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Prior to 509f6f62a4 (cache: update object ID functions for the_hash_algo, 2018-07-16), hashcmp() called memcmp() with a constant size of 20 bytes. Some compilers were able to turn that into a few quad-word comparisons, which is faster than actually calling memcmp(). In 509f6f62a4, we started using the_hash_algo->rawsz instead. Even though this will always be 20, the compiler doesn't know that while inlining hashcmp() and ends up just generating a call to memcmp(). Eventually we'll have to deal with multiple hash sizes, but for the upcoming v2.19, we can restore some of the original performance by asserting on the size. That gives the compiler enough information to know that the memcmp will always be called with a length of 20, and it performs the same optimization. Here are numbers for p0001.2 run against linux.git on a few versions. This is using -O2 with gcc 8.2.0. Test v2.18.0 v2.19.0-rc0 HEAD ------------------------------------------------------------------------------ 0001.2: 34.24(33.81+0.43) 34.83(34.42+0.40) +1.7% 33.90(33.47+0.42) -1.0% You can see that v2.19 is a little slower than v2.18. This commit ended up slightly faster than v2.18, but there's a fair bit of run-to-run noise (the generated code in the two cases is basically the same). This patch does seem to be consistently 1-2% faster than v2.19. I tried changing hashcpy(), which was also touched by 509f6f62a4, in the same way, but couldn't measure any speedup. Which makes sense, at least for this workload. A traversal of the whole commit graph requires looking up every entry of every tree via lookup_object(). That's many multiples of the numbers of objects in the repository (most of the lookups just return "yes, we already saw that object"). [jn: verified using "make object.s" that the memcmp call goes away.] Reported-by: Derrick Stolee <stolee@gmail.com> Signed-off-by: Jeff King <peff@peff.net Reviewed-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Git - fast, scalable, distributed revision control system
Git is a fast, scalable, distributed revision control system with an unusually rich command set that provides both high-level operations and full access to internals.
Git is an Open Source project covered by the GNU General Public License version 2 (some parts of it are under different licenses, compatible with the GPLv2). It was originally written by Linus Torvalds with help of a group of hackers around the net.
Please read the file INSTALL for installation instructions.
Many Git online resources are accessible from https://git-scm.com/ including full documentation and Git related tools.
See Documentation/gittutorial.txt to get started, then see
Documentation/giteveryday.txt for a useful minimum set of commands, and
Documentation/git-.txt for documentation of each command.
If git has been correctly installed, then the tutorial can also be
read with man gittutorial or git help tutorial, and the
documentation of each command with man git-<commandname> or git help <commandname>.
CVS users may also want to read Documentation/gitcvs-migration.txt
(man gitcvs-migration or git help cvs-migration if git is
installed).
The user discussion and development of Git take place on the Git mailing list -- everyone is welcome to post bug reports, feature requests, comments and patches to git@vger.kernel.org (read Documentation/SubmittingPatches for instructions on patch submission). To subscribe to the list, send an email with just "subscribe git" in the body to majordomo@vger.kernel.org. The mailing list archives are available at https://public-inbox.org/git/, http://marc.info/?l=git and other archival sites.
Issues which are security relevant should be disclosed privately to the Git Security mailing list git-security@googlegroups.com.
The maintainer frequently sends the "What's cooking" reports that list the current status of various development topics to the mailing list. The discussion following them give a good reference for project status, development direction and remaining tasks.
The name "git" was given by Linus Torvalds when he wrote the very first version. He described the tool as "the stupid content tracker" and the name as (depending on your mood):
- random three-letter combination that is pronounceable, and not actually used by any common UNIX command. The fact that it is a mispronunciation of "get" may or may not be relevant.
- stupid. contemptible and despicable. simple. Take your pick from the dictionary of slang.
- "global information tracker": you're in a good mood, and it actually works for you. Angels sing, and a light suddenly fills the room.
- "goddamn idiotic truckload of sh*t": when it breaks