[ValueTracking] Add logic for `add nuw x,y != 0` -> `x != 0 || y != 0`

[goldstein.w.n]

Summary

Alive2 Link:

https://alive2.llvm.org/ce/z/TKpqxc

[nikic]

llvm/lib/Analysis/ValueTracking.cpp

2780–2782

[goldstein.w.n]

Return NUW case unconditionally

[goldstein.w.n]

fixed

[nikic]

LGTM

[goldstein.w.n]

Rebase

[goldstein.w.n]

Rebase

llvm/lib/Analysis/ValueTracking.cpp

#endif
   case Instruction::UDiv:
   case Instruction::SDiv:
     // div exact can only produce a zero if the dividend is zero.
     if (cast<PossiblyExactOperator>(I)->isExact())
       return isKnownNonZero(I->getOperand(0), DemandedElts, Depth, Q);
     break;
   case Instruction::Add: {
     // X + Y.
+
+    // If Add has nuw wrap flag, then if either X or Y is non-zero the result is
+    // non-zero.
+    auto *BO = cast<OverflowingBinaryOperator>(V);
+    if (Q.IIQ.hasNoUnsignedWrap(BO))
+      return isKnownNonZero(I->getOperand(0), DemandedElts, Depth, Q) ||
+             isKnownNonZero(I->getOperand(1), DemandedElts, Depth, Q);
+
     KnownBits XKnown =
         computeKnownBits(I->getOperand(0), DemandedElts, Depth, Q);
     KnownBits YKnown =
         computeKnownBits(I->getOperand(1), DemandedElts, Depth, Q);
 
     // If X and Y are both non-negative (as signed values) then their sum is not
     // zero unless both X and Y are zero.
     if (XKnown.isNonNegative() && YKnown.isNonNegative())

llvm/test/Analysis/ValueTracking/known-non-zero.ll

;
   %r = icmp eq i8 %exp, 0
   ret i1 %r
 }
 
 define i1 @add_nonzero_nuw(i8 %x, i8 %y) {
 ; CHECK-LABEL: @add_nonzero_nuw(
 ; CHECK-NEXT:    [[X_NZ:%.*]] = icmp ne i8 [[X:%.*]], 0
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[X_NZ]])
-; CHECK-NEXT:    [[A:%.*]] = add nuw i8 [[X]], [[Y:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp eq i8 [[A]], 0
-; CHECK-NEXT:    ret i1 [[R]]
+; CHECK-NEXT:    ret i1 false
 ;
   %x_nz = icmp ne i8 %x, 0
   call void @llvm.assume(i1 %x_nz)
   %a = add nuw i8 %x, %y
   %r = icmp eq i8 %a, 0
   ret i1 %r
 }