[ConstantFolding] Fold scalable shufflevector of poison/undef. #143475
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@llvm/pr-subscribers-llvm-ir @llvm/pr-subscribers-llvm-transforms Author: Craig Topper (topperc) ChangesFull diff: https://github.com/llvm/llvm-project/pull/143475.diff 2 Files Affected:
diff --git a/llvm/lib/IR/ConstantFold.cpp b/llvm/lib/IR/ConstantFold.cpp
index b9db402fe9562..6c8850aae0476 100644
--- a/llvm/lib/IR/ConstantFold.cpp
+++ b/llvm/lib/IR/ConstantFold.cpp
@@ -463,10 +463,7 @@ Constant *llvm::ConstantFoldShuffleVectorInstruction(Constant *V1, Constant *V2,
Constant *Elt =
ConstantExpr::getExtractElement(V1, ConstantInt::get(Ty, 0));
- if (Elt->isNullValue()) {
- auto *VTy = VectorType::get(EltTy, MaskEltCount);
- return ConstantAggregateZero::get(VTy);
- } else if (!MaskEltCount.isScalable())
+ if (!MaskEltCount.isScalable() || Elt->isNullValue() || isa<UndefValue>(Elt))
return ConstantVector::getSplat(MaskEltCount, Elt);
}
diff --git a/llvm/test/Transforms/InstSimplify/shufflevector.ll b/llvm/test/Transforms/InstSimplify/shufflevector.ll
index 0442bd721974b..feab024a29213 100644
--- a/llvm/test/Transforms/InstSimplify/shufflevector.ll
+++ b/llvm/test/Transforms/InstSimplify/shufflevector.ll
@@ -337,3 +337,19 @@ define <4 x i32> @not_fold_identity2(<4 x i32> %x) {
%revshuf = shufflevector <4 x i32> %shuf, <4 x i32> poison, <4 x i32> <i32 3, i32 2, i32 1, i32 0>
ret <4 x i32> %revshuf
}
+
+define <vscale x 2 x i32> @scalable_splat_undef() {
+; CHECK-LABEL: @scalable_splat_undef(
+; CHECK-NEXT: ret <vscale x 2 x i32> undef
+;
+ %shuf = shufflevector <vscale x 2 x i32> undef, <vscale x 2 x i32> undef, <vscale x 2 x i32> zeroinitializer
+ ret <vscale x 2 x i32> %shuf
+}
+
+define <vscale x 2 x i32> @scalable_splat_poison() {
+; CHECK-LABEL: @scalable_splat_poison(
+; CHECK-NEXT: ret <vscale x 2 x i32> poison
+;
+ %shuf = shufflevector <vscale x 2 x i32> poison, <vscale x 2 x i32> poison, <vscale x 2 x i32> zeroinitializer
+ ret <vscale x 2 x i32> %shuf
+}
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You can test this locally with the following command:git-clang-format --diff HEAD~1 HEAD --extensions cpp -- llvm/lib/IR/ConstantFold.cppView the diff from clang-format here.diff --git a/llvm/lib/IR/ConstantFold.cpp b/llvm/lib/IR/ConstantFold.cpp
index d4ad21e69..9e8ae34a9 100644
--- a/llvm/lib/IR/ConstantFold.cpp
+++ b/llvm/lib/IR/ConstantFold.cpp
@@ -465,7 +465,8 @@ Constant *llvm::ConstantFoldShuffleVectorInstruction(Constant *V1, Constant *V2,
// For scalable vectors, make sure this doesn't fold back into a
// shufflevector.
- if (!MaskEltCount.isScalable() || Elt->isNullValue() || isa<UndefValue>(Elt))
+ if (!MaskEltCount.isScalable() || Elt->isNullValue() ||
+ isa<UndefValue>(Elt))
return ConstantVector::getSplat(MaskEltCount, Elt);
}
|
You can test this locally with the following command:git diff -U0 --pickaxe-regex -S '([^a-zA-Z0-9#_-]undef[^a-zA-Z0-9_-]|UndefValue::get)' 'HEAD~1' HEAD llvm/lib/IR/ConstantFold.cpp llvm/test/Transforms/InstSimplify/shufflevector.llThe following files introduce new uses of undef:
Undef is now deprecated and should only be used in the rare cases where no replacement is possible. For example, a load of uninitialized memory yields In tests, avoid using For example, this is considered a bad practice: define void @fn() {
...
br i1 undef, ...
}Please use the following instead: define void @fn(i1 %cond) {
...
br i1 %cond, ...
}Please refer to the Undefined Behavior Manual for more information. |
nikic
approved these changes
Jun 10, 2025
| auto *VTy = VectorType::get(EltTy, MaskEltCount); | ||
| return ConstantAggregateZero::get(VTy); | ||
| } else if (!MaskEltCount.isScalable()) | ||
| if (!MaskEltCount.isScalable() || Elt->isNullValue() || isa<UndefValue>(Elt)) |
There was a problem hiding this comment.
It took me a while to get why we need this condition. I'd add a comment like
// For scalable vectors, make sure this doesn't fold back into a shufflevector.
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LLVM Buildbot has detected a new failure on builder Full details are available at: https://lab.llvm.org/buildbot/#/builders/92/builds/20338 Here is the relevant piece of the build log for the reference |
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