[InstCombine] Check for poison instead of undef in splat shuffle fold

We can't canonicalize these to a splat shuffle, as doing so would
convert undef -> poison.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp

@@ -2214,19 +2214,19 @@ static Instruction *canonicalizeInsertSplat(ShuffleVectorInst &Shuf,
   uint64_t IndexC;
 
   // Match a shuffle that is a splat to a non-zero element.
-  if (!match(Op0, m_OneUse(m_InsertElt(m_Undef(), m_Value(X),
+  if (!match(Op0, m_OneUse(m_InsertElt(m_Poison(), m_Value(X),
                                        m_ConstantInt(IndexC)))) ||
-      !match(Op1, m_Undef()) || match(Mask, m_ZeroMask()) || IndexC == 0)
+      !match(Op1, m_Poison()) || match(Mask, m_ZeroMask()) || IndexC == 0)
     return nullptr;
 
   // Insert into element 0 of a poison vector.
   PoisonValue *PoisonVec = PoisonValue::get(Shuf.getType());
   Value *NewIns = Builder.CreateInsertElement(PoisonVec, X, (uint64_t)0);
 
-  // Splat from element 0. Any mask element that is undefined remains undefined.
+  // Splat from element 0. Any mask element that is poison remains poison.
   // For example:
-  // shuf (inselt undef, X, 2), _, <2,2,undef>
-  //   --> shuf (inselt undef, X, 0), poison, <0,0,undef>
+  // shuf (inselt poison, X, 2), _, <2,2,undef>
+  //   --> shuf (inselt poison, X, 0), poison, <0,0,undef>
   unsigned NumMaskElts =
       cast<FixedVectorType>(Shuf.getType())->getNumElements();
   SmallVector<int, 16> NewMask(NumMaskElts, 0);

llvm/test/Transforms/InstCombine/vec_shuffle.ll

@@ -2346,23 +2346,19 @@ define i16 @pr92887(<2 x i16> %v) {
   ret i16 %extract
 }
 
-; FIXME: This is a miscompile.
 define <2 x i32> @not_splat_shuffle1(i32 %x) {
 ; CHECK-LABEL: @not_splat_shuffle1(
-; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <2 x i32> poison, i32 [[X:%.*]], i64 0
-; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> poison, <2 x i32> zeroinitializer
+; CHECK-NEXT:    [[SHUF:%.*]] = insertelement <2 x i32> <i32 poison, i32 undef>, i32 [[X:%.*]], i64 0
 ; CHECK-NEXT:    ret <2 x i32> [[SHUF]]
 ;
   %vec = insertelement <2 x i32> undef, i32 %x, i32 1
   %shuf = shufflevector <2 x i32> %vec, <2 x i32> poison, <2 x i32> <i32 1, i32 0>
   ret <2 x i32> %shuf
 }
 
-; FIXME: This is a miscompile.
 define <2 x i32> @not_splat_shuffle2(i32 %x) {
 ; CHECK-LABEL: @not_splat_shuffle2(
-; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <2 x i32> poison, i32 [[X:%.*]], i64 0
-; CHECK-NEXT:    [[SHUF:%.*]] = shufflevector <2 x i32> [[TMP1]], <2 x i32> poison, <2 x i32> zeroinitializer
+; CHECK-NEXT:    [[SHUF:%.*]] = insertelement <2 x i32> <i32 poison, i32 undef>, i32 [[X:%.*]], i64 0
 ; CHECK-NEXT:    ret <2 x i32> [[SHUF]]
 ;
   %vec = insertelement <2 x i32> poison, i32 %x, i32 1