[PATCH] D76800: [InstCombine] Fix Incorrect fold of ashr+xor -> lshr w/ vectors

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In D76800#1942204 <https://reviews.llvm.org/D76800#1942204>, @lebedev.ri wrote:

> What we need to do here is "define" undef to be 0.
>
>   ----------------------------------------
>   define <4 x i5> @test_v4i5_not_ashr_negative_const_undef(<4 x i5> %a0) {
>   %0:
>     %1 = ashr <4 x i5> { 29, 27, undef, 23 }, %a0
>     %2 = xor <4 x i5> { 31, 31, 31, undef }, %1
>     ret <4 x i5> %2
>   }
>   =>
>   define <4 x i5> @test_v4i5_not_ashr_negative_const_undef(<4 x i5> %a0) {
>   %0:
>     %1 = lshr <4 x i5> { 2, 4, 0, 8 }, %a0
>     ret <4 x i5> %1
>   }
>   Transformation seems to be correct!
>  
>   Summary:
>     1 correct transformations
>     0 incorrect transformations
>     0 Alive2 errors
>
>
> There was some utility for that already, but i don't recall how it's called..


Probably want this:

  /// Some binary operators require special handling to avoid poison and undefined
  /// behavior. If a constant vector has undef elements, replace those undefs with
  /// identity constants if possible because those are always safe to execute.
  /// If no identity constant exists, replace undef with some other safe constant.
  static inline Constant *getSafeVectorConstantForBinop()


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