My today’s WTF on the topic. The following C++ returns incorrect value when the argument is 8 or greater:

    inline uint64_t makeRemainderMask( ptrdiff_t missingLanes )
    {
        // This is not a branch, compiles to conditional move
        missingLanes = std::max( missingLanes, (ptrdiff_t)0 );
        // Make a mask of 8 bytes
        // No need to clip for missingLanes <= 8 because the shift is already good, results in zero
        uint64_t mask = ~(uint64_t)0;
        mask >>= missingLanes * 8;
        return mask;
    }

TIL the language standard defines right shift operator for unsigned types in a weird way, making so for uint64_t argument, a >> b is equal to a >> ( b % 64 ). I expected zero on the output for b >= 64, not that.