commit f72dd4eca8dac84d819abccde4447755a7d9420d
author Craig Topper <craig.topper@sifive.com> Tue Jun 10 12:50:16 2025 -0700
committer GitHub <noreply@github.com> Tue Jun 10 12:50:16 2025 -0700
tree 39f25a88abbe7124f214098253fd2a969c52f0a1
parent be3c6a06d36d6210ce51bbeacc343f3590043b96

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

llvm/lib/IR/ConstantFold.cpp

diff --git a/llvm/lib/IR/ConstantFold.cpp b/llvm/lib/IR/ConstantFold.cpp
index b9db402..d4ad21e 100644
--- a/llvm/lib/IR/ConstantFold.cpp
+++ b/llvm/lib/IR/ConstantFold.cpp
@@ -463,10 +463,9 @@
     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);
   }
 

llvm/test/Transforms/InstSimplify/shufflevector.ll

diff --git a/llvm/test/Transforms/InstSimplify/shufflevector.ll b/llvm/test/Transforms/InstSimplify/shufflevector.ll
index 0442bd7..feab024 100644
--- a/llvm/test/Transforms/InstSimplify/shufflevector.ll
+++ b/llvm/test/Transforms/InstSimplify/shufflevector.ll
@@ -337,3 +337,19 @@
   %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
+}