[PATCH] D15857: [InstCombine] Defend against worst-case exponential execution time

[James Molloy via llvm-commits]

Hi David,

> Out of curiosity, is there a major benefit to canonicalizing this in the
middle-end vs doing this transform in code-gen prepare?

Major? no. My initial implementation was to do this in ISelLowering, but it
was remarked that matching bitreversals is a fairly generic concept that
other backends could benefit from.

The code already in instcombine (since at least 2010) to match bswaps
performs almost exactly the same steps as matching bitreversals (apart from
it looks at bytes rather than bits). I therefore thought the most obvious
thing to do would be to tweak the existing algorithm and improve it.

Do you think it's too heavyweight for instcombine?

James

On Mon, 4 Jan 2016 at 23:13 David Majnemer via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> majnemer added a comment.
>
> Out of curiosity, is there a major benefit to canonicalizing this in the
> middle-end vs doing this transform in code-gen prepare?
>
>
> Repository:
>   rL LLVM
>
> http://reviews.llvm.org/D15857
>
>
>
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