[llvm] f72dd4e - [ConstantFolding] Fold scalable shufflevector of poison/undef. (#143475)

[via llvm-commits]

Author: Craig Topper
Date: 2025-06-10T12:50:16-07:00
New Revision: f72dd4eca8dac84d819abccde4447755a7d9420d

URL: https://github.com/llvm/llvm-project/commit/f72dd4eca8dac84d819abccde4447755a7d9420d
DIFF: https://github.com/llvm/llvm-project/commit/f72dd4eca8dac84d819abccde4447755a7d9420d.diff

LOG: [ConstantFolding] Fold scalable shufflevector of poison/undef. (#143475)

Added: 
    

Modified: 
    llvm/lib/IR/ConstantFold.cpp
    llvm/test/Transforms/InstSimplify/shufflevector.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/IR/ConstantFold.cpp b/llvm/lib/IR/ConstantFold.cpp
index b9db402fe9562..d4ad21e69e848 100644
--- a/llvm/lib/IR/ConstantFold.cpp
+++ b/llvm/lib/IR/ConstantFold.cpp
@@ -463,10 +463,9 @@ 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())
+    // For scalable vectors, make sure this doesn't fold back into a
+    // shufflevector.
+    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
+}