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<div class="section" id="llvm-alias-analysis-infrastructure">
<h1>LLVM Alias Analysis Infrastructure<a class="headerlink" href="#llvm-alias-analysis-infrastructure" title="Permalink to this headline">¶</a></h1>
<div class="contents local topic" id="contents">
<ul class="simple">
<li><a class="reference internal" href="#introduction" id="id1">Introduction</a></li>
<li><a class="reference internal" href="#aliasanalysis-class-overview" id="id2"><code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> Class Overview</a><ul>
<li><a class="reference internal" href="#representation-of-pointers" id="id3">Representation of Pointers</a></li>
<li><a class="reference internal" href="#the-alias-method" id="id4">The <code class="docutils literal notranslate"><span class="pre">alias</span></code> method</a><ul>
<li><a class="reference internal" href="#must-may-and-no-alias-responses" id="id5">Must, May, and No Alias Responses</a></li>
</ul>
</li>
<li><a class="reference internal" href="#the-getmodrefinfo-methods" id="id6">The <code class="docutils literal notranslate"><span class="pre">getModRefInfo</span></code> methods</a></li>
<li><a class="reference internal" href="#other-useful-aliasanalysis-methods" id="id7">Other useful <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> methods</a><ul>
<li><a class="reference internal" href="#the-pointstoconstantmemory-method" id="id8">The <code class="docutils literal notranslate"><span class="pre">pointsToConstantMemory</span></code> method</a></li>
<li><a class="reference internal" href="#the-doesnotaccessmemory-and-onlyreadsmemory-methods" id="id9">The <code class="docutils literal notranslate"><span class="pre">doesNotAccessMemory</span></code> and <code class="docutils literal notranslate"><span class="pre">onlyReadsMemory</span></code> methods</a></li>
</ul>
</li>
</ul>
</li>
<li><a class="reference internal" href="#writing-a-new-aliasanalysis-implementation" id="id10">Writing a new <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> Implementation</a><ul>
<li><a class="reference internal" href="#different-pass-styles" id="id11">Different Pass styles</a></li>
<li><a class="reference internal" href="#required-initialization-calls" id="id12">Required initialization calls</a></li>
<li><a class="reference internal" href="#required-methods-to-override" id="id13">Required methods to override</a></li>
<li><a class="reference internal" href="#interfaces-which-may-be-specified" id="id14">Interfaces which may be specified</a></li>
<li><a class="reference internal" href="#aliasanalysis-chaining-behavior" id="id15"><code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> chaining behavior</a></li>
<li><a class="reference internal" href="#updating-analysis-results-for-transformations" id="id16">Updating analysis results for transformations</a><ul>
<li><a class="reference internal" href="#the-deletevalue-method" id="id17">The <code class="docutils literal notranslate"><span class="pre">deleteValue</span></code> method</a></li>
<li><a class="reference internal" href="#the-copyvalue-method" id="id18">The <code class="docutils literal notranslate"><span class="pre">copyValue</span></code> method</a></li>
<li><a class="reference internal" href="#the-replacewithnewvalue-method" id="id19">The <code class="docutils literal notranslate"><span class="pre">replaceWithNewValue</span></code> method</a></li>
<li><a class="reference internal" href="#the-addescapinguse-method" id="id20">The <code class="docutils literal notranslate"><span class="pre">addEscapingUse</span></code> method</a></li>
</ul>
</li>
<li><a class="reference internal" href="#efficiency-issues" id="id21">Efficiency Issues</a></li>
<li><a class="reference internal" href="#limitations" id="id22">Limitations</a></li>
</ul>
</li>
<li><a class="reference internal" href="#using-alias-analysis-results" id="id23">Using alias analysis results</a><ul>
<li><a class="reference internal" href="#using-the-memorydependenceanalysis-pass" id="id24">Using the <code class="docutils literal notranslate"><span class="pre">MemoryDependenceAnalysis</span></code> Pass</a></li>
<li><a class="reference internal" href="#using-the-aliassettracker-class" id="id25">Using the <code class="docutils literal notranslate"><span class="pre">AliasSetTracker</span></code> class</a><ul>
<li><a class="reference internal" href="#the-aliassettracker-implementation" id="id26">The AliasSetTracker implementation</a></li>
</ul>
</li>
<li><a class="reference internal" href="#using-the-aliasanalysis-interface-directly" id="id27">Using the <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> interface directly</a></li>
</ul>
</li>
<li><a class="reference internal" href="#existing-alias-analysis-implementations-and-clients" id="id28">Existing alias analysis implementations and clients</a><ul>
<li><a class="reference internal" href="#available-aliasanalysis-implementations" id="id29">Available <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> implementations</a><ul>
<li><a class="reference internal" href="#the-no-aa-pass" id="id30">The <code class="docutils literal notranslate"><span class="pre">-no-aa</span></code> pass</a></li>
<li><a class="reference internal" href="#the-basicaa-pass" id="id31">The <code class="docutils literal notranslate"><span class="pre">-basicaa</span></code> pass</a></li>
<li><a class="reference internal" href="#the-globalsmodref-aa-pass" id="id32">The <code class="docutils literal notranslate"><span class="pre">-globalsmodref-aa</span></code> pass</a></li>
<li><a class="reference internal" href="#the-steens-aa-pass" id="id33">The <code class="docutils literal notranslate"><span class="pre">-steens-aa</span></code> pass</a></li>
<li><a class="reference internal" href="#the-ds-aa-pass" id="id34">The <code class="docutils literal notranslate"><span class="pre">-ds-aa</span></code> pass</a></li>
<li><a class="reference internal" href="#the-scev-aa-pass" id="id35">The <code class="docutils literal notranslate"><span class="pre">-scev-aa</span></code> pass</a></li>
</ul>
</li>
<li><a class="reference internal" href="#alias-analysis-driven-transformations" id="id36">Alias analysis driven transformations</a><ul>
<li><a class="reference internal" href="#the-adce-pass" id="id37">The <code class="docutils literal notranslate"><span class="pre">-adce</span></code> pass</a></li>
<li><a class="reference internal" href="#the-licm-pass" id="id38">The <code class="docutils literal notranslate"><span class="pre">-licm</span></code> pass</a></li>
<li><a class="reference internal" href="#the-argpromotion-pass" id="id39">The <code class="docutils literal notranslate"><span class="pre">-argpromotion</span></code> pass</a></li>
<li><a class="reference internal" href="#the-gvn-memcpyopt-and-dse-passes" id="id40">The <code class="docutils literal notranslate"><span class="pre">-gvn</span></code>, <code class="docutils literal notranslate"><span class="pre">-memcpyopt</span></code>, and <code class="docutils literal notranslate"><span class="pre">-dse</span></code> passes</a></li>
</ul>
</li>
<li><a class="reference internal" href="#clients-for-debugging-and-evaluation-of-implementations" id="id41">Clients for debugging and evaluation of implementations</a><ul>
<li><a class="reference internal" href="#the-print-alias-sets-pass" id="id42">The <code class="docutils literal notranslate"><span class="pre">-print-alias-sets</span></code> pass</a></li>
<li><a class="reference internal" href="#the-aa-eval-pass" id="id43">The <code class="docutils literal notranslate"><span class="pre">-aa-eval</span></code> pass</a></li>
</ul>
</li>
</ul>
</li>
<li><a class="reference internal" href="#memory-dependence-analysis" id="id44">Memory Dependence Analysis</a></li>
</ul>
</div>
<div class="section" id="introduction">
<h2><a class="toc-backref" href="#id1">Introduction</a><a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
<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 class="reference internal" href="#must-may-or-no">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 class="reference external" href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</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.</p>
</div>
<div class="section" id="aliasanalysis-class-overview">
<h2><a class="toc-backref" href="#id2"><code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> Class Overview</a><a class="headerlink" href="#aliasanalysis-class-overview" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference external" href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>
class defines the interface that the various alias analysis implementations
should support. This class exports two important enums: <code class="docutils literal notranslate"><span class="pre">AliasResult</span></code> and
<code class="docutils literal notranslate"><span class="pre">ModRefResult</span></code> which represent the result of an alias query or a mod/ref
query, respectively.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> 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
<code class="docutils literal notranslate"><span class="pre">call</span></code> or <code class="docutils literal notranslate"><span class="pre">invoke</span></code> instructions that performs the call. The
<code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> interface also exposes some helper methods which allow you to
get mod/ref information for arbitrary instructions.</p>
<p>All <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> interfaces require that in queries involving multiple
values, values which are not <a class="reference internal" href="LangRef.html#constants"><span class="std std-ref">constants</span></a> are all
defined within the same function.</p>
<div class="section" id="representation-of-pointers">
<h3><a class="toc-backref" href="#id3">Representation of Pointers</a><a class="headerlink" href="#representation-of-pointers" title="Permalink to this headline">¶</a></h3>
<p>Most importantly, the <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> 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
<code class="docutils literal notranslate"><span class="pre">Value*</span></code>) 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="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="kt">int</span> <span class="n">i</span><span class="p">;</span>
<span class="kt">char</span> <span class="n">C</span><span class="p">[</span><span class="mi">2</span><span class="p">];</span>
<span class="kt">char</span> <span class="n">A</span><span class="p">[</span><span class="mi">10</span><span class="p">];</span>
<span class="cm">/* ... */</span>
<span class="k">for</span> <span class="p">(</span><span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o">!=</span> <span class="mi">10</span><span class="p">;</span> <span class="o">++</span><span class="n">i</span><span class="p">)</span> <span class="p">{</span>
<span class="n">C</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="n">A</span><span class="p">[</span><span class="n">i</span><span class="p">];</span> <span class="cm">/* One byte store */</span>
<span class="n">C</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="n">A</span><span class="p">[</span><span class="mi">9</span><span class="o">-</span><span class="n">i</span><span class="p">];</span> <span class="cm">/* One byte store */</span>
<span class="p">}</span>
</pre></div>
</div>
<p>In this case, the <code class="docutils literal notranslate"><span class="pre">basicaa</span></code> pass will disambiguate the stores to <code class="docutils literal notranslate"><span class="pre">C[0]</span></code> and
<code class="docutils literal notranslate"><span class="pre">C[1]</span></code> because they are accesses to two distinct locations one byte apart, and
the accesses are each one byte. In this case, the Loop Invariant Code Motion
(LICM) pass can use store motion to remove the stores from the loop. In
constrast, the following code:</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="kt">int</span> <span class="n">i</span><span class="p">;</span>
<span class="kt">char</span> <span class="n">C</span><span class="p">[</span><span class="mi">2</span><span class="p">];</span>
<span class="kt">char</span> <span class="n">A</span><span class="p">[</span><span class="mi">10</span><span class="p">];</span>
<span class="cm">/* ... */</span>
<span class="k">for</span> <span class="p">(</span><span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o">!=</span> <span class="mi">10</span><span class="p">;</span> <span class="o">++</span><span class="n">i</span><span class="p">)</span> <span class="p">{</span>
<span class="p">((</span><span class="kt">short</span><span class="o">*</span><span class="p">)</span><span class="n">C</span><span class="p">)[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="n">A</span><span class="p">[</span><span class="n">i</span><span class="p">];</span> <span class="cm">/* Two byte store! */</span>
<span class="n">C</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="o">=</span> <span class="n">A</span><span class="p">[</span><span class="mi">9</span><span class="o">-</span><span class="n">i</span><span class="p">];</span> <span class="cm">/* One byte store */</span>
<span class="p">}</span>
</pre></div>
</div>
<p>In this case, the two stores to C do alias each other, because the access to the
<code class="docutils literal notranslate"><span class="pre">&C[0]</span></code> 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="section" id="the-alias-method">
<span id="alias"></span><h3><a class="toc-backref" href="#id4">The <code class="docutils literal notranslate"><span class="pre">alias</span></code> method</a><a class="headerlink" href="#the-alias-method" title="Permalink to this headline">¶</a></h3>
<p>The <code class="docutils literal notranslate"><span class="pre">alias</span></code> method is the primary interface used to determine whether or not
two memory objects alias each other. It takes two memory objects as input and
returns MustAlias, PartialAlias, MayAlias, or NoAlias as appropriate.</p>
<p>Like all <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> interfaces, the <code class="docutils literal notranslate"><span class="pre">alias</span></code> method requires that either
the two pointer values be defined within the same function, or at least one of
the values is a <a class="reference internal" href="LangRef.html#constants"><span class="std std-ref">constant</span></a>.</p>
<div class="section" id="must-may-and-no-alias-responses">
<span id="must-may-or-no"></span><h4><a class="toc-backref" href="#id5">Must, May, and No Alias Responses</a><a class="headerlink" href="#must-may-and-no-alias-responses" title="Permalink to this headline">¶</a></h4>
<p>The <code class="docutils literal notranslate"><span class="pre">NoAlias</span></code> response may be used when there is never an immediate dependence
between any memory reference <em>based</em> on one pointer and any memory reference
<em>based</em> the other. The most obvious example is when the two pointers point to
non-overlapping memory ranges. Another is when the two pointers are only ever
used for reading memory. Another is when the memory is freed and reallocated
between accesses through one pointer and accesses through the other — in this
case, there is a dependence, but it’s mediated by the free and reallocation.</p>
<p>As an exception to this is with the <a class="reference internal" href="LangRef.html#noalias"><span class="std std-ref">noalias</span></a> keyword;
the “irrelevant” dependencies are ignored.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">MayAlias</span></code> response is used whenever the two pointers might refer to the
same object.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">PartialAlias</span></code> response is used when the two memory objects are known to
be overlapping in some way, regardless whether they start at the same address
or not.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">MustAlias</span></code> response may only be returned if the two memory objects are
guaranteed to always start at exactly the same location. A <code class="docutils literal notranslate"><span class="pre">MustAlias</span></code>
response does not imply that the pointers compare equal.</p>
</div>
</div>
<div class="section" id="the-getmodrefinfo-methods">
<h3><a class="toc-backref" href="#id6">The <code class="docutils literal notranslate"><span class="pre">getModRefInfo</span></code> methods</a><a class="headerlink" href="#the-getmodrefinfo-methods" title="Permalink to this headline">¶</a></h3>
<p>The <code class="docutils literal notranslate"><span class="pre">getModRefInfo</span></code> 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 instruction <strong>might</strong> read or write a location,
<code class="docutils literal notranslate"><span class="pre">ModRef</span></code> is returned.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> class also provides a <code class="docutils literal notranslate"><span class="pre">getModRefInfo</span></code> method for testing
dependencies between function calls. This method takes two call sites (<code class="docutils literal notranslate"><span class="pre">CS1</span></code>
& <code class="docutils literal notranslate"><span class="pre">CS2</span></code>), returns <code class="docutils literal notranslate"><span class="pre">NoModRef</span></code> if neither call writes to memory read or
written by the other, <code class="docutils literal notranslate"><span class="pre">Ref</span></code> if <code class="docutils literal notranslate"><span class="pre">CS1</span></code> reads memory written by <code class="docutils literal notranslate"><span class="pre">CS2</span></code>,
<code class="docutils literal notranslate"><span class="pre">Mod</span></code> if <code class="docutils literal notranslate"><span class="pre">CS1</span></code> writes to memory read or written by <code class="docutils literal notranslate"><span class="pre">CS2</span></code>, or <code class="docutils literal notranslate"><span class="pre">ModRef</span></code> if
<code class="docutils literal notranslate"><span class="pre">CS1</span></code> might read or write memory written to by <code class="docutils literal notranslate"><span class="pre">CS2</span></code>. Note that this
relation is not commutative.</p>
</div>
<div class="section" id="other-useful-aliasanalysis-methods">
<h3><a class="toc-backref" href="#id7">Other useful <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> methods</a><a class="headerlink" href="#other-useful-aliasanalysis-methods" title="Permalink to this headline">¶</a></h3>
<p>Several other tidbits of information are often collected by various alias
analysis implementations and can be put to good use by various clients.</p>
<div class="section" id="the-pointstoconstantmemory-method">
<h4><a class="toc-backref" href="#id8">The <code class="docutils literal notranslate"><span class="pre">pointsToConstantMemory</span></code> method</a><a class="headerlink" href="#the-pointstoconstantmemory-method" title="Permalink to this headline">¶</a></h4>
<p>The <code class="docutils literal notranslate"><span class="pre">pointsToConstantMemory</span></code> method returns true if and only if the analysis
can prove that the pointer only points to unchanging memory locations
(functions, constant global variables, and the null pointer). This information
can be used to refine mod/ref information: it is impossible for an unchanging
memory location to be modified.</p>
</div>
<div class="section" id="the-doesnotaccessmemory-and-onlyreadsmemory-methods">
<span id="never-access-memory-or-only-read-memory"></span><h4><a class="toc-backref" href="#id9">The <code class="docutils literal notranslate"><span class="pre">doesNotAccessMemory</span></code> and <code class="docutils literal notranslate"><span class="pre">onlyReadsMemory</span></code> methods</a><a class="headerlink" href="#the-doesnotaccessmemory-and-onlyreadsmemory-methods" title="Permalink to this headline">¶</a></h4>
<p>These methods are used to provide very simple mod/ref information for function
calls. The <code class="docutils literal notranslate"><span class="pre">doesNotAccessMemory</span></code> method returns true for a function if the
analysis can prove that the function never reads or writes to memory, or if the
function only reads from constant memory. Functions with this property are
side-effect free and only depend on their input arguments, allowing them to be
eliminated if they form common subexpressions or be hoisted out of loops. Many
common functions behave this way (e.g., <code class="docutils literal notranslate"><span class="pre">sin</span></code> and <code class="docutils literal notranslate"><span class="pre">cos</span></code>) but many others do
not (e.g., <code class="docutils literal notranslate"><span class="pre">acos</span></code>, which modifies the <code class="docutils literal notranslate"><span class="pre">errno</span></code> variable).</p>
<p>The <code class="docutils literal notranslate"><span class="pre">onlyReadsMemory</span></code> method returns true for a function if analysis can prove
that (at most) the function only reads from non-volatile memory. Functions with
this property are side-effect free, only depending on their input arguments and
the state of memory when they are called. This property allows calls to these
functions to be eliminated and moved around, as long as there is no store
instruction that changes the contents of memory. Note that all functions that
satisfy the <code class="docutils literal notranslate"><span class="pre">doesNotAccessMemory</span></code> method also satisfy <code class="docutils literal notranslate"><span class="pre">onlyReadsMemory</span></code>.</p>
</div>
</div>
</div>
<div class="section" id="writing-a-new-aliasanalysis-implementation">
<h2><a class="toc-backref" href="#id10">Writing a new <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> Implementation</a><a class="headerlink" href="#writing-a-new-aliasanalysis-implementation" title="Permalink to this headline">¶</a></h2>
<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 examples, take a
look at the <a class="reference internal" href="#various-alias-analysis-implementations">various alias analysis implementations</a> included with LLVM.</p>
<div class="section" id="different-pass-styles">
<h3><a class="toc-backref" href="#id11">Different Pass styles</a><a class="headerlink" href="#different-pass-styles" title="Permalink to this headline">¶</a></h3>
<p>The first step to determining what type of <a class="reference internal" href="WritingAnLLVMPass.html"><span class="doc">LLVM pass</span></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 class="arabic simple">
<li>If you require interprocedural analysis, it should be a <code class="docutils literal notranslate"><span class="pre">Pass</span></code>.</li>
<li>If you are a function-local analysis, subclass <code class="docutils literal notranslate"><span class="pre">FunctionPass</span></code>.</li>
<li>If you don’t need to look at the program at all, subclass <code class="docutils literal notranslate"><span class="pre">ImmutablePass</span></code>.</li>
</ol>
<p>In addition to the pass that you subclass, you should also inherit from the
<code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> interface, of course, and use the <code class="docutils literal notranslate"><span class="pre">RegisterAnalysisGroup</span></code>
template to register as an implementation of <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code>.</p>
</div>
<div class="section" id="required-initialization-calls">
<h3><a class="toc-backref" href="#id12">Required initialization calls</a><a class="headerlink" href="#required-initialization-calls" title="Permalink to this headline">¶</a></h3>
<p>Your subclass of <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> is required to invoke two methods on the
<code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> base class: <code class="docutils literal notranslate"><span class="pre">getAnalysisUsage</span></code> and
<code class="docutils literal notranslate"><span class="pre">InitializeAliasAnalysis</span></code>. In particular, your implementation of
<code class="docutils literal notranslate"><span class="pre">getAnalysisUsage</span></code> should explicitly call into the
<code class="docutils literal notranslate"><span class="pre">AliasAnalysis::getAnalysisUsage</span></code> method in addition to doing any declaring
any pass dependencies your pass has. Thus you should have something like this:</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span> <span class="nf">getAnalysisUsage</span><span class="p">(</span><span class="n">AnalysisUsage</span> <span class="o">&</span><span class="n">AU</span><span class="p">)</span> <span class="k">const</span> <span class="p">{</span>
<span class="n">AliasAnalysis</span><span class="o">::</span><span class="n">getAnalysisUsage</span><span class="p">(</span><span class="n">AU</span><span class="p">);</span>
<span class="c1">// declare your dependencies here.</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Additionally, your must invoke the <code class="docutils literal notranslate"><span class="pre">InitializeAliasAnalysis</span></code> method from your
analysis run method (<code class="docutils literal notranslate"><span class="pre">run</span></code> for a <code class="docutils literal notranslate"><span class="pre">Pass</span></code>, <code class="docutils literal notranslate"><span class="pre">runOnFunction</span></code> for a
<code class="docutils literal notranslate"><span class="pre">FunctionPass</span></code>, or <code class="docutils literal notranslate"><span class="pre">InitializePass</span></code> for an <code class="docutils literal notranslate"><span class="pre">ImmutablePass</span></code>). For example
(as part of a <code class="docutils literal notranslate"><span class="pre">Pass</span></code>):</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="kt">bool</span> <span class="nf">run</span><span class="p">(</span><span class="n">Module</span> <span class="o">&</span><span class="n">M</span><span class="p">)</span> <span class="p">{</span>
<span class="n">InitializeAliasAnalysis</span><span class="p">(</span><span class="k">this</span><span class="p">);</span>
<span class="c1">// Perform analysis here...</span>
<span class="k">return</span> <span class="nb">false</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="required-methods-to-override">
<h3><a class="toc-backref" href="#id13">Required methods to override</a><a class="headerlink" href="#required-methods-to-override" title="Permalink to this headline">¶</a></h3>
<p>You must override the <code class="docutils literal notranslate"><span class="pre">getAdjustedAnalysisPointer</span></code> method on all subclasses
of <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code>. An example implementation of this method would look like:</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="kt">void</span> <span class="o">*</span><span class="nf">getAdjustedAnalysisPointer</span><span class="p">(</span><span class="k">const</span> <span class="kt">void</span><span class="o">*</span> <span class="n">ID</span><span class="p">)</span> <span class="k">override</span> <span class="p">{</span>
<span class="k">if</span> <span class="p">(</span><span class="n">ID</span> <span class="o">==</span> <span class="o">&</span><span class="n">AliasAnalysis</span><span class="o">::</span><span class="n">ID</span><span class="p">)</span>
<span class="k">return</span> <span class="p">(</span><span class="n">AliasAnalysis</span><span class="o">*</span><span class="p">)</span><span class="k">this</span><span class="p">;</span>
<span class="k">return</span> <span class="k">this</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="interfaces-which-may-be-specified">
<h3><a class="toc-backref" href="#id14">Interfaces which may be specified</a><a class="headerlink" href="#interfaces-which-may-be-specified" title="Permalink to this headline">¶</a></h3>
<p>All of the <a class="reference external" href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a> virtual methods
default to providing <a class="reference internal" href="#aliasanalysis-chaining"><span class="std std-ref">chaining</span></a> to another alias
analysis implementation, which ends up returning 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.</p>
</div>
<div class="section" id="aliasanalysis-chaining-behavior">
<span id="aliasanalysis-chaining"></span><h3><a class="toc-backref" href="#id15"><code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> chaining behavior</a><a class="headerlink" href="#aliasanalysis-chaining-behavior" title="Permalink to this headline">¶</a></h3>
<p>With only one special exception (the <a class="reference internal" href="#aliasanalysis-no-aa"><span class="std std-ref">-no-aa</span></a> pass)
every alias analysis pass chains to another alias analysis implementation (for
example, the user can specify “<code class="docutils literal notranslate"><span class="pre">-basicaa</span> <span class="pre">-ds-aa</span> <span class="pre">-licm</span></code>” to get the maximum
benefit from both alias analyses). The alias analysis class automatically
takes care of most of this for methods that you don’t override. For methods
that you do override, in code paths that return a conservative MayAlias or
Mod/Ref result, simply return whatever the superclass computes. For example:</p>
<div class="highlight-c++ notranslate"><div class="highlight"><pre><span></span><span class="n">AliasResult</span> <span class="nf">alias</span><span class="p">(</span><span class="k">const</span> <span class="n">Value</span> <span class="o">*</span><span class="n">V1</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="n">V1Size</span><span class="p">,</span>
<span class="k">const</span> <span class="n">Value</span> <span class="o">*</span><span class="n">V2</span><span class="p">,</span> <span class="kt">unsigned</span> <span class="n">V2Size</span><span class="p">)</span> <span class="p">{</span>
<span class="k">if</span> <span class="p">(...)</span>
<span class="k">return</span> <span class="n">NoAlias</span><span class="p">;</span>
<span class="p">...</span>
<span class="c1">// Couldn't determine a must or no-alias result.</span>
<span class="k">return</span> <span class="n">AliasAnalysis</span><span class="o">::</span><span class="n">alias</span><span class="p">(</span><span class="n">V1</span><span class="p">,</span> <span class="n">V1Size</span><span class="p">,</span> <span class="n">V2</span><span class="p">,</span> <span class="n">V2Size</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
<p>In addition to analysis queries, you must make sure to unconditionally pass LLVM
<a class="reference internal" href="#update-notification">update notification</a> methods to the superclass as well if you override them,
which allows all alias analyses in a change to be updated.</p>
</div>
<div class="section" id="updating-analysis-results-for-transformations">
<span id="update-notification"></span><h3><a class="toc-backref" href="#id16">Updating analysis results for transformations</a><a class="headerlink" href="#updating-analysis-results-for-transformations" title="Permalink to this headline">¶</a></h3>
<p>Alias analysis information is initially computed for a static snapshot of the
program, but clients will use this information to make transformations to the
code. All but the most trivial forms of alias analysis will need to have their
analysis results updated to reflect the changes made by these transformations.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> interface exposes four methods which are used to
communicate program changes from the clients to the analysis implementations.
Various alias analysis implementations should use these methods to ensure that
their internal data structures are kept up-to-date as the program changes (for
example, when an instruction is deleted), and clients of alias analysis must be
sure to call these interfaces appropriately.</p>
<div class="section" id="the-deletevalue-method">
<h4><a class="toc-backref" href="#id17">The <code class="docutils literal notranslate"><span class="pre">deleteValue</span></code> method</a><a class="headerlink" href="#the-deletevalue-method" title="Permalink to this headline">¶</a></h4>
<p>The <code class="docutils literal notranslate"><span class="pre">deleteValue</span></code> method is called by transformations when they remove an
instruction or any other value from the program (including values that do not
use pointers). Typically alias analyses keep data structures that have entries
for each value in the program. When this method is called, they should remove
any entries for the specified value, if they exist.</p>
</div>
<div class="section" id="the-copyvalue-method">
<h4><a class="toc-backref" href="#id18">The <code class="docutils literal notranslate"><span class="pre">copyValue</span></code> method</a><a class="headerlink" href="#the-copyvalue-method" title="Permalink to this headline">¶</a></h4>
<p>The <code class="docutils literal notranslate"><span class="pre">copyValue</span></code> method is used when a new value is introduced into the
program. There is no way to introduce a value into the program that did not
exist before (this doesn’t make sense for a safe compiler transformation), so
this is the only way to introduce a new value. This method indicates that the
new value has exactly the same properties as the value being copied.</p>
</div>
<div class="section" id="the-replacewithnewvalue-method">
<h4><a class="toc-backref" href="#id19">The <code class="docutils literal notranslate"><span class="pre">replaceWithNewValue</span></code> method</a><a class="headerlink" href="#the-replacewithnewvalue-method" title="Permalink to this headline">¶</a></h4>
<p>This method is a simple helper method that is provided to make clients easier to
use. It is implemented by copying the old analysis information to the new
value, then deleting the old value. This method cannot be overridden by alias
analysis implementations.</p>
</div>
<div class="section" id="the-addescapinguse-method">
<h4><a class="toc-backref" href="#id20">The <code class="docutils literal notranslate"><span class="pre">addEscapingUse</span></code> method</a><a class="headerlink" href="#the-addescapinguse-method" title="Permalink to this headline">¶</a></h4>
<p>The <code class="docutils literal notranslate"><span class="pre">addEscapingUse</span></code> method is used when the uses of a pointer value have
changed in ways that may invalidate precomputed analysis information.
Implementations may either use this callback to provide conservative responses
for points whose uses have change since analysis time, or may recompute some or
all of their internal state to continue providing accurate responses.</p>
<p>In general, any new use of a pointer value is considered an escaping use, and
must be reported through this callback, <em>except</em> for the uses below:</p>
<ul class="simple">
<li>A <code class="docutils literal notranslate"><span class="pre">bitcast</span></code> or <code class="docutils literal notranslate"><span class="pre">getelementptr</span></code> of the pointer</li>
<li>A <code class="docutils literal notranslate"><span class="pre">store</span></code> through the pointer (but not a <code class="docutils literal notranslate"><span class="pre">store</span></code> <em>of</em> the pointer)</li>
<li>A <code class="docutils literal notranslate"><span class="pre">load</span></code> through the pointer</li>
</ul>
</div>
</div>
<div class="section" id="efficiency-issues">
<h3><a class="toc-backref" href="#id21">Efficiency Issues</a><a class="headerlink" href="#efficiency-issues" title="Permalink to this headline">¶</a></h3>
<p>From the LLVM perspective, the only thing you need to do to provide an efficient
alias analysis is to make sure that alias analysis <strong>queries</strong> 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>
</div>
<div class="section" id="limitations">
<h3><a class="toc-backref" href="#id22">Limitations</a><a class="headerlink" href="#limitations" title="Permalink to this headline">¶</a></h3>
<p>The AliasAnalysis infrastructure has several limitations which make writing a
new <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> implementation difficult.</p>
<p>There is no way to override the default alias analysis. It would be very useful
to be able to do something like “<code class="docutils literal notranslate"><span class="pre">opt</span> <span class="pre">-my-aa</span> <span class="pre">-O2</span></code>” and have it use <code class="docutils literal notranslate"><span class="pre">-my-aa</span></code>
for all passes which need AliasAnalysis, but there is currently no support for
that, short of changing the source code and recompiling. Similarly, there is
also no way of setting a chain of analyses as the default.</p>
<p>There is no way for transform passes to declare that they preserve
<code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> implementations. The <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> interface includes
<code class="docutils literal notranslate"><span class="pre">deleteValue</span></code> and <code class="docutils literal notranslate"><span class="pre">copyValue</span></code> methods which are intended to allow a pass to
keep an AliasAnalysis consistent, however there’s no way for a pass to declare
in its <code class="docutils literal notranslate"><span class="pre">getAnalysisUsage</span></code> that it does so. Some passes attempt to use
<code class="docutils literal notranslate"><span class="pre">AU.addPreserved<AliasAnalysis></span></code>, however this doesn’t actually have any
effect.</p>
<p>Similarly, the <code class="docutils literal notranslate"><span class="pre">opt</span> <span class="pre">-p</span></code> option introduces <code class="docutils literal notranslate"><span class="pre">ModulePass</span></code> passes between each
pass, which prevents the use of <code class="docutils literal notranslate"><span class="pre">FunctionPass</span></code> alias analysis passes.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> API does have functions for notifying implementations when
values are deleted or copied, however these aren’t sufficient. There are many
other ways that LLVM IR can be modified which could be relevant to
<code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> implementations which can not be expressed.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">AliasAnalysisDebugger</span></code> utility seems to suggest that <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code>
implementations can expect that they will be informed of any relevant <code class="docutils literal notranslate"><span class="pre">Value</span></code>
before it appears in an alias query. However, popular clients such as <code class="docutils literal notranslate"><span class="pre">GVN</span></code>
don’t support this, and are known to trigger errors when run with the
<code class="docutils literal notranslate"><span class="pre">AliasAnalysisDebugger</span></code>.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">AliasSetTracker</span></code> class (which is used by <code class="docutils literal notranslate"><span class="pre">LICM</span></code>) makes a
non-deterministic number of alias queries. This can cause debugging techniques
involving pausing execution after a predetermined number of queries to be
unreliable.</p>
<p>Many alias queries can be reformulated in terms of other alias queries. When
multiple <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> queries are chained together, it would make sense to
start those queries from the beginning of the chain, with care taken to avoid
infinite looping, however currently an implementation which wants to do this can
only start such queries from itself.</p>
</div>
</div>
<div class="section" id="using-alias-analysis-results">
<h2><a class="toc-backref" href="#id23">Using alias analysis results</a><a class="headerlink" href="#using-alias-analysis-results" title="Permalink to this headline">¶</a></h2>
<p>There are several different ways to use alias analysis results. In order of
preference, these are:</p>
<div class="section" id="using-the-memorydependenceanalysis-pass">
<h3><a class="toc-backref" href="#id24">Using the <code class="docutils literal notranslate"><span class="pre">MemoryDependenceAnalysis</span></code> Pass</a><a class="headerlink" href="#using-the-memorydependenceanalysis-pass" title="Permalink to this headline">¶</a></h3>
<p>The <code class="docutils literal notranslate"><span class="pre">memdep</span></code> pass uses alias analysis to provide high-level dependence
information about memory-using instructions. This will tell you which store
feeds into a load, for example. It uses caching and other techniques to be
efficient, and is used by Dead Store Elimination, GVN, and memcpy optimizations.</p>
</div>
<div class="section" id="using-the-aliassettracker-class">
<span id="aliassettracker"></span><h3><a class="toc-backref" href="#id25">Using the <code class="docutils literal notranslate"><span class="pre">AliasSetTracker</span></code> class</a><a class="headerlink" href="#using-the-aliassettracker-class" title="Permalink to this headline">¶</a></h3>
<p>Many transformations need information about alias <strong>sets</strong> that are active in
some scope, rather than information about pairwise aliasing. The
<a class="reference external" href="http://llvm.org/doxygen/classllvm_1_1AliasSetTracker.html">AliasSetTracker</a>
class is used to efficiently build these Alias Sets from the pairwise alias
analysis information provided by the <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> interface.</p>
<p>First you initialize the AliasSetTracker by using the “<code class="docutils literal notranslate"><span class="pre">add</span></code>” 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 <code class="docutils literal notranslate"><span class="pre">AliasSetTracker</span></code>
<code class="docutils literal notranslate"><span class="pre">begin()</span></code>/<code class="docutils literal notranslate"><span class="pre">end()</span></code> methods.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">AliasSet</span></code>s formed by the <code class="docutils literal notranslate"><span class="pre">AliasSetTracker</span></code> are guaranteed to be
disjoint, calculate mod/ref information and volatility 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>
<p>As an example user of this, the <a class="reference external" href="doxygen/structLICM.html">Loop Invariant Code Motion</a> pass uses <code class="docutils literal notranslate"><span class="pre">AliasSetTracker</span></code>s to calculate alias
sets for each loop nest. If an <code class="docutils literal notranslate"><span class="pre">AliasSet</span></code> 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 to <strong>and</strong> are must alias sets, then the stores may be sunk
to outside of the loop, promoting the memory location to a register for the
duration of the loop nest. Both of these transformations only apply if the
pointer argument is loop-invariant.</p>
<div class="section" id="the-aliassettracker-implementation">
<h4><a class="toc-backref" href="#id26">The AliasSetTracker implementation</a><a class="headerlink" href="#the-aliassettracker-implementation" title="Permalink to this headline">¶</a></h4>
<p>The AliasSetTracker class is implemented to be as efficient as possible. It
uses the union-find algorithm to efficiently merge AliasSets when a pointer is
inserted into the AliasSetTracker that aliases multiple sets. The primary data
structure is a hash table mapping pointers to the AliasSet they are in.</p>
<p>The AliasSetTracker class must maintain a list of all of the LLVM <code class="docutils literal notranslate"><span class="pre">Value*</span></code>s
that are in each AliasSet. Since the hash table already has entries for each
LLVM <code class="docutils literal notranslate"><span class="pre">Value*</span></code> of interest, the AliasesSets thread the linked list through
these hash-table nodes to avoid having to allocate memory unnecessarily, and to
make merging alias sets extremely efficient (the linked list merge is constant
time).</p>
<p>You shouldn’t need to understand these details if you are just a client of the
AliasSetTracker, but if you look at the code, hopefully this brief description
will help make sense of why things are designed the way they are.</p>
</div>
</div>
<div class="section" id="using-the-aliasanalysis-interface-directly">
<h3><a class="toc-backref" href="#id27">Using the <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> interface directly</a><a class="headerlink" href="#using-the-aliasanalysis-interface-directly" title="Permalink to this headline">¶</a></h3>
<p>If neither of these utility class are what your pass needs, you should use the
interfaces exposed by the <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> class directly. Try to use the
higher-level methods when possible (e.g., use mod/ref information instead of the
<a class="reference internal" href="#alias">alias</a> method directly if possible) to get the best precision and efficiency.</p>
</div>
</div>
<div class="section" id="existing-alias-analysis-implementations-and-clients">
<h2><a class="toc-backref" href="#id28">Existing alias analysis implementations and clients</a><a class="headerlink" href="#existing-alias-analysis-implementations-and-clients" title="Permalink to this headline">¶</a></h2>
<p>If you’re going to be working with the LLVM alias analysis infrastructure, you
should know what clients and implementations of alias analysis are available.
In particular, if you are implementing an alias analysis, you should be aware of
the <a class="reference internal" href="#the-clients">the clients</a> that are useful for monitoring and evaluating different
implementations.</p>
<div class="section" id="available-aliasanalysis-implementations">
<span id="various-alias-analysis-implementations"></span><h3><a class="toc-backref" href="#id29">Available <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> implementations</a><a class="headerlink" href="#available-aliasanalysis-implementations" title="Permalink to this headline">¶</a></h3>
<p>This section lists the various implementations of the <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code>
interface. With the exception of the <a class="reference internal" href="#aliasanalysis-no-aa"><span class="std std-ref">-no-aa</span></a>
implementation, all of these <a class="reference internal" href="#aliasanalysis-chaining"><span class="std std-ref">chain</span></a> to other
alias analysis implementations.</p>
<div class="section" id="the-no-aa-pass">
<span id="aliasanalysis-no-aa"></span><h4><a class="toc-backref" href="#id30">The <code class="docutils literal notranslate"><span class="pre">-no-aa</span></code> pass</a><a class="headerlink" href="#the-no-aa-pass" title="Permalink to this headline">¶</a></h4>
<p>The <code class="docutils literal notranslate"><span class="pre">-no-aa</span></code> pass 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.</p>
</div>
<div class="section" id="the-basicaa-pass">
<h4><a class="toc-backref" href="#id31">The <code class="docutils literal notranslate"><span class="pre">-basicaa</span></code> pass</a><a class="headerlink" href="#the-basicaa-pass" title="Permalink to this headline">¶</a></h4>
<p>The <code class="docutils literal notranslate"><span class="pre">-basicaa</span></code> pass is an aggressive local analysis that <em>knows</em> many
important facts:</p>
<ul class="simple">
<li>Distinct globals, stack allocations, and heap allocations can never alias.</li>
<li>Globals, stack allocations, and heap allocations never alias the null pointer.</li>
<li>Different fields of a structure do not alias.</li>
<li>Indexes into arrays with statically differing subscripts cannot alias.</li>
<li>Many common standard C library functions <a class="reference internal" href="#never-access-memory-or-only-read-memory">never access memory or only read
memory</a>.</li>
<li>Pointers that obviously point to constant globals “<code class="docutils literal notranslate"><span class="pre">pointToConstantMemory</span></code>”.</li>
<li>Function calls can not modify or references stack allocations if they never
escape from the function that allocates them (a common case for automatic
arrays).</li>
</ul>
</div>
<div class="section" id="the-globalsmodref-aa-pass">
<h4><a class="toc-backref" href="#id32">The <code class="docutils literal notranslate"><span class="pre">-globalsmodref-aa</span></code> pass</a><a class="headerlink" href="#the-globalsmodref-aa-pass" title="Permalink to this headline">¶</a></h4>
<p>This pass implements a simple context-sensitive mod/ref and alias analysis for
internal global variables that don’t “have their address taken”. If a global
does not have its address taken, the pass knows that no pointers alias the
global. This pass also keeps track of functions that it knows never access
memory or never read memory. This allows certain optimizations (e.g. GVN) to
eliminate call instructions entirely.</p>
<p>The real power of this pass is that it provides context-sensitive mod/ref
information for call instructions. This allows the optimizer to know that calls
to a function do not clobber or read the value of the global, allowing loads and
stores to be eliminated.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">This pass is somewhat limited in its scope (only support non-address taken
globals), but is very quick analysis.</p>
</div>
</div>
<div class="section" id="the-steens-aa-pass">
<h4><a class="toc-backref" href="#id33">The <code class="docutils literal notranslate"><span class="pre">-steens-aa</span></code> pass</a><a class="headerlink" href="#the-steens-aa-pass" title="Permalink to this headline">¶</a></h4>
<p>The <code class="docutils literal notranslate"><span class="pre">-steens-aa</span></code> pass implements a variation on the well-known “Steensgaard’s
algorithm” for interprocedural alias analysis. Steensgaard’s algorithm is a
unification-based, flow-insensitive, context-insensitive, and field-insensitive
alias analysis that is also very scalable (effectively linear time).</p>
<p>The LLVM <code class="docutils literal notranslate"><span class="pre">-steens-aa</span></code> pass implements a “speculatively field-<strong>sensitive</strong>”
version of Steensgaard’s algorithm using the Data Structure Analysis framework.
This gives it substantially more precision than the standard algorithm while
maintaining excellent analysis scalability.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last"><code class="docutils literal notranslate"><span class="pre">-steens-aa</span></code> is available in the optional “poolalloc” module. It is not part
of the LLVM core.</p>
</div>
</div>
<div class="section" id="the-ds-aa-pass">
<h4><a class="toc-backref" href="#id34">The <code class="docutils literal notranslate"><span class="pre">-ds-aa</span></code> pass</a><a class="headerlink" href="#the-ds-aa-pass" title="Permalink to this headline">¶</a></h4>
<p>The <code class="docutils literal notranslate"><span class="pre">-ds-aa</span></code> pass implements the full Data Structure Analysis algorithm. Data
Structure Analysis is a modular unification-based, flow-insensitive,
context-<strong>sensitive</strong>, and speculatively field-<strong>sensitive</strong> alias
analysis that is also quite scalable, usually at <code class="docutils literal notranslate"><span class="pre">O(n</span> <span class="pre">*</span> <span class="pre">log(n))</span></code>.</p>
<p>This algorithm is capable of responding to a full variety of alias analysis
queries, and can provide context-sensitive mod/ref information as well. The
only major facility not implemented so far is support for must-alias
information.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last"><code class="docutils literal notranslate"><span class="pre">-ds-aa</span></code> is available in the optional “poolalloc” module. It is not part of
the LLVM core.</p>
</div>
</div>
<div class="section" id="the-scev-aa-pass">
<h4><a class="toc-backref" href="#id35">The <code class="docutils literal notranslate"><span class="pre">-scev-aa</span></code> pass</a><a class="headerlink" href="#the-scev-aa-pass" title="Permalink to this headline">¶</a></h4>
<p>The <code class="docutils literal notranslate"><span class="pre">-scev-aa</span></code> pass implements AliasAnalysis queries by translating them into
ScalarEvolution queries. This gives it a more complete understanding of
<code class="docutils literal notranslate"><span class="pre">getelementptr</span></code> instructions and loop induction variables than other alias
analyses have.</p>
</div>
</div>
<div class="section" id="alias-analysis-driven-transformations">
<h3><a class="toc-backref" href="#id36">Alias analysis driven transformations</a><a class="headerlink" href="#alias-analysis-driven-transformations" title="Permalink to this headline">¶</a></h3>
<p>LLVM includes several alias-analysis driven transformations which can be used
with any of the implementations above.</p>
<div class="section" id="the-adce-pass">
<h4><a class="toc-backref" href="#id37">The <code class="docutils literal notranslate"><span class="pre">-adce</span></code> pass</a><a class="headerlink" href="#the-adce-pass" title="Permalink to this headline">¶</a></h4>
<p>The <code class="docutils literal notranslate"><span class="pre">-adce</span></code> pass, which implements Aggressive Dead Code Elimination uses the
<code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> interface to delete calls to functions that do not have
side-effects and are not used.</p>
</div>
<div class="section" id="the-licm-pass">
<h4><a class="toc-backref" href="#id38">The <code class="docutils literal notranslate"><span class="pre">-licm</span></code> pass</a><a class="headerlink" href="#the-licm-pass" title="Permalink to this headline">¶</a></h4>
<p>The <code class="docutils literal notranslate"><span class="pre">-licm</span></code> pass implements various Loop Invariant Code Motion related
transformations. It uses the <code class="docutils literal notranslate"><span class="pre">AliasAnalysis</span></code> interface for several different
transformations:</p>
<ul class="simple">
<li>It uses mod/ref information to hoist or sink load instructions out of loops if
there are no instructions in the loop that modifies the memory loaded.</li>
<li>It uses mod/ref information to hoist function calls out of loops that do not
write to memory and are loop-invariant.</li>
<li>It uses alias information to promote memory objects that are loaded and stored
to in loops to live in a register instead. It can do this if there are no may
aliases to the loaded/stored memory location.</li>
</ul>
</div>
<div class="section" id="the-argpromotion-pass">
<h4><a class="toc-backref" href="#id39">The <code class="docutils literal notranslate"><span class="pre">-argpromotion</span></code> pass</a><a class="headerlink" href="#the-argpromotion-pass" title="Permalink to this headline">¶</a></h4>
<p>The <code class="docutils literal notranslate"><span class="pre">-argpromotion</span></code> pass promotes by-reference arguments to be passed in
by-value instead. In particular, if pointer arguments are only loaded from it
passes in the value loaded instead of the address to the function. This pass
uses alias information to make sure that the value loaded from the argument
pointer is not modified between the entry of the function and any load of the
pointer.</p>
</div>
<div class="section" id="the-gvn-memcpyopt-and-dse-passes">
<h4><a class="toc-backref" href="#id40">The <code class="docutils literal notranslate"><span class="pre">-gvn</span></code>, <code class="docutils literal notranslate"><span class="pre">-memcpyopt</span></code>, and <code class="docutils literal notranslate"><span class="pre">-dse</span></code> passes</a><a class="headerlink" href="#the-gvn-memcpyopt-and-dse-passes" title="Permalink to this headline">¶</a></h4>
<p>These passes use AliasAnalysis information to reason about loads and stores.</p>
</div>
</div>
<div class="section" id="clients-for-debugging-and-evaluation-of-implementations">
<span id="the-clients"></span><h3><a class="toc-backref" href="#id41">Clients for debugging and evaluation of implementations</a><a class="headerlink" href="#clients-for-debugging-and-evaluation-of-implementations" title="Permalink to this headline">¶</a></h3>
<p>These passes are useful for evaluating the various alias analysis
implementations. You can use them with commands like:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>% opt -ds-aa -aa-eval foo.bc -disable-output -stats
</pre></div>
</div>
<div class="section" id="the-print-alias-sets-pass">
<h4><a class="toc-backref" href="#id42">The <code class="docutils literal notranslate"><span class="pre">-print-alias-sets</span></code> pass</a><a class="headerlink" href="#the-print-alias-sets-pass" title="Permalink to this headline">¶</a></h4>
<p>The <code class="docutils literal notranslate"><span class="pre">-print-alias-sets</span></code> pass is exposed as part of the <code class="docutils literal notranslate"><span class="pre">opt</span></code> tool to print
out the Alias Sets formed by the <a class="reference internal" href="#aliassettracker">AliasSetTracker</a> class. This is useful if
you’re using the <code class="docutils literal notranslate"><span class="pre">AliasSetTracker</span></code> class. To use it, use something like:</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>% opt -ds-aa -print-alias-sets -disable-output
</pre></div>
</div>
</div>
<div class="section" id="the-aa-eval-pass">
<h4><a class="toc-backref" href="#id43">The <code class="docutils literal notranslate"><span class="pre">-aa-eval</span></code> pass</a><a class="headerlink" href="#the-aa-eval-pass" title="Permalink to this headline">¶</a></h4>
<p>The <code class="docutils literal notranslate"><span class="pre">-aa-eval</span></code> 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 indicating the percent of no/may/must aliases found (a more precise
algorithm will have a lower number of may aliases).</p>
</div>
</div>
</div>
<div class="section" id="memory-dependence-analysis">
<h2><a class="toc-backref" href="#id44">Memory Dependence Analysis</a><a class="headerlink" href="#memory-dependence-analysis" title="Permalink to this headline">¶</a></h2>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<p class="last">We are currently in the process of migrating things from
<code class="docutils literal notranslate"><span class="pre">MemoryDependenceAnalysis</span></code> to <a class="reference internal" href="MemorySSA.html"><span class="doc">MemorySSA</span></a>. Please try to use
that instead.</p>
</div>
<p>If you’re just looking to be a client of alias analysis information, consider
using the Memory Dependence Analysis interface instead. MemDep is a lazy,
caching layer on top of alias analysis that is able to answer the question of
what preceding memory operations a given instruction depends on, either at an
intra- or inter-block level. Because of its laziness and caching policy, using
MemDep can be a significant performance win over accessing alias analysis
directly.</p>
</div>
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