PassManager support for alternative AliasAnalysis implementation -has some issues.
+The AliasAnalysis infrastructure has several limitations which make +writing a new AliasAnalysis implementation difficult.
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 @@ -558,6 +558,40 @@ silently route alias analysis queries directly to passes between each pass, which prevents the use of FunctionPass alias analysis passes.
+The AliasAnalysis 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 AliasAnalysis +implementations which can not be expressed.
+ +The AliasAnalysisDebugger utility seems to suggest that +AliasAnalysis implementations can expect that they will be +informed of any relevant Value before it appears in an +alias query. However, popular clients such as GVN don't +support this, and are known to trigger errors when run with the +AliasAnalysisDebugger.
+ +Due to several of the above limitations, the most obvious use for +the AliasAnalysisCounter utility, collecting stats on all +alias queries in a compilation, doesn't work, even if the +AliasAnalysis implementations don't use FunctionPass. +There's no way to set a default, much less a default sequence, +and there's no way to preserve it.
+ +The AliasSetTracker class (which is used by LICM +makes a non-deterministic number of alias queries. This can cause stats +collected by AliasAnalysisCounter 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.
+ +Many alias queries can be reformulated in terms of other alias +queries. When multiple AliasAnalysis 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.
+