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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
+<html><head><title>Alias Analysis Infrastructure in LLVM</title></head>
+
+<body bgcolor=white>
+
+<table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
+<tr><td> <font size=+3 color="#EEEEFF" face="Georgia,Palatino,Times,Roman"><b>Alias Analysis Infrastructure in LLVM</b></font></td>
+</tr></table>
+
+<ol>
+ <li><a href="#introduction">Introduction</a>
+
+ <li><a href="#overview">AliasAnalysis Overview</a>
+ <ul>
+ <li><a href="#pointers">Representation of Pointers</a>
+ <li><a href="#MustMayNo">Must, May, and No Alias Responses</a>
+ <li><a href="#ModRefInfo">The <tt>getModRefInfo</tt> methods</a>
+ </ul>
+
+ <li><a href="#writingnew">Writing a new AliasAnalysis Implementation</a>
+ <ul>
+ <li><a href="#passsubclasses">Different Pass styles</a>
+ <li><a href="#requiredcalls">Required initialization calls</a>
+ <li><a href="#interfaces">Interfaces which may be specified</a>
+ <li><a href="#chaining">The AliasAnalysis chaining behavior</a>
+ <li><a href="#implefficiency">Efficiency Issues</a>
+ </ul>
+
+ <li><a href="#using">Using AliasAnalysis results</a>
+ <ul>
+ <li><a href="#loadvn">Using the <tt>-load-vn</tt> Pass</a>
+ <li><a href="#ast">Using the <tt>AliasSetTracker</tt> class</a>
+ <li><a href="#direct">Using the AliasAnalysis interface directly</a>
+ </ul>
+ <li><a href="#tools">Helpful alias analysis related tools</a>
+ <ul>
+ <li><a href="#no-aa">The <tt>-no-aa</tt> pass</a>
+ <li><a href="#print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
+ <li><a href="#count-aa">The <tt>-count-aa</tt> pass</a>
+ <li><a href="#aa-eval">The <tt>-aa-eval</tt> pass</a>
+ </ul>
+ </ul>
+
+ <p><b>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></b><p>
+</ol><p>
+
+
+
+<!-- *********************************************************************** -->
+<table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
+<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
+<a name="introduction">Introduction
+</b></font></td></tr></table><ul>
+<!-- *********************************************************************** -->
+
+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>
+
+The <a href="/doxygen/classAliasAnalysis.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>
+
+This document contains information neccesary 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>
+
+
+<!-- *********************************************************************** -->
+</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
+<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
+<a name="overview">AliasAnalysis Overview
+</b></font></td></tr></table><ul>
+<!-- *********************************************************************** -->
+
+The <a href="/doxygen/classAliasAnalysis.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>
+
+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>
+
+<!-- ======================================================================= -->
+</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
+<tr><td> </td><td width="100%">
+<font color="#EEEEFF" face="Georgia,Palatino"><b>
+<a name="pointers">Representation of Pointers
+</b></font></td></tr></table><ul>
+
+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>
+
+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>
+
+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>
+
+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>
+
+
+<!-- ======================================================================= -->
+</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
+<tr><td> </td><td width="100%">
+<font color="#EEEEFF" face="Georgia,Palatino"><b>
+<a name="MustMayNo">Must, May, and No Alias Responses
+</b></font></td></tr></table><ul>
+
+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>
+
+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>
+
+
+<!-- ======================================================================= -->
+</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
+<tr><td> </td><td width="100%">
+<font color="#EEEEFF" face="Georgia,Palatino"><b>
+<a name="ModRefInfo">The <tt>getModRefInfo</tt> methods
+</b></font></td></tr></table><ul>
+
+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>
+
+
+
+<!-- *********************************************************************** -->
+</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
+<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
+<a name="writingnew">Writing a new AliasAnalysis Implementation
+</b></font></td></tr></table><ul>
+<!-- *********************************************************************** -->
+
+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/structNoAA.html"><tt>no-aa</tt></a>
+implementation.<p>
+
+
+<!-- ======================================================================= -->
+</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
+<tr><td> </td><td width="100%">
+<font color="#EEEEFF" face="Georgia,Palatino"><b>
+<a name="passsubclasses">Different Pass styles
+</b></font></td></tr></table><ul>
+
+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>If you are a global analysis, subclass <tt>FunctionPass</tt>.
+<li>If you are a local pass, subclass <tt>BasicBlockPass</tt>.
+<li>If you don't need to look at the program at all, subclass
+ <tt>ImmutablePass</tt>.
+</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>
+
+
+<!-- ======================================================================= -->
+</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
+<tr><td> </td><td width="100%">
+<font color="#EEEEFF" face="Georgia,Palatino"><b>
+<a name="requiredcalls">Required initialization calls
+</b></font></td></tr></table><ul>
+
+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>
+
+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>
+
+
+<!-- ======================================================================= -->
+</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
+<tr><td> </td><td width="100%">
+<font color="#EEEEFF" face="Georgia,Palatino"><b>
+<a name="interfaces">Interfaces which may be specified
+</b></font></td></tr></table><ul>
+
+All of the <a href="/doxygen/classAliasAnalysis.html">AliasAnalysis</a> virtual
+methods default to providing conservatively correct information (returning "May"
+Alias and "Mod/Ref" for alias and mod/ref queries respectively). Depending on
+the capabilities of the analysis you are implementing, you just override the
+interfaces you can improve.
+
+
+<!-- ======================================================================= -->
+</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
+<tr><td> </td><td width="100%">
+<font color="#EEEEFF" face="Georgia,Palatino"><b>
+<a name="chaining">The AliasAnalysis chaining behavior
+</b></font></td></tr></table><ul>
+
+With only two special exceptions (the <tt>basicaa</tt> and <a
+href="#no-aa"><tt>no-aa</tt></a> passes) every alias analysis pass should chain
+to another alias analysis implementation (for example, you could specify
+"<tt>-basic-aa -ds-aa -andersens-aa -licm</tt>" to get the maximum benefit from
+the three alias analyses). To do this, simply "Require" AliasAnalysis in your
+<tt>getAnalysisUsage</tt> method, and if you need to return a conservative
+MayAlias or Mod/Ref result, simply chain to a lower analysis.<p>
+
+
+<!-- ======================================================================= -->
+</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
+<tr><td> </td><td width="100%">
+<font color="#EEEEFF" face="Georgia,Palatino"><b>
+<a name="implefficiency">Efficiency Issues
+</b></font></td></tr></table><ul>
+
+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>
+
+
+<!-- *********************************************************************** -->
+</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
+<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
+<a name="using">Using AliasAnalysis results
+</b></font></td></tr></table><ul>
+<!-- *********************************************************************** -->
+
+There are several different ways to use alias analysis results. In order of
+preference, these are...<p>
+
+<!-- ======================================================================= -->
+</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
+<tr><td> </td><td width="100%">
+<font color="#EEEEFF" face="Georgia,Palatino"><b>
+<a name="loadvn">Using the <tt>-load-vn</tt> Pass
+</b></font></td></tr></table><ul>
+
+The <tt>load-vn</tt> pass uses alias analysis to provide value numbering
+information for <tt>load</tt> instructions. If your analysis or transformation
+can be modelled in a form that uses value numbering information, you don't have
+to do anything special to handle load instructions: just use the
+<tt>load-vn</tt> pass, which uses alias analysis.<p>
+
+
+<!-- ======================================================================= -->
+</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
+<tr><td> </td><td width="100%">
+<font color="#EEEEFF" face="Georgia,Palatino"><b>
+<a name="ast">Using the <tt>AliasSetTracker</tt> class
+</b></font></td></tr></table><ul>
+
+Many transformations need information about alias <b>sets</b> that are active in
+some scope, rather than information about pairwise aliasing. The <tt><a
+href="/doxygen/classAliasSetTracker.html">AliasSetTracker</a></tt> class is used
+to efficiently build these Alias Sets from the pairwise alias analysis
+information provided by the AliasAnalysis interface.<p>
+
+First you initialize the AliasSetTracker by use the "<tt>add</tt>" methods to
+add information about various potentially aliasing instructions in the scope you
+are interested in. Once all of the alias sets are completed, your pass should
+simply iterate through the constructed alias sets, using the AliasSetTracker
+<tt>begin()</tt>/<tt>end()</tt> methods.<p>
+
+The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed to
+be disjoint, calculate mod/ref information for the set, and keep track of
+whether or not all of the pointers in the set are Must aliases. The
+AliasSetTracker also makes sure that sets are properly folded due to call
+instructions, and can provide a list of pointers in each set.<p>
+
+As an example user of this, the <a href="/doxygen/structLICM.html">Loop
+Invariant Code Motion</a> pass uses AliasSetTrackers to build alias information
+about each loop nest. If an AliasSet in a loop is not modified, then all load
+instructions from that set may be hoisted out of the loop. If any alias sets
+are stored <b>and</b> are must alias sets, then the stores may be sunk to
+outside of the loop. Both of these transformations obviously only apply if the
+pointer argument is loop-invariant.<p>
+
+
+<!-- ======================================================================= -->
+</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
+<tr><td> </td><td width="100%">
+<font color="#EEEEFF" face="Georgia,Palatino"><b>
+<a name="direct">Using the AliasAnalysis interface directly
+</b></font></td></tr></table><ul>
+
+As a last resort, your pass could use the AliasAnalysis interface directly to
+service your pass. If you find the need to do this, please <a
+href="mailto:sabre@nondot.org">let me know</a> so I can see if something new
+needs to be added to LLVM.<p>
+
+
+<!-- *********************************************************************** -->
+</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0>
+<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b>
+<a name="tools">Helpful alias analysis related tools
+</b></font></td></tr></table><ul>
+<!-- *********************************************************************** -->
+
+If you're going to be working with the AliasAnalysis infrastructure, there are
+several nice tools that may be useful for you and are worth knowing about...<p>
+
+<!-- ======================================================================= -->
+</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
+<tr><td> </td><td width="100%">
+<font color="#EEEEFF" face="Georgia,Palatino"><b>
+<a name="no-aa">The <tt>-no-aa</tt> pass
+</b></font></td></tr></table><ul>
+
+The <tt>-no-aa</tt> analysis is just like what it sounds: an alias analysis that
+never returns any useful information. This pass can be useful if you think that
+alias analysis is doing something wrong and are trying to narrow down a problem.
+If you don't specify an alias analysis, the default will be to use the
+<tt>basicaa</tt> pass which does quite a bit of disambiguation on its own.<p>
+
+
+<!-- ======================================================================= -->
+</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
+<tr><td> </td><td width="100%">
+<font color="#EEEEFF" face="Georgia,Palatino"><b>
+<a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass
+</b></font></td></tr></table><ul>
+
+The <tt>-print-alias-sets</tt> pass is exposed as part of the <tt>analyze</tt>
+tool to print out the Alias Sets formed by the <a
+href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
+the <tt>AliasSetTracker</tt>.<p>
+
+
+<!-- ======================================================================= -->
+</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
+<tr><td> </td><td width="100%">
+<font color="#EEEEFF" face="Georgia,Palatino"><b>
+<a name="count-aa">The <tt>-count-aa</tt> pass</a>
+</b></font></td></tr></table><ul>
+
+The <tt>-count-aa</tt> pass is useful to see how many queries a particular pass
+is making and what kinds of responses are returned by the alias analysis. An
+example usage is:<p>
+
+<pre>
+ $ opt -basicaa -count-aa -ds-aa -count-aa -licm
+</pre>
+
+Which will print out how many queries (and what responses are returned) by the
+<tt>-licm</tt> pass (of the <tt>-ds-aa</tt> pass) and how many queries are made
+of the <tt>-basicaa</tt> pass by the <tt>-ds-aa</tt> pass. This can be useful
+when evaluating an alias analysis for precision.<p>
+
+<!-- ======================================================================= -->
+</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0>
+<tr><td> </td><td width="100%">
+<font color="#EEEEFF" face="Georgia,Palatino"><b>
+<a name="aa-eval">The <tt>-aa-eval</tt> pass
+</b></font></td></tr></table><ul>
+
+The <tt>-aa-eval</tt> pass simply iterates through all pairs of pointers in a
+function and asks an alias analysis whether or not the pointers alias. This
+gives an indication of the precision of the alias analysis. Statistics are
+printed.<p>
+
+
+<!-- *********************************************************************** -->
+</ul>
+<!-- *********************************************************************** -->
+
+<hr><font size=-1>
+<address><a href="mailto:sabre@nondot.org">Chris Lattner</a></address>
+<!-- Created: Wed Feb 26 10:40:50 CST 2003 -->
+<!-- hhmts start -->
+Last modified: Tue Mar 4 13:36:53 CST 2003
+<!-- hhmts end -->
+</font></body></html>