From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on dcvr.yhbt.net X-Spam-Level: X-Spam-ASN: AS31976 209.132.180.0/23 X-Spam-Status: No, score=-5.5 required=3.0 tests=AWL,BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,RCVD_IN_DNSWL_HI,RP_MATCHES_RCVD shortcircuit=no autolearn=ham autolearn_force=no version=3.4.0 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by dcvr.yhbt.net (Postfix) with ESMTP id D92D11FF40 for ; Thu, 1 Dec 2016 04:27:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758032AbcLAE0y convert rfc822-to-8bit (ORCPT ); Wed, 30 Nov 2016 23:26:54 -0500 Received: from dmz-mailsec-scanner-2.mit.edu ([18.9.25.13]:45550 "EHLO dmz-mailsec-scanner-2.mit.edu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757998AbcLAE0x (ORCPT ); Wed, 30 Nov 2016 23:26:53 -0500 X-AuditID: 1209190d-8f7ff7000000320f-74-583fa686a785 Received: from mailhub-auth-2.mit.edu ( [18.7.62.36]) (using TLS with cipher DHE-RSA-AES256-SHA (256/256 bits)) (Client did not present a certificate) by (Symantec Messaging Gateway) with SMTP id 96.54.12815.686AF385; Wed, 30 Nov 2016 23:26:47 -0500 (EST) Received: from outgoing.mit.edu (outgoing-auth-1.mit.edu [18.9.28.11]) by mailhub-auth-2.mit.edu (8.13.8/8.9.2) with ESMTP id uB14QkCR000754; Wed, 30 Nov 2016 23:26:46 -0500 Received: from localhost (buzzword-bingo.mit.edu [18.9.64.24]) (authenticated bits=0) (User authenticated as andersk@ATHENA.MIT.EDU) by outgoing.mit.edu (8.13.8/8.12.4) with ESMTP id uB14QhxM019398 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES128-SHA bits=128 verify=NOT); Wed, 30 Nov 2016 23:26:44 -0500 Date: Wed, 30 Nov 2016 23:26:43 -0500 (EST) From: Anders Kaseorg To: Jeff King cc: Junio C Hamano , git@vger.kernel.org, Thomas Rast Subject: [PATCH] xdiff: Do not enable XDL_FAST_HASH by default Message-ID: User-Agent: Alpine 2.10 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=UTF-8 Content-Transfer-Encoding: 8BIT X-Brightmail-Tracker: H4sIAAAAAAAAA+NgFvrMIsWRmVeSWpSXmKPExsUixG6notu+zD7C4NdtdYuuK91MFg29V5gt frT0MFus3LWQyYHF41nvHkaPi5eUPeZ+WsTq8XmTXABLFJdNSmpOZllqkb5dAldGS9Md1oJ3 khX3OqcwNjBOFu1i5OSQEDCROPnjAGsXIxeHkEAbk8SvLRcYIZyNjBKbt/+Gyuxmklj+6Cw7 SAuLgLbExmYIm01ATeLD0a+sILaIgKzE98MbGUFsZoFMiXmH1rGB2MICdhLnVp9hAbF5BTwk 7l5eARYXFdCVOPTvDxtEXFDi5MwnLBC96hIHPl2EmqMtcf9mG9sERr5ZSMpmISmbhaRsASPz KkbZlNwq3dzEzJzi1GTd4uTEvLzUIl0jvdzMEr3UlNJNjKDw5JTk3cH4767XIUYBDkYlHt6E 23YRQqyJZcWVuYcYJTmYlER5y0rsI4T4kvJTKjMSizPii0pzUosPMUpwMCuJ8L5bApTjTUms rEotyodJSXOwKInz/nf7Gi4kkJ5YkpqdmlqQWgSTleHgUJLgbQBpFCxKTU+tSMvMKUFIM3Fw ggznARoetRhkeHFBYm5xZjpE/hSjLsebXS8fMAmx5OXnpUqJ8xaDDBIAKcoozYObA04rnA4S rxjFgd4S5j0FUsUDTElwk14BLWECWvL2tTXIkpJEhJRUA+PVW3EqX7/fuRNtdiz75Ce/FWIG a65k1RTO/LluZe6+7l8WVR9qnu3PYL2aaZ2kt+F4H4eOcNndUinhKt9LrkZfZdb8n7ekvvD6 ktZQ/QcLzrw/uKZnX0We3iK+4/PT9V1TLKuy1zgtjp6rXaecHVdQI/j9KduXk0febfkxSY3d i/GNvWr9hLVKLMUZiYZazEXFiQCdtWMBBgMAAA== Sender: git-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org Although XDL_FAST_HASH computes hashes slightly faster on some architectures, its collision characteristics are much worse, resulting in some pathological diffs running over 100x slower (http://public-inbox.org/git/20141222041944.GA441@peff.net/). Furthermore, it was being enabled when ‘uname -m’ returns x86_64, even if we are cross-compiling for a different architecture. This mistake was also causing the Debian build reproducibility test to fail (https://tests.reproducible-builds.org/debian/index_variations.html). Future architecture-specific definitions should be based on compiler macros such as __x86_64__ rather than uname. Signed-off-by: Anders Kaseorg --- On Wed, 30 Nov 2016, Jeff King wrote: > However, I think this might be the tip of the iceberg. There are lots of > Makefile knobs whose defaults are tweaked based on uname output. This > one caught you because you are cross-compiling across architectures, but > in theory you could cross-compile for FreeBSD from Linux, or whatever. > > So I suspect a better strategy in general is to just override the > uname_* variables when cross-compiling. The specific case of an i386 userspace on an x86_64 kernel is important independently of the general cross compilation problem (in fact, the words “cross compilation” may not even really apply here). And I don’t think one should have to manually tweak the build for this setup, especially since the compiler already has the needed information. > All that being said, I actually think an easier fix for this particular > case might be to drop XDL_FAST_HASH entirely. Works for me. Anders Makefile | 1 - config.mak.uname | 5 ----- 2 files changed, 6 deletions(-) diff --git a/Makefile b/Makefile index f53fcc90d..c237d4f91 100644 --- a/Makefile +++ b/Makefile @@ -341,7 +341,6 @@ all:: # Define XDL_FAST_HASH to use an alternative line-hashing method in # the diff algorithm. It gives a nice speedup if your processor has # fast unaligned word loads. Does NOT work on big-endian systems! -# Enabled by default on x86_64. # # Define GIT_USER_AGENT if you want to change how git identifies itself during # network interactions. The default is "git/$(GIT_VERSION)". diff --git a/config.mak.uname b/config.mak.uname index b232908f8..2831a68c3 100644 --- a/config.mak.uname +++ b/config.mak.uname @@ -1,10 +1,8 @@ # Platform specific Makefile tweaks based on uname detection uname_S := $(shell sh -c 'uname -s 2>/dev/null || echo not') -uname_M := $(shell sh -c 'uname -m 2>/dev/null || echo not') uname_O := $(shell sh -c 'uname -o 2>/dev/null || echo not') uname_R := $(shell sh -c 'uname -r 2>/dev/null || echo not') -uname_P := $(shell sh -c 'uname -p 2>/dev/null || echo not') uname_V := $(shell sh -c 'uname -v 2>/dev/null || echo not') ifdef MSVC @@ -17,9 +15,6 @@ endif # because maintaining the nesting to match is a pain. If # we had "elif" things would have been much nicer... -ifeq ($(uname_M),x86_64) - XDL_FAST_HASH = YesPlease -endif ifeq ($(uname_S),OSF1) # Need this for u_short definitions et al BASIC_CFLAGS += -D_OSF_SOURCE -- 2.11.0