From: Derrick Stolee <stolee@gmail.com>
Date: Wed, 22 Aug 2018 16:49:34 GMT
Subject: Re: [ANNOUNCE] Git v2.19.0-rc0
Message-ID: <7ea416cf-b043-1274-e161-85a8780b8e1c@gmail.com>
In-Reply-To: <20180822162609.GA11904@sigill.intra.peff.net>

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:

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)