From: Derrick Stolee <stolee@gmail.com>
To: "brian m. carlson" <sandals@crustytoothpaste.net>,
Jeff King <peff@peff.net>, Junio C Hamano <gitster@pobox.com>,
git@vger.kernel.org
Subject: Re: [ANNOUNCE] Git v2.19.0-rc0
Date: Wed, 22 Aug 2018 10:28:56 -0400 [thread overview]
Message-ID: <459f138f-78ba-6465-0f2b-b2419bbc36ff@gmail.com> (raw)
In-Reply-To: <20180822053626.GB535143@genre.crustytoothpaste.net>
On 8/22/2018 1:36 AM, brian m. carlson wrote:
> On Tue, Aug 21, 2018 at 11:03:44PM -0400, Jeff King wrote:
>> So I wonder if there's some other way to tell the compiler that we'll
>> only have a few values. An enum comes to mind, though I don't think the
>> enum rules are strict enough to make this guarantee (after all, it's OK
>> to bitwise-OR enums, so they clearly don't specify all possible values).
> I was thinking about this:
>
> diff --git a/cache.h b/cache.h
> index 1398b2a4e4..1f5c6e9319 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -1033,7 +1033,14 @@ extern const struct object_id null_oid;
>
> static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
> {
> - return memcmp(sha1, sha2, the_hash_algo->rawsz);
> + switch (the_hash_algo->rawsz) {
> + case 20:
> + return memcmp(sha1, sha2, 20);
> + case 32:
> + return memcmp(sha1, sha2, 32);
> + default:
> + assert(0);
> + }
> }
>
> static inline int oidcmp(const struct object_id *oid1, const struct object_id *oid2)
>
> That would make it obvious that there are at most two options.
> Unfortunately, gcc for me determines that the buffer in walker.c is 20
> bytes in size and steadfastly refuses to compile because it doesn't know
> that the value will never be 32 in our codebase currently. I'd need to
> send in more patches before it would compile.
>
> I don't know if something like this is an improvement or now, but this
> seems to at least compile:
>
> diff --git a/cache.h b/cache.h
> index 1398b2a4e4..3207f74771 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -1033,7 +1033,13 @@ extern const struct object_id null_oid;
>
> static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
> {
> - return memcmp(sha1, sha2, the_hash_algo->rawsz);
> + switch (the_hash_algo->rawsz) {
> + case 20:
> + case 32:
> + return memcmp(sha1, sha2, the_hash_algo->rawsz);
> + default:
> + assert(0);
> + }
> }
In my testing, I've had the best luck with this change:
diff --git a/cache.h b/cache.h
index b1fd3d58ab..6c8b51c390 100644
--- a/cache.h
+++ b/cache.h
@@ -1023,7 +1023,14 @@ extern const struct object_id null_oid;
static inline int hashcmp(const unsigned char *sha1, const unsigned
char *sha2)
{
- return memcmp(sha1, sha2, the_hash_algo->rawsz);
+ switch (the_hash_algo->rawsz) {
+ case 20:
+ return memcmp(sha1, sha2, 20);
+ case 32:
+ return memcmp(sha1, sha2, 32);
+ default:
+ assert(0);
+ }
}
The fact that '20' and '32' are constants here may be helpful to the
compiler. Can someone else test the perf?
Thanks,
-Stolee
next prev parent reply other threads:[~2018-08-22 14:29 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
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 [this message]
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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