[PATCH] D23908: LSV: Always try to adjust the alloca alignment

Matt Arsenault via llvm-commits llvm-commits at lists.llvm.org
Mon Aug 29 13:22:26 PDT 2016

arsenm added a comment.

In https://reviews.llvm.org/D23908#528067, @jlebar wrote:

> accessIsMisaligned returns false if the TTI reports that the access is not "fast".  But here we seem to be ignoring that and aligning the access anyway.
> Is there prior art for ignoring TTI and assuming that unaligned accesses are slower than aligned accesses?  Alternatively, could you just make your target return false for "fast"?

I think what "fast" means is ambiguous. The way this is used changes the meaning of fast in the context of the uses. For example in DAGCombiner, this is used to check if it's OK to merge a store if it creates a less-aligned vector store. That's a bit different than the question of changing the underlying alignment. The unaligned operation might be faster to some unaligned expansion, but that doesn't mean it's still not better still to have the aligned access


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