[llvm] r257875 - [InstCombine] Rewrite bswap/bitreverse handling completely.

[James Molloy via llvm-commits]

Hi Hans,

Yes, as long as David is happy please merge this, along with the followup that fixes msan.

I was waiting for it to soak through the stage2 bots before suggesting it myself.

James



> On 15 Jan 2016, at 17:59, Hans Wennborg <hans at chromium.org> wrote:
>
> It was suggested that this be merged to 3.8. David, I think you're the
> code owner here; OK for merge?
>
> On Fri, Jan 15, 2016 at 1:20 AM, James Molloy via llvm-commits
> <llvm-commits at lists.llvm.org> wrote:
>> Author: jamesm
>> Date: Fri Jan 15 03:20:19 2016
>> New Revision: 257875
>>
>> URL: http://llvm.org/viewvc/llvm-project?rev=257875&view=rev
>> Log:
>> [InstCombine] Rewrite bswap/bitreverse handling completely.
>>
>> There are several requirements that ended up with this design;
>>  1. Matching bitreversals is too heavyweight for InstCombine and doesn't really need to be done so early.
>>  2. Bitreversals and byteswaps are very related in their matching logic.
>>  3. We want to implement support for matching more advanced bswap/bitreverse patterns like partial bswaps/bitreverses.
>>  4. Bswaps are best matched early in InstCombine.
>>
>> The result of these is that a new utility function is created in Transforms/Utils/Local.h that can be configured to search for bswaps, bitreverses or both. InstCombine uses it to find only bswaps, CGP uses it to find only bitreversals.
>>
>> We can then extend the matching logic in one place only.
>
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