c578e29ba0
Our put_be32() routine and its variants (get_be32(), put_be64(), etc) has two implementations: on some platforms we cast memory in place and use nothl()/htonl(), which can cause unaligned memory access. And on others, we pick out the individual bytes using bitshifts. This introduces extra complexity, and sometimes causes compilers to generate warnings about type-punning. And it's not clear there's any performance advantage. This split goes back to 660231aa97 (block-sha1: support for architectures with memory alignment restrictions, 2009-08-12). The unaligned versions were part of the original block-sha1 code in d7c208a92e (Add new optimized C 'block-sha1' routines, 2009-08-05), which says it is: Based on the mozilla SHA1 routine, but doing the input data accesses a word at a time and with 'htonl()' instead of loading bytes and shifting. Back then, Linus provided timings versus the mozilla code which showed a 27% improvement: https://lore.kernel.org/git/alpine.LFD.2.01.0908051545000.3390@localhost.localdomain/ However, the unaligned loads were either not the useful part of that speedup, or perhaps compilers and processors have changed since then. Here are times for computing the sha1 of 4GB of random data, with and without -DNO_UNALIGNED_LOADS (and BLK_SHA1=1, of course). This is with gcc 10, -O2, and the processor is a Core i9-9880H. [stock] Benchmark #1: t/helper/test-tool sha1 <foo.rand Time (mean ± σ): 6.638 s ± 0.081 s [User: 6.269 s, System: 0.368 s] Range (min … max): 6.550 s … 6.841 s 10 runs [-DNO_UNALIGNED_LOADS] Benchmark #1: t/helper/test-tool sha1 <foo.rand Time (mean ± σ): 6.418 s ± 0.015 s [User: 6.058 s, System: 0.360 s] Range (min … max): 6.394 s … 6.447 s 10 runs And here's the same test run on an AMD A8-7600, using gcc 8. [stock] Benchmark #1: t/helper/test-tool sha1 <foo.rand Time (mean ± σ): 11.721 s ± 0.113 s [User: 10.761 s, System: 0.951 s] Range (min … max): 11.509 s … 11.861 s 10 runs [-DNO_UNALIGNED_LOADS] Benchmark #1: t/helper/test-tool sha1 <foo.rand Time (mean ± σ): 11.744 s ± 0.066 s [User: 10.807 s, System: 0.928 s] Range (min … max): 11.637 s … 11.863 s 10 runs So the unaligned loads don't seem to help much, and actually make things worse. It's possible there are platforms where they provide more benefit, but: - the non-x86 platforms for which we use this code are old and obscure (powerpc and s390). - the main caller that cares about performance is block-sha1. But these days it is rarely used anyway, in favor of sha1dc (which is already much slower, and nobody seems to have cared that much). Let's just drop unaligned versions entirely in the name of simplicity. Signed-off-by: Jeff King <peff@peff.net> 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-<commandname>.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://lore.kernel.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