Re: [ANNOUNCE] Git v2.19.0-rc0
On 8/22/2018 12:26 PM, Jeff King wrote:
Show 26 quoted lines
> 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)
Derrick Stolee <stolee@gmail.com>