[PATCH] D23432: [AliasSetTracker] Degrade AliasSetTracker results when may-alias sets get too large.

[Xinliang David Li via llvm-commits]

I understand your concern here, and  performance cliff is definitely
something we should try to avoid. However dropping alias info in
situation like this != performance cliff.  I am sure we can come up
with hand-created examples to show  performance damage with dropped
alias info, in real programs, when a function reaches such a state,
the alias query results will be already so conservative that doing
memory disambiguation busily any further will likely be just waste of
compile time, so 'gracefully' lowering alias precision or dropping
aliasing info to the floor makes no difference practically.  I have
not seen performance regressions due to the use of cutoff limits
elsewhere in LLVM.

David


On Sat, Aug 20, 2016 at 12:24 PM, Philip Reames
<listmail at philipreames.com> wrote:
> reames added a subscriber: reames.
> reames added a comment.
>
> I am not actively objecting to this patch, but I really don't like the overall direction here.  Having a threshold where our ability to optimize falls off a cliff just seems really undesirable.  As Hal pointed out, there are likely options for summarizing alias sets to allow quicker AA queries.  How much have we explored that design space?
>
>
> Repository:
>   rL LLVM
>
> https://reviews.llvm.org/D23432
>
>
>