mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-10-29 23:12:55 +01:00
452 lines
20 KiB
HTML
452 lines
20 KiB
HTML
<!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>
|