+</div>
+
+<!-- ======================================================================= -->
+<h3>
+ <a name="limitations">Limitations</a>
+</h3>
+
+<div>
+
+<p>The AliasAnalysis infrastructure has several limitations which make
+writing a new <tt>AliasAnalysis</tt> 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 "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.</p>
+
+<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>
+
+<p>The <tt>AliasAnalysis</tt> 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 <tt>AliasAnalysis</tt>
+implementations which can not be expressed.</p>
+
+<p>The <tt>AliasAnalysisDebugger</tt> utility seems to suggest that
+<tt>AliasAnalysis</tt> implementations can expect that they will be
+informed of any relevant <tt>Value</tt> before it appears in an
+alias query. However, popular clients such as <tt>GVN</tt> don't
+support this, and are known to trigger errors when run with the
+<tt>AliasAnalysisDebugger</tt>.</p>
+
+<p>Due to several of the above limitations, the most obvious use for
+the <tt>AliasAnalysisCounter</tt> utility, collecting stats on all
+alias queries in a compilation, doesn't work, even if the
+<tt>AliasAnalysis</tt> implementations don't use <tt>FunctionPass</tt>.
+There's no way to set a default, much less a default sequence,
+and there's no way to preserve it.</p>
+
+<p>The <tt>AliasSetTracker</tt> class (which is used by <tt>LICM</tt>
+makes a non-deterministic number of alias queries. This can cause stats
+collected by <tt>AliasAnalysisCounter</tt> to have fluctuations among
+identical runs, for example. Another consequence is that debugging
+techniques involving pausing execution after a predetermined number
+of queries can be unreliable.</p>
+
+<p>Many alias queries can be reformulated in terms of other alias
+queries. When multiple <tt>AliasAnalysis</tt> 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>