Document problems that I encountered when working on -scev-aa.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@106776 91177308-0d34-0410-b5e6-96231b3b80d8

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>
 
@@ -502,6 +503,45 @@
 
 </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&lt;AliasAnalysis&gt;</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>