From: Junio C Hamano <gitster@pobox.com>
Date: Wed, 22 Aug 2018 17:02:59 GMT
Subject: Re: [ANNOUNCE] Git v2.19.0-rc0
Message-ID: <xmqqtvnmmj0c.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <20180822165923.GA14878@sigill.intra.peff.net>

Jeff King <peff@peff.net> writes:

> On Wed, Aug 22, 2018 at 12:49:34PM -0400, Derrick Stolee wrote:
>
>> > Yes, that was what I meant. We actually did switch to that hand-rolled
>> > loop, but later we went back to memcmp in 0b006014c8 (hashcmp: use
>> > memcmp instead of open-coded loop, 2017-08-09).
>> 
>> Looking at that commit, I'm surprised the old logic was just a for
>> loop, instead of a word-based approach, such as the following:
>> [...]
>> +struct object_id_20 {
>> +       uint64_t data0;
>> +       uint64_t data1;
>> +       uint32_t data2;
>> +};
>> +
>>  static inline int hashcmp(const unsigned char *sha1, const unsigned char
>> *sha2)
>>  {
>> -       return memcmp(sha1, sha2, the_hash_algo->rawsz);
>> +       if (the_hash_algo->rawsz == 20) {
>> +               struct object_id_20 *obj1 = (struct object_id_20 *)sha1;
>> +               struct object_id_20 *obj2 = (struct object_id_20 *)sha2;
>
> I wonder if you're potentially running afoul of alignment requirements
> here.

Yup, and I think that all was discussed in that old thread ;-)