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

Author: topperc

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);
   }
 

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
+}