From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on dcvr.yhbt.net X-Spam-Level: X-Spam-ASN: AS31976 209.132.180.0/23 X-Spam-Status: No, score=-4.0 required=3.0 tests=AWL,BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,RCVD_IN_DNSWL_HI shortcircuit=no autolearn=ham autolearn_force=no version=3.4.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by dcvr.yhbt.net (Postfix) with ESMTP id 814951F954 for ; Wed, 22 Aug 2018 15:14:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728736AbeHVSj0 (ORCPT ); Wed, 22 Aug 2018 14:39:26 -0400 Received: from cloud.peff.net ([104.130.231.41]:51622 "HELO cloud.peff.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1728568AbeHVSj0 (ORCPT ); Wed, 22 Aug 2018 14:39:26 -0400 Received: (qmail 18293 invoked by uid 109); 22 Aug 2018 15:14:09 -0000 Received: from Unknown (HELO peff.net) (10.0.1.2) by cloud.peff.net (qpsmtpd/0.94) with SMTP; Wed, 22 Aug 2018 15:14:09 +0000 Authentication-Results: cloud.peff.net; auth=none Received: (qmail 374 invoked by uid 111); 22 Aug 2018 15:14:15 -0000 Received: from sigill.intra.peff.net (HELO sigill.intra.peff.net) (10.0.0.7) by peff.net (qpsmtpd/0.94) with (ECDHE-RSA-AES256-GCM-SHA384 encrypted) SMTP; Wed, 22 Aug 2018 11:14:15 -0400 Authentication-Results: peff.net; auth=none Received: by sigill.intra.peff.net (sSMTP sendmail emulation); Wed, 22 Aug 2018 11:14:07 -0400 Date: Wed, 22 Aug 2018 11:14:07 -0400 From: Jeff King To: =?utf-8?B?w4Z2YXIgQXJuZmrDtnLDsA==?= Bjarmason Cc: "brian m. carlson" , Derrick Stolee , Junio C Hamano , Git Mailing List Subject: Re: [ANNOUNCE] Git v2.19.0-rc0 Message-ID: <20180822151407.GA32630@sigill.intra.peff.net> References: <1b20b754-987c-a712-2594-235b845bc5d0@gmail.com> <20180821212923.GB24431@sigill.intra.peff.net> <20180822004815.GA535143@genre.crustytoothpaste.net> <20180822030344.GA14684@sigill.intra.peff.net> <20180822053626.GB535143@genre.crustytoothpaste.net> <20180822060735.GA13195@sigill.intra.peff.net> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org On Wed, Aug 22, 2018 at 09:39:57AM +0200, Ævar Arnfjörð Bjarmason wrote: > > I don't have a good option. The assert() thing works until I add in the > > "32" branch, but that's just punting the issue off until you add support > > for the new hash. > > > > Hand-rolling our own asm or C is a portability headache, and we need to > > change all of the callsites to use a new hasheq(). > > > > Hiding it behind a per-hash function is conceptually cleanest, but not > > quite as fast. And it also requires hasheq(). > > > > So all of the solutions seem non-trivial. Again, I'm starting to wonder > > if it's worth chasing this few percent. > > Did you try __builtin_expect? It's a GCC builtin for these sorts of > situations, and sometimes helps: > https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html > > I.e. you'd tell GCC we expect to have the 20 there with: > > if (__builtin_expect(the_hash_algo->rawsz == 20, 1)) { ... } > > The perl codebase has LIKELY() and UNLIKELY() macros for this which if > the feature isn't available fall back on just plain C code: > https://github.com/Perl/perl5/blob/v5.27.7/perl.h#L3335-L3344 Sadly, no, this doesn't seem to change anything. We still end up with a single call to memcmp. I also tried "hiding" the fallback call like this: diff --git a/cache.h b/cache.h index b1fd3d58ab..7808bf3d6b 100644 --- a/cache.h +++ b/cache.h @@ -1021,9 +1021,13 @@ 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; +int super_secret_memcmp(const void *a, const void *b, size_t len); + 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) + return memcmp(sha1, sha2, 20); + return super_secret_memcmp(sha1, sha2, the_hash_algo->rawsz); } static inline int oidcmp(const struct object_id *oid1, const struct object_id *oid2) diff --git a/sha1-file.c b/sha1-file.c index 97b7423848..5cd0a4b73f 100644 --- a/sha1-file.c +++ b/sha1-file.c @@ -2280,3 +2280,8 @@ int read_loose_object(const char *path, munmap(map, mapsize); return ret; } + +int super_secret_memcmp(const void *a, const void *b, size_t len) +{ + return memcmp(a, b, len); +} but that just results in calling memcmp and super_secret_memcmp on the two codepaths (with or without the __builtin_expect). -Peff