[InstCombine] Let shrinkSplatShuffle act on vectors of different lengths (#148593)

shrinkSplatShuffle in InstCombine would only move truncs up through
shuffles if those shuffles inputs had the exact same type as their
output, this PR weakens this constraint to only requiring that the
scalar type of the input and output match.

1 files changed, 6 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 033ef8b..a43a6ee 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -708,10 +708,14 @@ static Instruction *shrinkSplatShuffle(TruncInst &Trunc,
   auto *Shuf = dyn_cast<ShuffleVectorInst>(Trunc.getOperand(0));
   if (Shuf && Shuf->hasOneUse() && match(Shuf->getOperand(1), m_Undef()) &&
       all_equal(Shuf->getShuffleMask()) &&
-      Shuf->getType() == Shuf->getOperand(0)->getType()) {
+      ElementCount::isKnownGE(Shuf->getType()->getElementCount(),
+                              cast<VectorType>(Shuf->getOperand(0)->getType())
+                                  ->getElementCount())) {
     // trunc (shuf X, Undef, SplatMask) --> shuf (trunc X), Poison, SplatMask
     // trunc (shuf X, Poison, SplatMask) --> shuf (trunc X), Poison, SplatMask
-    Value *NarrowOp = Builder.CreateTrunc(Shuf->getOperand(0), Trunc.getType());
+    Type *NewTruncTy = Shuf->getOperand(0)->getType()->getWithNewType(
+        Trunc.getType()->getScalarType());
+    Value *NarrowOp = Builder.CreateTrunc(Shuf->getOperand(0), NewTruncTy);
     return new ShuffleVectorInst(NarrowOp, Shuf->getShuffleMask());
   }