diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp --- a/llvm/lib/Analysis/ValueTracking.cpp +++ b/llvm/lib/Analysis/ValueTracking.cpp @@ -2884,7 +2884,13 @@ return XKnown.isNonZero() || isKnownNonZero(I->getOperand(0), DemandedElts, Depth, Q); - return KnownBits::mul(XKnown, YKnown).isNonZero(); + // If there exists any subset of X (sX) and subset of Y (sY) s.t sX * sY is + // non-zero, then X * Y is non-zero. We can find sX and sY by just taking + // the the lowest known One of X and Y. If they are non-zero, the result + // must be non-zero. We can check if LSB(X) * LSB(Y) != 0 by doing + // X.CountLeadingZeros + Y.CountLeadingZeros < BitWidth. + return (XKnown.countMaxTrailingZeros() + YKnown.countMaxTrailingZeros()) < + BitWidth; } case Instruction::Select: // (C ? X : Y) != 0 if X != 0 and Y != 0. diff --git a/llvm/test/Analysis/ValueTracking/known-non-zero.ll b/llvm/test/Analysis/ValueTracking/known-non-zero.ll --- a/llvm/test/Analysis/ValueTracking/known-non-zero.ll +++ b/llvm/test/Analysis/ValueTracking/known-non-zero.ll @@ -1099,11 +1099,7 @@ define i1 @mul_nonzero_contains_nonzero_mul(i8 %x, i8 %y) { ; CHECK-LABEL: @mul_nonzero_contains_nonzero_mul( -; CHECK-NEXT: [[XX:%.*]] = or i8 [[X:%.*]], 16 -; CHECK-NEXT: [[YY:%.*]] = or i8 [[Y:%.*]], 8 -; CHECK-NEXT: [[XY:%.*]] = mul i8 [[XX]], [[YY]] -; CHECK-NEXT: [[NZ:%.*]] = icmp ne i8 [[XY]], 0 -; CHECK-NEXT: ret i1 [[NZ]] +; CHECK-NEXT: ret i1 true ; %xx = or i8 %x, 16 %yy = or i8 %y, 8