[ConstantFold] Support scalable constant splats in ConstantFoldCastInstruction (#133207)

Previously only fixed vector splats were handled. This adds supports for
scalable vectors too by allowing ConstantExpr splats.

We need to add the extra V->getType()->isVectorTy() check because a
ConstantExpr might be a scalar to vector bitcast.

By allowing ConstantExprs this also allow fixed vector ConstantExprs to
be folded, which causes the diffs in
llvm/test/Analysis/ValueTracking/known-bits-from-operator-constexpr.ll
and llvm/test/Transforms/InstSimplify/ConstProp/cast-vector.ll. I can
remove them from this PR if reviewers would prefer.

Fixes #132922

1 files changed, 5 insertions, 4 deletions

diff --git a/llvm/lib/IR/ConstantFold.cpp b/llvm/lib/IR/ConstantFold.cpp
index b577f69..7e5fda2 100644
--- a/llvm/lib/IR/ConstantFold.cpp
+++ b/llvm/lib/IR/ConstantFold.cpp
@@ -160,10 +160,9 @@ Constant *llvm::ConstantFoldCastInstruction(unsigned opc, Constant *V,
   // If the cast operand is a constant vector, perform the cast by
   // operating on each element. In the cast of bitcasts, the element
   // count may be mismatched; don't attempt to handle that here.
-  if ((isa<ConstantVector>(V) || isa<ConstantDataVector>(V)) &&
-      DestTy->isVectorTy() &&
-      cast<FixedVectorType>(DestTy)->getNumElements() ==
-          cast<FixedVectorType>(V->getType())->getNumElements()) {
+  if (DestTy->isVectorTy() && V->getType()->isVectorTy() &&
+      cast<VectorType>(DestTy)->getElementCount() ==
+          cast<VectorType>(V->getType())->getElementCount()) {
     VectorType *DestVecTy = cast<VectorType>(DestTy);
     Type *DstEltTy = DestVecTy->getElementType();
     // Fast path for splatted constants.
@@ -174,6 +173,8 @@ Constant *llvm::ConstantFoldCastInstruction(unsigned opc, Constant *V,
       return ConstantVector::getSplat(
           cast<VectorType>(DestTy)->getElementCount(), Res);
     }
+    if (isa<ScalableVectorType>(DestTy))
+      return nullptr;
     SmallVector<Constant *, 16> res;
     Type *Ty = IntegerType::get(V->getContext(), 32);
     for (unsigned i = 0,