Ingo Molnar wrote:
> * Jonathan Nieder <jrnieder@xxxxxxxxx> wrote:
>> E.g., how would something like
>>
>> const unsigned int *start1 = (const unsigned int *) sha1;
>> const unsigned int *start2 = (const unsigned int *) sha2;
>>
>> if (likely(*start1 != *start2)) {
>> if (*start1 < *start2)
>> return -1;
>> return +1;
>> }
>> return memcmp(sha1 + 4, sha2 + 4, 16);
>>
>> perform?
>
> Note that this function wont work on like 99% of the systems out there due to
> endianness assumptions in Git.
Yes, I was greedy and broke the semantics, and my suggestion was
nonsensical for other reasons (e.g., alignment), too. I should have
written something like:
if (likely(*sha1 != *sha2)) {
if (*sha1 < *sha2)
return -1;
return +1;
}
return memcmp(sha1, sha2, 20);
since speeding it up 255/256 times seems good enough already.
> Also, your hypothetical smart compiler would recognize the above as equivalent
> to memcmp(sha1, sha2, 20) and could rewrite it again - so we'd be back to
> square 1.
True. The real point is a "likely" to explain to human readers what
is happening.
> Having said that, it would be nice if someone could test these two patches on a
> modern AMD box, using the perf stat from here:
>
> http://people.redhat.com/mingo/tip.git/README
>
> cd tools/perf/
> make -j install
>
> and do something like this to test git gc's performance:
>
> $ perf stat --sync --repeat 10 ./git gc
>
> ... to see whether these speedups are generic, or somehow Intel CPU specific.
Sounds like fun. Will try to find time to play around with this in
the next few days.
> Well i messed up endianness in an early version of this patch and 'git gc' was
> eminently unhappy about it! I have not figured out which part of Git relies on
> the comparison result though - most places seem to use the result as a boolean.
I think hashcmp is used to run binary searches within a packfile
index. Thanks for explaining.
Regards,
Jonathan
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