Answer comments

Author: Adar Dagan

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

@@ -708,17 +708,15 @@ 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()->getScalarType() ==
-          Shuf->getOperand(0)->getType()->getScalarType()) {
+      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
-    auto *const NewTruncTy = VectorType::get(
-        Trunc.getType()->getScalarType(),
-        cast<VectorType>(Shuf->getOperand(0)->getType())->getElementCount());
-    Value *NarrowOp =
-        Builder.CreateTrunc(Shuf->getOperand(0), NewTruncTy, Trunc.getName());
-    return new ShuffleVectorInst(NarrowOp, Shuf->getShuffleMask(),
-                                 Shuf->getName());
+    Type *const NewTruncTy = Shuf->getOperand(0)->getType()->getWithNewType(
+        Trunc.getType()->getScalarType());
+    Value *NarrowOp = Builder.CreateTrunc(Shuf->getOperand(0), NewTruncTy);
+    return new ShuffleVectorInst(NarrowOp, Shuf->getShuffleMask());
   }
 
   return nullptr;

llvm/test/Transforms/InstCombine/trunc.ll

@@ -969,6 +969,19 @@ define <8 x i8> @wide_lengthening_splat(<4 x i16> %v) {
   ret <8 x i8> %tr
 }
 
+; This is a negative test, we expect the trunc to remain after the shuffle as it
+; might not be beneficial to preform trunc on a wider type
+define <4 x i8> @wide_shortening_splat(<8 x i16> %v) {
+; CHECK-LABEL: @wide_shortening_splat(
+; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <8 x i16> [[V:%.*]], <8 x i16> poison, <4 x i32> zeroinitializer
+; CHECK-NEXT:    [[TR:%.*]] = trunc <4 x i16> [[SHUF]] to <4 x i8>
+; CHECK-NEXT:    ret <4 x i8> [[TR]]
+;
+  %shuf = shufflevector <8 x i16> %v, <8 x i16> %v, <4 x i32> zeroinitializer
+  %tr = trunc <4 x i16> %shuf to <4 x i8>
+  ret <4 x i8> %tr
+}
+
 define <2 x i8> @narrow_add_vec_constant(<2 x i32> %x) {
 ; CHECK-LABEL: @narrow_add_vec_constant(
 ; CHECK-NEXT:    [[TMP1:%.*]] = trunc <2 x i32> [[X:%.*]] to <2 x i8>