[PATCH] D123408: [InstCombine] Limit folding of cast into PHI

[Zaara Syeda via Phabricator via llvm-commits]

syzaara added a comment.

In D123408#3571242 <https://reviews.llvm.org/D123408#3571242>, @syzaara wrote:

> In D123408#3571038 <https://reviews.llvm.org/D123408#3571038>, @spatel wrote:
>
>> In D123408#3570605 <https://reviews.llvm.org/D123408#3570605>, @syzaara wrote:
>>
>>> @spatel Could you please provide some feedback about this change to InstCombine.
>>
>> In general, making exceptions to transforms will not solve the general problem. I'm not sure if this is enough, but instcombine seems to miss a narrowing transform like this:
>> https://alive2.llvm.org/ce/z/hRy3rE
>>
>> We do already have a variation of that fold, so it should be a small enhancement. I can take a shot at that.
>> A demanded bits solution within instcombine might be better, but I'm not seeing how to make that work in general since we have to create 3 instructions from the pattern.

Oh nevermind, please ignore above comment, I see what you are saying now! That we can add a transformation which can convert:

>   %a.0 = phi i32 [ 0, %entry ], [ %phi.cast, %for.body ]
>   %conv = zext i8 %0 to i32
>   %add = add nuw nsw i32 %a.0, %conv 
>   %phi.cast = and i32 %add, 255

Back to:

>   %a.0 = phi i32 [ 0, %entry ], [ %phi.cast, %for.body ]
>   %conv = trunc i32 %a.0 to i8
>   %add = add i8 %0, %conv
>   %conv2 = zext i8 %add to i32

So we don't need to disable the phi cast optimization, but will still get the i8 add.


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  https://reviews.llvm.org/D123408/new/

https://reviews.llvm.org/D123408