[InstCombine] Do not perform binop-of-shuffle when mask is poison

A miscompilation issue has been addressed with refined checking.
Shuffle masks operand may be turned into `poison` if this does
not lead to observable changes. This however may not guarantee
binop to binop-of-shuffle replacement to be sound anymore.

Fixes: #82052.

Author: antoniofrighetto

llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp

@@ -1870,14 +1870,16 @@ Value *InstCombinerImpl::SimplifyDemandedVectorElts(Value *V,
         Value *ShufOp = MatchShufAsOp0 ? X : Y;
         Value *OtherOp = MatchShufAsOp0 ? Y : X;
         for (User *U : OtherOp->users()) {
-          auto Shuf = m_Shuffle(m_Specific(ShufOp), m_Value(), m_ZeroMask());
+          ArrayRef<int> Mask;
+          auto Shuf = m_Shuffle(m_Specific(ShufOp), m_Value(), m_Mask(Mask));
           if (BO->isCommutative()
                   ? match(U, m_c_BinOp(Opcode, Shuf, m_Specific(OtherOp)))
                   : MatchShufAsOp0
                         ? match(U, m_BinOp(Opcode, Shuf, m_Specific(OtherOp)))
                         : match(U, m_BinOp(Opcode, m_Specific(OtherOp), Shuf)))
-            if (DT.dominates(U, I))
-              return U;
+            if (match(Mask, m_ZeroMask()) && Mask[0] != PoisonMaskElem)
+              if (DT.dominates(U, I))
+                return U;
         }
         return nullptr;
       };

llvm/test/Transforms/InstCombine/vec_demanded_elts.ll

@@ -1159,3 +1159,53 @@ define i4 @common_binop_demand_via_extelt_op0_mismatch_elt1(<2 x i4> %x, <2 x i4
   call void @use(<2 x i4> %b_xshuf_y)
   ret i4 %b_xy0
 }
+
+define <2 x i8> @common_binop_demand_via_splat_mask_poison(<2 x i8> %x, <2 x i8> %y) {
+; CHECK-LABEL: @common_binop_demand_via_splat_mask_poison(
+; CHECK-NEXT:    [[YSPLAT:%.*]] = shufflevector <2 x i8> [[Y:%.*]], <2 x i8> poison, <2 x i32> <i32 0, i32 poison>
+; CHECK-NEXT:    [[VV:%.*]] = add <2 x i8> [[YSPLAT]], [[X:%.*]]
+; CHECK-NEXT:    [[MSPLAT:%.*]] = shufflevector <2 x i8> [[VV]], <2 x i8> poison, <2 x i32> zeroinitializer
+; CHECK-NEXT:    [[RES:%.*]] = add <2 x i8> [[VV]], [[MSPLAT]]
+; CHECK-NEXT:    ret <2 x i8> [[RES]]
+;
+  %ysplat = shufflevector <2 x i8> %y, <2 x i8> poison, <2 x i32> <i32 0, i32 poison>                       ; <y0, poison>
+  %vv = add <2 x i8> %x, %ysplat                                                                            ; <x0+y0, poison>
+  %m = add <2 x i8> %x, %y                                                                                  ; <x0+y0, x1+y1>
+  %msplat = shufflevector <2 x i8> %m, <2 x i8> poison, <2 x i32> <i32 0, i32 0>      ; LeftDemanded = 1    ; <x0+y0, x0+y0>
+  %res = add <2 x i8> %vv, %msplat                                                                          ; <x0+y0+x0+y0, poison>
+  ret <2 x i8> %res
+}
+
+define <2 x i8> @common_binop_demand_via_splat_mask_poison_2(<2 x i8> %x, <2 x i8> %y) {
+; CHECK-LABEL: @common_binop_demand_via_splat_mask_poison_2(
+; CHECK-NEXT:    [[YSPLAT:%.*]] = shufflevector <2 x i8> [[Y:%.*]], <2 x i8> poison, <2 x i32> <i32 poison, i32 0>
+; CHECK-NEXT:    [[VV:%.*]] = add <2 x i8> [[YSPLAT]], [[X:%.*]]
+; CHECK-NEXT:    [[M:%.*]] = add <2 x i8> [[X]], [[Y]]
+; CHECK-NEXT:    [[MSPLAT:%.*]] = shufflevector <2 x i8> [[M]], <2 x i8> [[Y]], <2 x i32> <i32 0, i32 2>
+; CHECK-NEXT:    [[RES:%.*]] = add <2 x i8> [[VV]], [[MSPLAT]]
+; CHECK-NEXT:    ret <2 x i8> [[RES]]
+;
+  %ysplat = shufflevector <2 x i8> %y, <2 x i8> poison, <2 x i32> <i32 poison, i32 0>
+  %vv = add <2 x i8> %x, %ysplat
+  %m = add <2 x i8> %x, %y
+  %msplat = shufflevector <2 x i8> %m, <2 x i8> %y, <2 x i32> <i32 0, i32 2>           ; LeftDemanded = 1, RightDemanded = 1
+  %res = add <2 x i8> %vv, %msplat
+  ret <2 x i8> %res
+}
+
+define <2 x i8> @common_binop_demand_via_splat_mask_poison_3(<2 x i8> %x, <2 x i8> %y) {
+; CHECK-LABEL: @common_binop_demand_via_splat_mask_poison_3(
+; CHECK-NEXT:    [[YSPLAT:%.*]] = shufflevector <2 x i8> [[Y:%.*]], <2 x i8> poison, <2 x i32> <i32 poison, i32 0>
+; CHECK-NEXT:    [[VV:%.*]] = add <2 x i8> [[YSPLAT]], [[X:%.*]]
+; CHECK-NEXT:    [[M:%.*]] = add <2 x i8> [[X]], [[Y]]
+; CHECK-NEXT:    [[MSPLAT:%.*]] = shufflevector <2 x i8> [[M]], <2 x i8> poison, <2 x i32> zeroinitializer
+; CHECK-NEXT:    [[RES:%.*]] = add <2 x i8> [[VV]], [[MSPLAT]]
+; CHECK-NEXT:    ret <2 x i8> [[RES]]
+;
+  %ysplat = shufflevector <2 x i8> %y, <2 x i8> poison, <2 x i32> <i32 poison, i32 0>
+  %vv = add <2 x i8> %x, %ysplat
+  %m = add <2 x i8> %x, %y
+  %msplat = shufflevector <2 x i8> %m, <2 x i8> poison, <2 x i32> <i32 0, i32 0>       ; LeftDemanded = 1
+  %res = add <2 x i8> %vv, %msplat
+  ret <2 x i8> %res
+}