Subject: Re: [PATCH v2] fsm-listen-darwin: combine bit operations

From: Junio C Hamano <[email-masked]>
Date: Fri, 20 Jan 2023 09:52:38 -0800

Jeff Hostetler <[email-masked]> writes:

>>     static int ef_is_dropped(const FSEventStreamEventFlags ef)
>>   {
>> -	return (ef & kFSEventStreamEventFlagMustScanSubDirs ||
>> -		ef & kFSEventStreamEventFlagKernelDropped ||
>> -		ef & kFSEventStreamEventFlagUserDropped);
>> +	return (ef & (kFSEventStreamEventFlagMustScanSubDirs |
>> +		      kFSEventStreamEventFlagKernelDropped |
>> +		      kFSEventStreamEventFlagUserDropped));
>>   }
>
> Technically, the returned value is slightly different, but
> the only caller is just checking for non-zero, so it doesn't
> matter.
>
> So this is fine.

But is it worth the code churn and reviewer bandwidth?  Don't we
have better things to spend our time on?

I would not be surprised if a smart enough compiler used the same
transformartion as this patch does manually as an optimization.

Then it matters more which one of the two is more readable by our
developers.  And the original matches how we humans would think, I
would imagine.  ef might have MustScanSubdirs bit, KernelDropped
bit, or UserDropped bit and in these cases we want to say that ef is
dropped.  Arguably, the original is more readble, and it would be a
good change to adopt if there is an upside, like the updated code
resulting in markedly more efficient binary.

So, this might be technically fine, but I am not enthused to see
these kind of code churning patches with dubious upside.  An
optimization patch should be able to demonstrate its benefit with a
solid benchmark, or at least a clear difference in generated code.

In fact.

Compiler explorer godbolt.org tells me that gcc 12 with -O2 compiles
the following two functions into identical assembly.  The !! prefix
used in the second example is different from the postimage of what
Seija posted, but this being a file-scope static function, I would
expect the compiler to notice that the actual value would not matter
to the callers, only the truth value, does.

* Input *
int one(unsigned int num) {
    return ((num & 01) ||
            (num & 02) || (num & 04));
}

int two(unsigned int num) {
    return !!((num) & (01|02|04));
}

* Assembly *
one(unsigned int):
        xor     eax, eax
        and     edi, 7
        setne   al
        ret
two(unsigned int):
        xor     eax, eax
        and     edi, 7
        setne   al
        ret