Re: [ANNOUNCE] Git v2.19.0-rc0

> On Wed, Aug 22, 2018 at 6:49 PM Derrick Stolee <stolee@gmail.com> wrote:
>> On 8/22/2018 12:26 PM, Jeff King wrote:
>>> On Wed, Aug 22, 2018 at 06:14:24PM +0200, Duy Nguyen wrote:
>>>
>>>> On Wed, Aug 22, 2018 at 6:08 PM Duy Nguyen <pclouds@gmail.com> wrote:
>>>>> On Wed, Aug 22, 2018 at 6:03 PM Jeff King <peff@peff.net> wrote:
>>>>>> On Wed, Aug 22, 2018 at 07:14:42AM -0400, Derrick Stolee wrote:
>>>>>>
>>>>>>> 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?
>>>>>> I tried various sprinkling of "const" around the declarations to make it
>>>>>> clear that the values wouldn't change once we saw them. But I couldn't
>>>>>> detect any difference. At most I think that would let us hoist the "if"
>>>>>> out of the loop, but gcc still seems unwilling to expand the memcmp when
>>>>>> there are other branches.
>>>>>>
>>>>>> I think if that's the thing we want to have happen, we really do need to
>>>>>> just write it out on that branch rather than saying "memcmp".
>>>>> This reminds me of an old discussion about memcpy() vs doing explicit
>>>>> compare loop with lots of performance measurements..
>>>> Ah found it. Not sure if it is still relevant in light of multiple hash support
>>>>
>>>> https://public-inbox.org/git/20110427225114.GA16765@elte.hu/
>>> 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:
> Might work on x86 but it breaks on cpu architectures with stricter
> alignment. I don't think we have a guarantee that object_id is always
> 8 byte aligned.