[InstCombine] Avoid simplifying bitcast of undef to a zeroinitializer…

… vector (llvm#108872)

In some cases, if an undef value is the product of another instcombine
simplification, a bitcast of undef is simplified to a zeroinitializer
vector instead of undef.

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

@@ -2323,6 +2323,11 @@ static Value *optimizeIntegerToVectorInsertions(BitCastInst &CI,
  auto *DestVecTy = cast<FixedVectorType>(CI.getType());
  Value *IntInput = CI.getOperand(0);
 
+ // if the int input is just an undef value do not try to optimize to vector
+ // insertions as it will prevent undef propagation
+ if (isa<UndefValue>(IntInput))
+ return nullptr;
+
  SmallVector<Value*, 8> Elements(DestVecTy->getNumElements());
  if (!collectInsertionElements(IntInput, 0, Elements,
  DestVecTy->getElementType(),

llvm/test/Transforms/InstCombine/bitcast.ll

@@ -879,3 +879,26 @@ define half @copysign_idiom_constant_wrong_type2(bfloat %x, i16 %mag) {
  %y = bitcast i16 %res to half
  ret half %y
 }
+
+define i16 @bitcast_undef_to_vector() {
+; CHECK-LABEL: @bitcast_undef_to_vector(
+; CHECK-NEXT: entry:
+; CHECK-NEXT: br label [[END:%.*]]
+; CHECK: unreachable:
+; CHECK-NEXT: br label [[END]]
+; CHECK: end:
+; CHECK-NEXT: ret i16 undef
+;
+entry:
+ br label %end
+
+unreachable: ; No predecessors!
+ %0 = extractvalue { i32, i32 } zeroinitializer, 1
+ br label %end
+
+end: ; preds = %unreachable, %entry
+ %1 = phi i32 [ %0, %unreachable ], [ undef, %entry ]
+ %2 = bitcast i32 %1 to <2 x i16>
+ %3 = extractelement <2 x i16> %2, i64 0
+ ret i16 %3
+}