From: Derrick Stolee <stolee@gmail.com>
Date: Wed, 22 Aug 2018 11:14:42 GMT
Subject: Re: [ANNOUNCE] Git v2.19.0-rc0
Message-ID: <d1fafc75-d6e4-d363-d600-579c200aca43@gmail.com>
In-Reply-To: <CACBZZX7Cmp8d=UKF2nk36fL7mR+umdKwKZAKNZSkyP0NXvquhw@mail.gmail.com>


On 8/22/2018 3:39 AM, Ævar Arnfjörð Bjarmason wrote:
> On Wed, Aug 22, 2018 at 8:20 AM Jeff King <peff@peff.net> wrote:
>> On Wed, Aug 22, 2018 at 05:36:26AM +0000, brian m. carlson wrote:
>>
>>> On Tue, Aug 21, 2018 at 11:03:44PM -0400, Jeff King wrote:
>>> I don't know if something like this is an improvement or now, but this
>>> seems to at least compile:
>>>
>>> diff --git a/cache.h b/cache.h
>>> index 1398b2a4e4..3207f74771 100644
>>> --- a/cache.h
>>> +++ b/cache.h
>>> @@ -1033,7 +1033,13 @@ extern const struct object_id null_oid;
>>>
>>>   static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
>>>   {
>>> -     return memcmp(sha1, sha2, the_hash_algo->rawsz);
>>> +     switch (the_hash_algo->rawsz) {
>>> +             case 20:
>>> +             case 32:
>>> +                     return memcmp(sha1, sha2, the_hash_algo->rawsz);
>>> +             default:
>>> +                     assert(0);
>>> +     }
>> I think that would end up with the same slow code, as gcc would rather
>> call memcmp than expand out the two sets of asm.
>>
>>> I won't have time to sit down and test this out until tomorrow afternoon
>>> at the earliest.  If you want to send in something in the mean time,
>>> even if that limits things to just 20 for now, that's fine.
>> I don't have a good option. The assert() thing works until I add in the
>> "32" branch, but that's just punting the issue off until you add support
>> for the new hash.
>>
>> Hand-rolling our own asm or C is a portability headache, and we need to
>> change all of the callsites to use a new hasheq().
>>
>> Hiding it behind a per-hash function is conceptually cleanest, but not
>> quite as fast. And it also requires hasheq().
>>
>> So all of the solutions seem non-trivial.  Again, I'm starting to wonder
>> if it's worth chasing this few percent.
> Did you try __builtin_expect? It's a GCC builtin for these sorts of
> situations, and sometimes helps:
> https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
>
> I.e. you'd tell GCC we expect to have the 20 there with:
>
>      if (__builtin_expect(the_hash_algo->rawsz == 20, 1)) { ... }
>
> The perl codebase has LIKELY() and UNLIKELY() macros for this which if
> the feature isn't available fall back on just plain C code:
> https://github.com/Perl/perl5/blob/v5.27.7/perl.h#L3335-L3344
The other thing I was going to recommend (and I'll try to test this out 
myself later) is to see if 'the_hash_algo->rawsz' is being treated as a 
volatile variable, since it is being referenced through a pointer. 
Perhaps storing the value locally and then casing on it would help?