implementations</a></li>
</ul>
</li>
+ <li><a href="#memdep">Memory Dependence Analysis</a></li>
</ol>
<div class="doc_author">
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 href="#MustNoMay">Must, May, or No</a> alias response,
+to a query with a <a href="#MustMayNo">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
-href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
+href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></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
<div class="doc_text">
<p>The <a
-href="http://llvm.cs.uiuc.edu/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
+href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html"><tt>AliasAnalysis</tt></a>
class defines the interface that the various 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
</div>
<div class="doc_text">
+<p>The NoAlias response is used when the two pointers refer to distinct objects,
+regardless of whether the pointers compare equal. For example, freed pointers
+don't alias any pointers that were allocated afterwards. As a degenerate case,
+pointers returned by malloc(0) have no bytes for an object, and are considered
+NoAlias even when malloc returns the same pointer. The same rule applies to
+NULL pointers.</p>
-<p>An Alias Analysis implementation can return one of three responses:
-MustAlias, MayAlias, and NoAlias. The No and May alias results are obvious: if
-the two pointers can never equal each other, return NoAlias, if they might,
-return MayAlias.</p>
+<p>The MayAlias response is used whenever the two pointers might refer to the
+same object. If the two memory objects overlap, but do not start at the same
+location, return MayAlias.</p>
-<p>The MustAlias 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, return MayAlias.</p>
+<p>The MustAlias response may only be returned if the two memory objects are
+guaranteed to always start at exactly the same location. A MustAlias response
+implies that the pointers compare equal.</p>
</div>
Structure Analysis framework. This gives it substantially more precision than
the standard algorithm while maintaining excellent analysis scalability.</p>
+<p>Note that <tt>-steens-aa</tt> is available in the optional "poolalloc"
+module, it is not part of the LLVM core.</p>
+
</div>
<!-- _______________________________________________________________________ -->
only major facility not implemented so far is support for must-alias
information.</p>
+<p>Note that <tt>-ds-aa</tt> is available in the optional "poolalloc"
+module, it is not part of the LLVM core.</p>
+
</div>
<div class="doc_text">
<p>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
+<tt>
opt</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> class.</p>
+the <tt>AliasSetTracker</tt> class. To use it, use something like:</p>
+
+<div class="doc_code">
+<pre>
+% opt -ds-aa -print-alias-sets -disable-output
+</pre>
+</div>
</div>
</div>
+<!-- *********************************************************************** -->
+<div class="doc_section">
+ <a name="memdep">Memory Dependence Analysis</a>
+</div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<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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<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
- <a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
+ <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
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