[InstCombine] Drop samesign flags in foldLogOpOfMaskedICmps_NotAllZeros_BMask_Mixed (#120373)

Counterexamples: https://alive2.llvm.org/ce/z/6Ks8Qz
Closes #120361.

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp

@@ -455,14 +455,20 @@ static Value *foldLogOpOfMaskedICmps_NotAllZeros_BMask_Mixed(
   // RHS. For example,
   // (icmp ne (A & 255), 0) & (icmp eq (A & 15), 8) -> (icmp eq (A & 15), 8).
   // (icmp ne (A & 15), 0) & (icmp eq (A & 15), 8) -> (icmp eq (A & 15), 8).
-  if (IsSuperSetOrEqual(BCst, DCst))
+  if (IsSuperSetOrEqual(BCst, DCst)) {
+    // We can't guarantee that samesign hold after this fold.
+    RHS->setSameSign(false);
     return RHS;
+  }
   // Otherwise, B is a subset of D. If B and E have a common bit set,
   // ie. (B & E) != 0, then LHS is subsumed by RHS. For example.
   // (icmp ne (A & 12), 0) & (icmp eq (A & 15), 8) -> (icmp eq (A & 15), 8).
   assert(IsSubSetOrEqual(BCst, DCst) && "Precondition due to above code");
-  if ((*BCst & ECst) != 0)
+  if ((*BCst & ECst) != 0) {
+    // We can't guarantee that samesign hold after this fold.
+    RHS->setSameSign(false);
     return RHS;
+  }
   // Otherwise, LHS and RHS contradict and the whole expression becomes false
   // (or true if negated.) For example,
   // (icmp ne (A & 7), 0) & (icmp eq (A & 15), 8) -> false.

llvm/test/Transforms/InstCombine/icmp-logical.ll

@@ -1900,3 +1900,28 @@ define i1 @masked_icmps_bmask_notmixed_not_subset_notoptimized(i32 %A) {
   %res = and i1 %tst1, %tst2
   ret i1 %res
 }
+
+define i1 @pr120361(i8 %x, i8 %y) {
+; CHECK-LABEL: @pr120361(
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp eq i8 [[X:%.*]], -1
+; CHECK-NEXT:    ret i1 [[CMP1]]
+;
+  %cmp1 = icmp samesign eq i8 %x, -1
+  %cmp2 = icmp ne i8 %x, 0
+  %result = select i1 %cmp2, i1 %cmp1, i1 false
+  ret i1 %result
+}
+
+define i1 @pr120361_v2(i32 %x) {
+; CHECK-LABEL: @pr120361_v2(
+; CHECK-NEXT:    [[AND2:%.*]] = and i32 [[X:%.*]], -113
+; CHECK-NEXT:    [[CMP2:%.*]] = icmp eq i32 [[AND2]], 15
+; CHECK-NEXT:    ret i1 [[CMP2]]
+;
+  %and1 = and i32 %x, 15
+  %cmp1 = icmp ne i32 %and1, 0
+  %and2 = and i32 %x, -113
+  %cmp2 = icmp samesign eq i32 %and2, 15
+  %and = select i1 %cmp1, i1 %cmp2, i1 false
+  ret i1 %and
+}