From: Derrick Stolee <stolee@gmail.com>
To: Jeff King <peff@peff.net>, Duy Nguyen <pclouds@gmail.com>
Cc: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
"brian m. carlson" <sandals@crustytoothpaste.net>,
"Junio C Hamano" <gitster@pobox.com>,
"Git Mailing List" <git@vger.kernel.org>
Subject: Re: [ANNOUNCE] Git v2.19.0-rc0
Date: Wed, 22 Aug 2018 12:49:34 -0400 [thread overview]
Message-ID: <7ea416cf-b043-1274-e161-85a8780b8e1c@gmail.com> (raw)
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)
next prev parent reply other threads:[~2018-08-22 16:49 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-20 22:13 [ANNOUNCE] Git v2.19.0-rc0 Junio C Hamano
2018-08-20 22:41 ` Stefan Beller
2018-08-20 23:39 ` Jonathan Nieder
2018-08-21 0:27 ` Jonathan Nieder
2018-08-21 0:46 ` Stefan Beller
2018-08-21 20:41 ` Derrick Stolee
2018-08-21 21:29 ` Jeff King
2018-08-22 0:48 ` brian m. carlson
2018-08-22 3:03 ` Jeff King
2018-08-22 3:36 ` Jeff King
2018-08-22 11:11 ` Derrick Stolee
2018-08-22 5:36 ` brian m. carlson
2018-08-22 6:07 ` Jeff King
2018-08-22 7:39 ` Ævar Arnfjörð Bjarmason
2018-08-22 11:14 ` Derrick Stolee
2018-08-22 15:17 ` Jeff King
2018-08-22 16:08 ` Duy Nguyen
2018-08-22 16:14 ` Duy Nguyen
2018-08-22 16:26 ` Jeff King
2018-08-22 16:49 ` Derrick Stolee [this message]
2018-08-22 16:58 ` Duy Nguyen
2018-08-22 17:04 ` Derrick Stolee
2018-08-22 16:59 ` Jeff King
2018-08-22 17:02 ` Junio C Hamano
2018-08-22 15:14 ` Jeff King
2018-08-22 14:28 ` Derrick Stolee
2018-08-22 15:24 ` Jeff King
2018-08-22 12:42 ` Paul Smith
2018-08-22 15:23 ` Jeff King
2018-08-23 1:23 ` Jonathan Nieder
2018-08-23 2:16 ` Jeff King
2018-08-23 2:27 ` Jonathan Nieder
2018-08-23 5:02 ` Jeff King
2018-08-23 5:09 ` brian m. carlson
2018-08-23 5:10 ` Jonathan Nieder
2018-08-23 13:20 ` Junio C Hamano
2018-08-23 16:31 ` wide t/perf output, was " Jeff King
2018-08-23 3:47 ` brian m. carlson
2018-08-23 5:04 ` Jeff King
2018-08-23 10:26 ` Derrick Stolee
2018-08-23 13:16 ` Junio C Hamano
2018-08-23 16:14 ` Jeff King
2018-08-23 23:30 ` Jacob Keller
2018-08-23 23:40 ` Jeff King
2018-08-24 0:06 ` Jeff King
2018-08-24 0:16 ` Jeff King
2018-08-24 2:48 ` Jacob Keller
2018-08-24 2:59 ` Jeff King
2018-08-24 6:45 ` Jeff King
2018-08-24 11:04 ` Derrick Stolee
2018-08-27 19:36 ` Junio C Hamano
2018-08-23 18:53 ` Jeff King
2018-08-23 20:59 ` Derrick Stolee
2018-08-24 6:56 ` Jeff King
2018-08-24 7:57 ` Ævar Arnfjörð Bjarmason
2018-08-24 16:45 ` Derrick Stolee
2018-08-25 8:26 ` Jeff King
2018-09-02 18:53 ` Kaartic Sivaraam
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