[InstCombine] Fold (icmp eq/ne (add nuw x, y), 0) -> (icmp eq/ne (or x, y), 0)

`(icmp eq/ne (or x, y), 0)` is probably easier to analyze than `(icmp
eq/ne x, -y)`

Proof: https://alive2.llvm.org/ce/z/2-VTb6

Author: goldsteinn

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -3453,6 +3453,11 @@ Instruction *InstCombinerImpl::foldICmpBinOpEqualityWithConstant(
       if (Value *NegVal = dyn_castNegVal(BOp0))
         return new ICmpInst(Pred, NegVal, BOp1);
       if (BO->hasOneUse()) {
+        // (add nuw A, B) != 0 -> (or A, B) != 0
+        if (match(BO, m_NUWAdd(m_Value(), m_Value()))) {
+          Value *Or = Builder.CreateOr(BOp0, BOp1);
+          return new ICmpInst(Pred, Or, Constant::getNullValue(BO->getType()));
+        }
         Value *Neg = Builder.CreateNeg(BOp1);
         Neg->takeName(BO);
         return new ICmpInst(Pred, BOp0, Neg);

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

@@ -9,10 +9,8 @@ declare void @use(i32)
 define i1 @cvt_icmp_0_zext_plus_zext_eq_i16(i16 %arg, i16 %arg1) {
 ; CHECK-LABEL: @cvt_icmp_0_zext_plus_zext_eq_i16(
 ; CHECK-NEXT:  bb:
-; CHECK-NEXT:    [[I:%.*]] = zext i16 [[ARG:%.*]] to i32
-; CHECK-NEXT:    [[I2:%.*]] = zext i16 [[ARG1:%.*]] to i32
-; CHECK-NEXT:    [[I3:%.*]] = sub nsw i32 0, [[I]]
-; CHECK-NEXT:    [[I4:%.*]] = icmp eq i32 [[I2]], [[I3]]
+; CHECK-NEXT:    [[TMP0:%.*]] = or i16 [[ARG1:%.*]], [[ARG:%.*]]
+; CHECK-NEXT:    [[I4:%.*]] = icmp eq i16 [[TMP0]], 0
 ; CHECK-NEXT:    ret i1 [[I4]]
 ;
 bb:
@@ -27,10 +25,8 @@ bb:
 define i1 @cvt_icmp_0_zext_plus_zext_eq_i8(i8 %arg, i8 %arg1) {
 ; CHECK-LABEL: @cvt_icmp_0_zext_plus_zext_eq_i8(
 ; CHECK-NEXT:  bb:
-; CHECK-NEXT:    [[I:%.*]] = zext i8 [[ARG:%.*]] to i32
-; CHECK-NEXT:    [[I2:%.*]] = zext i8 [[ARG1:%.*]] to i32
-; CHECK-NEXT:    [[I3:%.*]] = sub nsw i32 0, [[I]]
-; CHECK-NEXT:    [[I4:%.*]] = icmp eq i32 [[I2]], [[I3]]
+; CHECK-NEXT:    [[TMP0:%.*]] = or i8 [[ARG1:%.*]], [[ARG:%.*]]
+; CHECK-NEXT:    [[I4:%.*]] = icmp eq i8 [[TMP0]], 0
 ; CHECK-NEXT:    ret i1 [[I4]]
 ;
 bb:
@@ -3005,8 +3001,8 @@ define i1 @icmp_dec_notnonzero(i8 %x) {
 
 define i1 @icmp_addnuw_nonzero(i8 %x, i8 %y) {
 ; CHECK-LABEL: @icmp_addnuw_nonzero(
-; CHECK-NEXT:    [[I:%.*]] = sub i8 0, [[Y:%.*]]
-; CHECK-NEXT:    [[C:%.*]] = icmp eq i8 [[I]], [[X:%.*]]
+; CHECK-NEXT:    [[TMP1:%.*]] = or i8 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[C:%.*]] = icmp eq i8 [[TMP1]], 0
 ; CHECK-NEXT:    ret i1 [[C]]
 ;
   %i = add nuw i8 %x, %y