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Diffstat (limited to 'docs/AliasAnalysis.html')
| -rw-r--r-- | docs/AliasAnalysis.html | 88 |
1 files changed, 78 insertions, 10 deletions
diff --git a/docs/AliasAnalysis.html b/docs/AliasAnalysis.html index 0413622..a23d908 100644 --- a/docs/AliasAnalysis.html +++ b/docs/AliasAnalysis.html @@ -31,6 +31,7 @@ <li><a href="#chaining"><tt>AliasAnalysis</tt> chaining behavior</a></li> <li><a href="#updating">Updating analysis results for transformations</a></li> <li><a href="#implefficiency">Efficiency Issues</a></li> + <li><a href="#passmanager">Pass Manager Issues</a></li> </ul> </li> @@ -116,6 +117,11 @@ as the actual <tt>call</tt> or <tt>invoke</tt> instructions that performs the call. The <tt>AliasAnalysis</tt> interface also exposes some helper methods which allow you to get mod/ref information for arbitrary instructions.</p> +<p>All <tt>AliasAnalysis</tt> interfaces require that in queries involving +multiple values, values which are not +<a href="LangRef.html#constants">constants</a> are all defined within the +same function.</p> + </div> <!-- ======================================================================= --> @@ -180,9 +186,13 @@ that the accesses alias.</p> </div> <div class="doc_text"> -The <tt>alias</tt> 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, MayAlias, or NoAlias as appropriate. +<p>The <tt>alias</tt> 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, MayAlias, or NoAlias as appropriate.</p> + +<p>Like all <tt>AliasAnalysis</tt> interfaces, the <tt>alias</tt> method requires +that either the two pointer values be defined within the same function, or at +least one of the values is a <a href="LangRef.html#constants">constant</a>.</p> </div> <!-- _______________________________________________________________________ --> @@ -191,12 +201,18 @@ and returns MustAlias, MayAlias, or NoAlias as appropriate. </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>The NoAlias response may be used when there is never an immediate dependence +between any memory reference <i>based</i> on one pointer and any memory +reference <i>based</i> 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 href="LangRef.html#noalias"><tt>noalias</tt></a> keyword; the "irrelevant" +dependencies are ignored.</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 @@ -502,6 +518,45 @@ method as possible (within reason).</p> </div> +<!-- ======================================================================= --> +<div class="doc_subsection"> + <a name="passmanager">Pass Manager Issues</a> +</div> + +<div class="doc_text"> + +<p>PassManager support for alternative AliasAnalysis implementation +has some issues.</p> + +<p>There is no way to override the default alias analysis. It would +be very useful to be able to do something like "opt -my-aa -O2" and +have it use -my-aa 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 +<tt>AliasAnalysis</tt> implementations. The <tt>AliasAnalysis</tt> +interface includes <tt>deleteValue</tt> and <tt>copyValue</tt> 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 +<tt>getAnalysisUsage</tt> that it does so. Some passes attempt to use +<tt>AU.addPreserved<AliasAnalysis></tt>, however this doesn't +actually have any effect.</tt> + +<p><tt>AliasAnalysisCounter</tt> (<tt>-count-aa</tt>) and <tt>AliasDebugger</tt> +(<tt>-debug-aa</tt>) are implemented as <tt>ModulePass</tt> classes, so if your +alias analysis uses <tt>FunctionPass</tt>, it won't be able to use +these utilities. If you try to use them, the pass manager will +silently route alias analysis queries directly to +<tt>BasicAliasAnalysis</tt> instead.</p> + +<p>Similarly, the <tt>opt -p</tt> option introduces <tt>ModulePass</tt> +passes between each pass, which prevents the use of <tt>FunctionPass</tt> +alias analysis passes.</p> + +</div> + <!-- *********************************************************************** --> <div class="doc_section"> <a name="using">Using alias analysis results</a> @@ -749,6 +804,19 @@ module, it is not part of the LLVM core.</p> </div> +<!-- _______________________________________________________________________ --> +<div class="doc_subsubsection"> + <a name="scev-aa">The <tt>-scev-aa</tt> pass</a> +</div> + +<div class="doc_text"> + +<p>The <tt>-scev-aa</tt> pass implements AliasAnalysis queries by +translating them into ScalarEvolution queries. This gives it a +more complete understanding of <tt>getelementptr</tt> instructions +and loop induction variables than other alias analyses have.</p> + +</div> <!-- ======================================================================= --> <div class="doc_subsection"> @@ -930,7 +998,7 @@ analysis directly.</p> <a href="mailto:sabre@nondot.org">Chris Lattner</a><br> <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br> - Last modified: $Date: 2010-05-07 02:28:04 +0200 (Fri, 07 May 2010) $ + Last modified: $Date: 2010-07-07 16:27:09 +0200 (Wed, 07 Jul 2010) $ </address> </body> |
