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@xxxxxxxx/). 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 <andersk@xxxxxxx> --- [Oops, also resending for Thomas’s new email address. Sorry for the spam.] 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