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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Alias Analysis Infrastructure in LLVM</title>
<link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<body>
<div class="doc_title">
Alias Analysis Infrastructure in LLVM
</div>
<ol>
<li><a href="#introduction">Introduction</a></li>
<li><a href="#overview">AliasAnalysis Overview</a>
<ul>
<li><a href="#pointers">Representation of Pointers</a></li>
<li><a href="#MustMayNo">Must, May, and No Alias Responses</a></li>
<li><a href="#ModRefInfo">The <tt>getModRefInfo</tt> methods</a></li>
</ul></li>
<li><a href="#writingnew">Writing a new AliasAnalysis 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">The AliasAnalysis chaining behavior</a></li>
<li><a href="#implefficiency">Efficiency Issues</a></li>
</ul></li>
<li><a href="#using">Using AliasAnalysis results</a>
<ul>
<li><a href="#loadvn">Using the <tt>-load-vn</tt> Pass</a></li>
<li><a href="#ast">Using the <tt>AliasSetTracker</tt> class</a></li>
<li><a href="#direct">Using the AliasAnalysis interface directly</a></li>
</ul></li>
<li><a href="#tools">Helpful alias analysis related tools</a>
<ul>
<li><a href="#no-aa">The <tt>-no-aa</tt> pass</a></li>
<li><a href="#print-alias-sets">The <tt>-print-alias-sets</tt> pass</a></li>
<li><a href="#count-aa">The <tt>-count-aa</tt> pass</a></li>
<li><a href="#aa-eval">The <tt>-aa-eval</tt> pass</a></li>
</ul></li>
</ol>
<div class="doc_text">
<p><b>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></b></p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="introduction">Introduction</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>Alias Analysis (or Pointer Analysis) is a technique which attempts to
determine whether or not two pointers ever can point to the same object in
memory. Traditionally, Alias Analyses respond to a query with either a <a
href="#MustNoMay">Must, May, or No</a> alias response, indicating that two
pointers do point to the same object, might point to the same object, or are
known not to point to the same object.</p>
<p>The <a href="/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a> class is the
centerpiece of the LLVM Alias Analysis related infrastructure. This class is
the common interface between clients of alias analysis information and the
implementations providing it. 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">AliasAnalysis Overview</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>The <a href="/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a> class
defines the interface that 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 AliasAnalysis 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
AliasAnalysis 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 AliasAnalysis class provides several methods which are
used to query whether or not pointers alias, whether function calls can modify
or read memory, etc.</p>
<p>Representing memory objects as a starting address and a size is critically
important for precise Alias Analyses. For example, consider this (silly) C
code:</p>
<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>
<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>
<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>
<p>In this case, the two stores to C do alias each other, because the access to
the <tt>&C[0]</tt> element is a two byte access. If size information wasn't
available in the query, even the first case would have to conservatively assume
that the accesses alias.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="MustMayNo">Must, May, and No Alias Responses</a>
</div>
<div class="doc_text">
<p>An Alias Analysis implementation can return one of three responses:
MustAlias, MayAlias, and NoAlias. The No and May alias results are obvious: if
the two pointers may never equal each other, return NoAlias, if they might,
return MayAlias.</p>
<p>The Must Alias response is trickier though. In LLVM, the Must Alias response
may only be returned if the two memory objects are guaranteed to always start at
exactly the same location. If two memory objects overlap, but do not start at
the same location, MayAlias must be returned.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="ModRefInfo">The <tt>getModRefInfo</tt> methods</a>
</div>
<div class="doc_text">
<p>The <tt>getModRefInfo</tt> methods return information about whether the
execution of an instruction can read or modify a memory location. Mod/Ref
information is always conservative: if an action <b>may</b> read a location, Ref
is returned.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="writingnew">Writing a new AliasAnalysis Implementation</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>Writing a new alias analysis implementation for LLVM is quite
straight-forward. There are already several implementations that you can use
for examples, and the following information should help fill in any details.
For a minimal example, take a look at the <a
href="/doxygen/structllvm_1_1NoAA.html"><tt>no-aa</tt></a> implementation.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="passsubclasses">Different Pass styles</a>
</div>
<div class="doc_text">
<p>The first step to determining what type of <a
href="WritingAnLLVMPass.html">LLVM pass</a> you need to use for your Alias
Analysis. As is the case with most other analyses and transformations, the
answer should be fairly obvious from what type of problem you are trying to
solve:</p>
<ol>
<li>If you require interprocedural analysis, it should be a
<tt>Pass</tt>.</li>
<li>If you are a global analysis, subclass <tt>FunctionPass</tt>.</li>
<li>If you are a local pass, subclass <tt>BasicBlockPass</tt>.</li>
<li>If you don't need to look at the program at all, subclass
<tt>ImmutablePass</tt>.</li>
</ol>
<p>In addition to the pass that you subclass, you should also inherit from the
<tt>AliasAnalysis</tt> interface, of course, and use the
<tt>RegisterAnalysisGroup</tt> template to register as an implementation of
<tt>AliasAnalysis</tt>.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="requiredcalls">Required initialization calls</a>
</div>
<div class="doc_text">
<p>Your subclass of AliasAnalysis is required to invoke two methods on the
AliasAnalysis base class: <tt>getAnalysisUsage</tt> and
<tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
<tt>getAnalysisUsage</tt> should explicitly call into the
<tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
declaring any pass dependencies your pass has. Thus you should have something
like this:</p>
<pre>
void getAnalysisUsage(AnalysisUsage &AU) const {
AliasAnalysis::getAnalysisUsage(AU);
<i>// declare your dependencies here.</i>
}
</pre>
<p>Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method
from your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
<tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, <tt>runOnBasicBlock</tt> for
a <tt>BasicBlockPass</tt>, or <tt>InitializeAliasAnalysis</tt> for an
<tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):</p>
<pre>
bool run(Module &M) {
InitializeAliasAnalysis(this);
<i>// Perform analysis here...</i>
return false;
}
</pre>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="interfaces">Interfaces which may be specified</a>
</div>
<div class="doc_text">
<p>All of the <a href=&qu
|
