[PATCH] D127119: [SLP]Fix undef handling in gather function.

[Nikita Popov via Phabricator via llvm-commits]

nikic added inline comments.


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Comment at: llvm/test/Transforms/SLPVectorizer/X86/diamond_broadcast_extra_shuffle.ll:37
 ; CHECK-NEXT:    [[TMP0:%.*]] = insertelement <4 x i32> poison, i32 [[LD]], i32 0
-; CHECK-NEXT:    [[SHUFFLE:%.*]] = shufflevector <4 x i32> [[TMP0]], <4 x i32> poison, <4 x i32> zeroinitializer
-; CHECK-NEXT:    [[TMP1:%.*]] = mul <4 x i32> [[SHUFFLE]], [[SHUFFLE]]
+; CHECK-NEXT:    [[SHUFFLE:%.*]] = shufflevector <4 x i32> [[TMP0]], <4 x i32> poison, <4 x i32> <i32 0, i32 0, i32 0, i32 undef>
+; CHECK-NEXT:    [[SHUFFLE1:%.*]] = shufflevector <4 x i32> [[TMP0]], <4 x i32> poison, <4 x i32> zeroinitializer
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hvdijk wrote:
> vporpo wrote:
> > Doesn't an undef here mean that lane 4 can potentially be poison? Wouldn't that be incorrect?
> A mask of undef means the result is undef, not poison, even if the input contains poison elements. See https://llvm.org/docs/LangRef.html#shufflevector-instruction (which is less clear than I would have liked, it used to explicitly say undef, it now says "undefined" to be more readable, but it's not obvious that it is still meant to refer to undef rather than poison)
This will be switched to return a poison element in the future, so it would be best not to rely on it. (cc @nlopes).


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D127119/new/

https://reviews.llvm.org/D127119