When memmem() was imported from glibc 2.2 into compat/, an optimization
was dropped in the process, in order to make the code smaller and simpler.
It was OK because memmem() wasn't used in performance-critical code. Now
the situation has changed and we can benefit from this optimization.
The trick is to avoid calling memcmp() if the first character of the needle
already doesn't match. Checking one character directly is much cheaper
than the function call overhead. We keep the first character of the needle
in the variable named point and the rest in the one named tail.
The following commands were run in a Linux kernel repository and timed, the
best of five results is shown:
$ STRING='Ensure that the real time constraints are schedulable.'
$ git log -S"$STRING" HEAD -- kernel/sched.c >/dev/null
On Windows Vista x64, before:
real 0m8.470s
user 0m0.000s
sys 0m0.000s
And after the patch:
real 0m1.887s
user 0m0.000s
sys 0m0.000s
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
memmem() is a nice GNU extension for searching a length limited string
in another one.
This compat version is based on the version found in glibc 2.2 (GPL 2);
I only removed the optimization of checking the first char by hand, and
generally tried to keep the code simple. We can add it back if memcmp
shows up high in a profile, but for now I prefer to keep it (almost
trivially) simple.
Since I don't really know which platforms beside those with a glibc
have their own memmem(), I used a heuristic: if NO_STRCASESTR is set,
then NO_MEMMEM is set, too.
Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
René Scharfe