Re: [ANNOUNCE] Git v2.19.0-rc0

[Duy Nguyen <pclouds@gmail.com>]

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.

>
> diff --git a/cache.h b/cache.h
> index b1fd3d58ab..5e5819ad49 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -1021,9 +1021,41 @@ extern int find_unique_abbrev_r(char *hex, const
> struct object_id *oid, int len)
>   extern const unsigned char null_sha1[GIT_MAX_RAWSZ];
>   extern const struct object_id null_oid;
>
> +static inline int word_cmp_32(uint32_t a, uint32_t b)
> +{
> +       return memcmp(&a, &b, sizeof(uint32_t));
> +}
> +
> +static inline int word_cmp_64(uint64_t a, uint64_t b)
> +{
> +       return memcmp(&a, &b, sizeof(uint64_t));
> +}
> +
> +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;
> +
> +               if (obj1->data0 == obj2->data0) {
> +                       if (obj1->data1 == obj2->data1) {
> +                               if (obj1->data2 == obj2->data2) {
> +                                       return 0;
> +                               }
> +                               return word_cmp_32(obj1->data2,
> obj2->data2);
> +                       }
> +                       return word_cmp_64(obj1->data1, obj2->data1);
> +               }
> +               return word_cmp_64(obj1->data0, obj2->data0);
> +       }
> +
> +       assert(0);
>   }
>
>   static inline int oidcmp(const struct object_id *oid1, const struct
> object_id *oid2)
>
>


-- 
Duy