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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html><head><title>Alias Analysis Infrastructure in LLVM</title></head> <html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<link rel="stylesheet" href="llvm.css" type="text/css" media="screen" />
<title>Alias Analysis Infrastructure in LLVM</title>
</head>
<body bgcolor=white> <body>
<table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0> <div class="doc_title">
<tr><td>&nbsp; <font size=+3 color="#EEEEFF" face="Georgia,Palatino,Times,Roman"><b>Alias Analysis Infrastructure in LLVM</b></font></td> Alias Analysis Infrastructure in LLVM
</tr></table> </div>
<ol> <ol>
<li><a href="#introduction">Introduction</a> <li><a href="#introduction">Introduction</a>
@@ -45,70 +50,80 @@
</ol><p> </ol><p>
<!-- *********************************************************************** --> <!-- *********************************************************************** -->
<table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0> <div class="doc_section">
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b> <a name="introduction">Introduction</a>
<a name="introduction">Introduction </div>
</b></font></td></tr></table><ul>
<!-- *********************************************************************** --> <!-- *********************************************************************** -->
<div class="doc_text">
<p>
Alias Analysis (or Pointer Analysis) is a technique which attempts to determine 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. 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 Traditionally, Alias Analyses respond to a query with either a <a
href="#MustNoMay">Must, May, or No</a> alias response, indicating that two 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 pointers do point to the same object, might point to the same object, or are
known not to point to the same object.<p> known not to point to the same object.
</p>
<p>
The <a href="/doxygen/classAliasAnalysis.html">AliasAnalysis</a> class is the The <a href="/doxygen/classAliasAnalysis.html">AliasAnalysis</a> class is the
centerpiece of the LLVM Alias Analysis related infrastructure. This class is centerpiece of the LLVM Alias Analysis related infrastructure. This class is
the common interface between clients of alias analysis information and the the common interface between clients of alias analysis information and the
implementations providing it. In addition to simple alias analysis information, implementations providing it. In addition to simple alias analysis information,
this class exposes Mod/Ref information from those implementations which can this class exposes Mod/Ref information from those implementations which can
provide it, allowing for powerful analyses and transformations to work well provide it, allowing for powerful analyses and transformations to work well
together.<p> together.
</p>
<p>
This document contains information necessary to successfully implement this This document contains information necessary to successfully implement this
interface, use it, and to test both sides. It also explains some of the finer 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 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> or should be added, please <a href="mailto:sabre@nondot.org">let me
know</a>.
</p>
</div>
<!-- *********************************************************************** --> <!-- *********************************************************************** -->
</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0> <div class="doc_section">
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b> <a name="overview">AliasAnalysis Overview</a>
<a name="overview">AliasAnalysis Overview </div>
</b></font></td></tr></table><ul>
<!-- *********************************************************************** --> <!-- *********************************************************************** -->
<div class="doc_text">
<p>
The <a href="/doxygen/classAliasAnalysis.html">AliasAnalysis</a> class defines The <a href="/doxygen/classAliasAnalysis.html">AliasAnalysis</a> class defines
the interface that Alias Analysis implementations should support. This class the interface that Alias Analysis implementations should support. This class
exports two important enums: <tt>AliasResult</tt> and <tt>ModRefResult</tt> exports two important enums: <tt>AliasResult</tt> and <tt>ModRefResult</tt>
which represent the result of an alias query or a mod/ref query, which represent the result of an alias query or a mod/ref query,
respectively.<p> respectively.
</p>
<p>
The AliasAnalysis interface exposes information about memory, represented in The AliasAnalysis interface exposes information about memory, represented in
several different ways. In particular, memory objects are represented as a several different ways. In particular, memory objects are represented as a
starting address and size, and function calls are represented as the actual 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>call</tt> or <tt>invoke</tt> instructions that performs the call. The
AliasAnalysis interface also exposes some helper methods which allow you to get AliasAnalysis interface also exposes some helper methods which allow you to get
mod/ref information for arbitrary instructions.<p> mod/ref information for arbitrary instructions.
</p>
</div>
<!-- ======================================================================= --> <!-- ======================================================================= -->
</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0> <div class="doc_subsection">
<tr><td>&nbsp;</td><td width="100%">&nbsp; <a name="pointers">Representation of Pointers</a>
<font color="#EEEEFF" face="Georgia,Palatino"><b> </div>
<a name="pointers">Representation of Pointers
</b></font></td></tr></table><ul>
<div class="doc_text">
<p>
Most importantly, the AliasAnalysis class provides several methods which are Most importantly, the AliasAnalysis class provides several methods which are
used to query whether or not pointers alias, whether function calls can modify used to query whether or not pointers alias, whether function calls can modify
or read memory, etc.<p> or read memory, etc.
</p>
<p>
Representing memory objects as a starting address and a size is critically Representing memory objects as a starting address and a size is critically
important for precise Alias Analyses. For example, consider this (silly) C important for precise Alias Analyses. For example, consider this (silly) C
code:<p> code:
</p>
<p>
<pre> <pre>
int i; int i;
char C[2]; char C[2];
@@ -119,13 +134,15 @@ code:<p>
C[1] = A[9-i]; /* One byte store */ C[1] = A[9-i]; /* One byte store */
} }
</pre> </pre>
</p>
<p>
In this case, the <tt>basicaa</tt> pass will disambiguate the stores to 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 <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 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 LICM pass can use store motion to remove the stores from the loop. In
constrast, the following code:<p> constrast, the following code:
</p>
<p>
<pre> <pre>
int i; int i;
char C[2]; char C[2];
@@ -136,113 +153,126 @@ constrast, the following code:<p>
C[1] = A[9-i]; /* One byte store */ C[1] = A[9-i]; /* One byte store */
} }
</pre> </pre>
</p>
<p>
In this case, the two stores to C do alias each other, because the access to the In this case, the two stores to C do alias each other, because the access to the
<tt>&amp;C[0]</tt> element is a two byte access. If size information wasn't <tt>&amp;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 available in the query, even the first case would have to conservatively assume
that the accesses alias.<p> that the accesses alias.
</p>
</div>
<!-- ======================================================================= --> <!-- ======================================================================= -->
</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0> <div class="doc_subsection">
<tr><td>&nbsp;</td><td width="100%">&nbsp; <a name="MustMayNo">Must, May, and No Alias Responses</a>
<font color="#EEEEFF" face="Georgia,Palatino"><b> </div>
<a name="MustMayNo">Must, May, and No Alias Responses
</b></font></td></tr></table><ul>
<div class="doc_text">
<p>
An Alias Analysis implementation can return one of three responses: MustAlias, 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 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 pointers may never equal each other, return NoAlias, if they might, return
MayAlias.<p> MayAlias.
</p>
<p>
The Must Alias response is trickier though. In LLVM, the Must Alias response 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 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 exactly the same location. If two memory objects overlap, but do not start at
the same location, MayAlias must be returned.<p> the same location, MayAlias must be returned.
</p>
</div>
<!-- ======================================================================= --> <!-- ======================================================================= -->
</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0> <div class="doc_subsection">
<tr><td>&nbsp;</td><td width="100%">&nbsp; <a name="ModRefInfo">The <tt>getModRefInfo</tt> methods</a>
<font color="#EEEEFF" face="Georgia,Palatino"><b> </div>
<a name="ModRefInfo">The <tt>getModRefInfo</tt> methods
</b></font></td></tr></table><ul>
<div class="doc_text">
<p>
The <tt>getModRefInfo</tt> methods return information about whether the The <tt>getModRefInfo</tt> methods return information about whether the
execution of an instruction can read or modify a memory location. Mod/Ref 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 information is always conservative: if an action <b>may</b> read a location, Ref
is returned.<p> is returned.
</p>
</div>
<!-- *********************************************************************** --> <!-- *********************************************************************** -->
</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0> <div class="doc_section">
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b> <a name="writingnew">Writing a new AliasAnalysis Implementation</a>
<a name="writingnew">Writing a new AliasAnalysis Implementation </div>
</b></font></td></tr></table><ul>
<!-- *********************************************************************** --> <!-- *********************************************************************** -->
<div class="doc_text">
<p>
Writing a new alias analysis implementation for LLVM is quite straight-forward. 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 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, 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> take a look at the <a href="/doxygen/structNoAA.html"><tt>no-aa</tt></a>
implementation.<p> implementation.
</p>
</div>
<!-- ======================================================================= --> <!-- ======================================================================= -->
</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0> <div class="doc_subsection">
<tr><td>&nbsp;</td><td width="100%">&nbsp; <a name="passsubclasses">Different Pass styles</a>
<font color="#EEEEFF" face="Georgia,Palatino"><b> </div>
<a name="passsubclasses">Different Pass styles
</b></font></td></tr></table><ul>
<div class="doc_text">
<p>
The first step to determining what type of <a href="WritingAnLLVMPass.html">LLVM 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 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 other analyses and transformations, the answer should be fairly obvious from
what type of problem you are trying to solve:<p> what type of problem you are trying to solve:
</p>
<p>
<ol> <ol>
<li>If you require interprocedural analysis, it should be a <tt>Pass</tt>. <li>If you require interprocedural analysis, it should be a
<li>If you are a global analysis, subclass <tt>FunctionPass</tt>. <tt>Pass</tt>.</li>
<li>If you are a local pass, subclass <tt>BasicBlockPass</tt>. <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 <li>If you don't need to look at the program at all, subclass
<tt>ImmutablePass</tt>. <tt>ImmutablePass</tt>.</li>
</ol><p> </ol>
</p>
<p>
In addition to the pass that you subclass, you should also inherit from the In addition to the pass that you subclass, you should also inherit from the
<tt>AliasAnalysis</tt> interface, of course, and use the <tt>AliasAnalysis</tt> interface, of course, and use the
<tt>RegisterAnalysisGroup</tt> template to register as an implementation of <tt>RegisterAnalysisGroup</tt> template to register as an implementation of
<tt>AliasAnalysis</tt>.<p> <tt>AliasAnalysis</tt>.
</p>
</div>
<!-- ======================================================================= --> <!-- ======================================================================= -->
</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0> <div class="doc_subsection">
<tr><td>&nbsp;</td><td width="100%">&nbsp; <a name="requiredcalls">Required initialization calls</a>
<font color="#EEEEFF" face="Georgia,Palatino"><b> </div>
<a name="requiredcalls">Required initialization calls
</b></font></td></tr></table><ul>
<div class="doc_text">
<p>
Your subclass of AliasAnalysis is required to invoke two methods on the Your subclass of AliasAnalysis is required to invoke two methods on the
AliasAnalysis base class: <tt>getAnalysisUsage</tt> and AliasAnalysis base class: <tt>getAnalysisUsage</tt> and
<tt>InitializeAliasAnalysis</tt>. In particular, your implementation of <tt>InitializeAliasAnalysis</tt>. In particular, your implementation of
<tt>getAnalysisUsage</tt> should explicitly call into the <tt>getAnalysisUsage</tt> should explicitly call into the
<tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any <tt>AliasAnalysis::getAnalysisUsage</tt> method in addition to doing any
declaring any pass dependencies your pass has. Thus you should have something declaring any pass dependencies your pass has. Thus you should have something
like this:<p> like this:
</p>
<p>
<pre> <pre>
void getAnalysisUsage(AnalysisUsage &amp;AU) const { void getAnalysisUsage(AnalysisUsage &amp;AU) const {
AliasAnalysis::getAnalysisUsage(AU); AliasAnalysis::getAnalysisUsage(AU);
<i>// declare your dependencies here.</i> <i>// declare your dependencies here.</i>
} }
</pre> </pre>
</p>
<p>
Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method from Additionally, your must invoke the <tt>InitializeAliasAnalysis</tt> method from
your analysis run method (<tt>run</tt> for a <tt>Pass</tt>, your analysis run method (<tt>run</tt> for a <tt>Pass</tt>,
<tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, <tt>runOnBasicBlock</tt> for <tt>runOnFunction</tt> for a <tt>FunctionPass</tt>, <tt>runOnBasicBlock</tt> for
a <tt>BasicBlockPass</tt>, or <tt>InitializeAliasAnalysis</tt> for an a <tt>BasicBlockPass</tt>, or <tt>InitializeAliasAnalysis</tt> for an
<tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):<p> <tt>ImmutablePass</tt>). For example (as part of a <tt>Pass</tt>):
</p>
<p>
<pre> <pre>
bool run(Module &amp;M) { bool run(Module &amp;M) {
InitializeAliasAnalysis(this); InitializeAliasAnalysis(this);
@@ -250,196 +280,218 @@ a <tt>BasicBlockPass</tt>, or <tt>InitializeAliasAnalysis</tt> for an
return false; return false;
} }
</pre> </pre>
</p>
</div>
<!-- ======================================================================= --> <!-- ======================================================================= -->
</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0> <div class="doc_subsection">
<tr><td>&nbsp;</td><td width="100%">&nbsp; <a name="interfaces">Interfaces which may be specified</a>
<font color="#EEEEFF" face="Georgia,Palatino"><b> </div>
<a name="interfaces">Interfaces which may be specified
</b></font></td></tr></table><ul>
<div class="doc_text">
<p>
All of the <a href="/doxygen/classAliasAnalysis.html">AliasAnalysis</a> virtual All of the <a href="/doxygen/classAliasAnalysis.html">AliasAnalysis</a> virtual
methods default to providing conservatively correct information (returning "May" methods default to providing conservatively correct information (returning "May"
Alias and "Mod/Ref" for alias and mod/ref queries respectively). Depending on 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 the capabilities of the analysis you are implementing, you just override the
interfaces you can improve. interfaces you can improve.
</p>
</div>
<!-- ======================================================================= --> <!-- ======================================================================= -->
</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0> <div class="doc_subsection">
<tr><td>&nbsp;</td><td width="100%">&nbsp; <a name="chaining">The AliasAnalysis chaining behavior</a>
<font color="#EEEEFF" face="Georgia,Palatino"><b> </div>
<a name="chaining">The AliasAnalysis chaining behavior
</b></font></td></tr></table><ul>
<div class="doc_text">
<p>
With only two special exceptions (the <tt>basicaa</tt> and <a 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 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 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 "<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 the three alias analyses). To do this, simply "Require" AliasAnalysis in your
<tt>getAnalysisUsage</tt> method, and if you need to return a conservative <tt>getAnalysisUsage</tt> method, and if you need to return a conservative
MayAlias or Mod/Ref result, simply chain to a lower analysis.<p> MayAlias or Mod/Ref result, simply chain to a lower analysis.
</p>
</div>
<!-- ======================================================================= --> <!-- ======================================================================= -->
</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0> <div class="doc_subsection">
<tr><td>&nbsp;</td><td width="100%">&nbsp; <a name="implefficiency">Efficiency Issues</a>
<font color="#EEEEFF" face="Georgia,Palatino"><b> </div>
<a name="implefficiency">Efficiency Issues
</b></font></td></tr></table><ul>
<div class="doc_text">
<p>
From the LLVM perspective, the only thing you need to do to provide an efficient 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 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" quickly. The actual calculation of the alias analysis results (the "run"
method) is only performed once, but many (perhaps duplicate) queries may be 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 performed. Because of this, try to move as much computation to the run method
as possible (within reason).<p> as possible (within reason).
</p>
</div>
<!-- *********************************************************************** --> <!-- *********************************************************************** -->
</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0> <div class="doc_section">
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b> <a name="using">Using AliasAnalysis results</a>
<a name="using">Using AliasAnalysis results </div>
</b></font></td></tr></table><ul>
<!-- *********************************************************************** --> <!-- *********************************************************************** -->
<div class="doc_text">
<p>
There are several different ways to use alias analysis results. In order of There are several different ways to use alias analysis results. In order of
preference, these are...<p> preference, these are...
</p>
</div>
<!-- ======================================================================= --> <!-- ======================================================================= -->
</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0> <div class="doc_subsection">
<tr><td>&nbsp;</td><td width="100%">&nbsp; <a name="loadvn">Using the <tt>-load-vn</tt> Pass</a>
<font color="#EEEEFF" face="Georgia,Palatino"><b> </div>
<a name="loadvn">Using the <tt>-load-vn</tt> Pass
</b></font></td></tr></table><ul>
<div class="doc_text">
<p>
The <tt>load-vn</tt> pass uses alias analysis to provide value numbering The <tt>load-vn</tt> pass uses alias analysis to provide value numbering
information for <tt>load</tt> instructions. If your analysis or transformation 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 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 to do anything special to handle load instructions: just use the
<tt>load-vn</tt> pass, which uses alias analysis.<p> <tt>load-vn</tt> pass, which uses alias analysis.
</p>
</div>
<!-- ======================================================================= --> <!-- ======================================================================= -->
</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0> <div class="doc_subsection">
<tr><td>&nbsp;</td><td width="100%">&nbsp; <a name="ast">Using the <tt>AliasSetTracker</tt> class</a>
<font color="#EEEEFF" face="Georgia,Palatino"><b> </div>
<a name="ast">Using the <tt>AliasSetTracker</tt> class
</b></font></td></tr></table><ul>
<div class="doc_text">
<p>
Many transformations need information about alias <b>sets</b> that are active in Many transformations need information about alias <b>sets</b> that are active in
some scope, rather than information about pairwise aliasing. The <tt><a some scope, rather than information about pairwise aliasing. The <tt><a
href="/doxygen/classAliasSetTracker.html">AliasSetTracker</a></tt> class is used href="/doxygen/classAliasSetTracker.html">AliasSetTracker</a></tt> class is used
to efficiently build these Alias Sets from the pairwise alias analysis to efficiently build these Alias Sets from the pairwise alias analysis
information provided by the AliasAnalysis interface.<p> information provided by the AliasAnalysis interface.
</p>
<p>
First you initialize the AliasSetTracker by use the "<tt>add</tt>" methods to First you initialize the AliasSetTracker by use the "<tt>add</tt>" methods to
add information about various potentially aliasing instructions in the scope you 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 are interested in. Once all of the alias sets are completed, your pass should
simply iterate through the constructed alias sets, using the AliasSetTracker simply iterate through the constructed alias sets, using the AliasSetTracker
<tt>begin()</tt>/<tt>end()</tt> methods.<p> <tt>begin()</tt>/<tt>end()</tt> methods.
</p>
<p>
The <tt>AliasSet</tt>s formed by the <tt>AliasSetTracker</tt> are guaranteed to 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 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 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 AliasSetTracker also makes sure that sets are properly folded due to call
instructions, and can provide a list of pointers in each set.<p> 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 As an example user of this, the <a href="/doxygen/structLICM.html">Loop
Invariant Code Motion</a> pass uses AliasSetTrackers to build alias information 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 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 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 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 outside of the loop. Both of these transformations obviously only apply if the
pointer argument is loop-invariant.<p> pointer argument is loop-invariant.
</p>
</div>
<!-- ======================================================================= --> <!-- ======================================================================= -->
</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0> <div class="doc_subsection">
<tr><td>&nbsp;</td><td width="100%">&nbsp; <a name="direct">Using the AliasAnalysis interface directly</a>
<font color="#EEEEFF" face="Georgia,Palatino"><b> </div>
<a name="direct">Using the AliasAnalysis interface directly
</b></font></td></tr></table><ul>
<div class="doc_text">
<p>
As a last resort, your pass could use the AliasAnalysis interface directly to 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 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 href="mailto:sabre@nondot.org">let me know</a> so I can see if something new
needs to be added to LLVM.<p> needs to be added to LLVM.
</p>
</div>
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</ul><table width="100%" bgcolor="#330077" border=0 cellpadding=4 cellspacing=0> <div class="doc_section">
<tr><td align=center><font color="#EEEEFF" size=+2 face="Georgia,Palatino"><b> <a name="tools">Helpful alias-analysis-related tools</a>
<a name="tools">Helpful alias analysis related tools </div>
</b></font></td></tr></table><ul>
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<div class="doc_text">
<p>
If you're going to be working with the AliasAnalysis infrastructure, there are 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> several nice tools that may be useful for you and are worth knowing about...
</p>
</div>
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</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0> <div class="doc_subsection">
<tr><td>&nbsp;</td><td width="100%">&nbsp; <a name="no-aa">The <tt>-no-aa</tt> pass</a>
<font color="#EEEEFF" face="Georgia,Palatino"><b> </div>
<a name="no-aa">The <tt>-no-aa</tt> pass
</b></font></td></tr></table><ul>
<div class="doc_text">
<p>
The <tt>-no-aa</tt> analysis is just like what it sounds: an alias analysis that 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 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. 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 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> <tt>basicaa</tt> pass which does quite a bit of disambiguation on its own.
</p>
</div>
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</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0> <div class="doc_subsection">
<tr><td>&nbsp;</td><td width="100%">&nbsp; <a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass</a>
<font color="#EEEEFF" face="Georgia,Palatino"><b> </div>
<a name="print-alias-sets">The <tt>-print-alias-sets</tt> pass
</b></font></td></tr></table><ul>
<div class="doc_text">
<p>
The <tt>-print-alias-sets</tt> pass is exposed as part of the <tt>analyze</tt> 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 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 href="#ast"><tt>AliasSetTracker</tt></a> class. This is useful if you're using
the <tt>AliasSetTracker</tt>.<p> the <tt>AliasSetTracker</tt>.
</p>
</div>
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</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0> <div class="doc_subsection">
<tr><td>&nbsp;</td><td width="100%">&nbsp;
<font color="#EEEEFF" face="Georgia,Palatino"><b>
<a name="count-aa">The <tt>-count-aa</tt> pass</a> <a name="count-aa">The <tt>-count-aa</tt> pass</a>
</b></font></td></tr></table><ul> </div>
<div class="doc_text">
<p>
The <tt>-count-aa</tt> pass is useful to see how many queries a particular pass 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 is making and what kinds of responses are returned by the alias analysis. An
example usage is:<p> example usage is:
</p>
<p>
<pre> <pre>
$ opt -basicaa -count-aa -ds-aa -count-aa -licm $ opt -basicaa -count-aa -ds-aa -count-aa -licm
</pre> </pre>
</p>
<p>
Which will print out how many queries (and what responses are returned) by the 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 <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 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> when evaluating an alias analysis for precision.
</p>
</div>
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</ul><table width="50%" bgcolor="#441188" border=0 cellpadding=4 cellspacing=0> <div class="doc_subsection">
<tr><td>&nbsp;</td><td width="100%">&nbsp; <a name="aa-eval">The <tt>-aa-eval</tt> pass</a>
<font color="#EEEEFF" face="Georgia,Palatino"><b> </div>
<a name="aa-eval">The <tt>-aa-eval</tt> pass
</b></font></td></tr></table><ul>
<div class="doc_text">
<p>
The <tt>-aa-eval</tt> pass simply iterates through all pairs of pointers in a 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 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 gives an indication of the precision of the alias analysis. Statistics are
printed.<p> printed.
</p>
</div>
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</ul>
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