to test both sides. It also explains some of the finer
points about what exactly results mean. If you feel that something is unclear
or should be added, please <a href="mailto:sabre@nondot.org">let me
know</a>.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="overview"><tt>AliasAnalysis</tt> Class Overview</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>The <a
href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
class defines the interface that the various alias analysis implementations
should support. This class exports two important enums: <tt>AliasResult</tt>
and <tt>ModRefResult</tt> which represent the result of an alias query or a
mod/ref query, respectively.</p>
<p>The <tt>AliasAnalysis</tt> interface exposes information about memory,
represented in several different ways. In particular, memory objects are
represented as a starting address and size, and function calls are represented
as the actual <tt>call</tt> or <tt>invoke</tt> instructions that performs the
call. The <tt>AliasAnalysis</tt> interface also exposes some helper methods
which allow you to get mod/ref information for arbitrary instructions.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="pointers">Representation of Pointers</a>
</div>
<div class="doc_text">
<p>Most importantly, the <tt>AliasAnalysis</tt> class provides several methods
which are used to query whether or not two memory objects alias, whether
function calls can modify or read a memory object, etc. For all of these
queries, memory objects are represented as a pair of their starting address (a
symbolic LLVM <tt>Value*</tt>) and a static size.</p>
<p>Representing memory objects as a starting address and a size is critically
important for correct Alias Analyses. For example, consider this (silly, but
possible) C code:</p>
<div class="doc_code">
<pre>
int i;
char C[2];
char A[10];
/* ... */
for (i = 0; i != 10; ++i) {
C[0] = A[i]; /* One byte store */
C[1] = A[9-i]; /* One byte store */
}
</pre>
</div>
<p>In this case, the <tt>basicaa</tt> pass will disambiguate the stores to
<tt>C[0]</tt> and <tt>C[1]</tt> because they are accesses to two distinct
locations one byte apart, and the accesses are each one byte. In this case, the
LICM pass can use store motion to remove the stores from the loop. In
constrast, the following code:</p>
<div class="doc_code">
<pre>
int i;
char C[2];
char A[10];
/* ... */
for (i = 0; i != 10; ++i) {
((short*)C)[0] = A[i]; /* Two byte store! */
C[1] = A[9-i]; /* One byte store */
}
</pre>
</div>
<p>In this case, the two stores to C do alias each other, because the access to
the <tt
|
