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

[Harald van Dijk via Phabricator via llvm-commits]

hvdijk 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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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)


Repository:
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  https://reviews.llvm.org/D127119/new/

https://reviews.llvm.org/D127119