[LLVMdev] Comparison of Alias Analysis in LLVM

[David Gardner]

Jianzhou Zhao <jianzhou <at> seas.upenn.edu> writes:
> At this level, I can understand how it works. I was confused because I
> have been looking at the source code for implementing them. All the
> globalmodref, scev-aa, steenaa and ds-aa are only subclasses of the
> AliasAnalysis class, so I cannot see how ds-aa can automatically call
> basicaa.

There's some magic in the pass registration which adds them with a 
`previous' link between AA passes, so the base AliasAnalysis class 
ends up calling the previous one via the "AliasAnalysis *AA" member.

> What I guess is that the order of the flags matters. This means if I
> want to chain a simple AA (say, -basicaa) and a fancier one (ds-aa), I
> should add the  -basicaa before -ds-aa to define the chain, which the
> AnalysisGroup can understand. If I add ds-aa before basicaa, does that
> mean basicaa will chain with ds-aa backward?

It will affect the order of the usage of the particular alias analysis
passes, but the last one specified is called first (so -basicaa -ds-aa
will cause ds-aa to be used first, then chain to basicaa as the previous
AA pass).  However, I believe that it should not affect the accuracy of
the alias information as the chaining will stop when a definite answer
is reached, whatever the order of the chained passes.