Re: Make GIT_USE_LOOKUP default?

On Tue, Mar 19, 2013 at 10:43:40PM +0700, Nguyen Thai Ngoc Duy wrote:

quoted

I could not replicate his benchmarks at all. In fact, my measurements
showed a slight slowdown with 1a812f3 (hashcmp(): inline memcmp() by
hand to optimize, 2011-04-28).

[...]

What gcc and glibc versions are you using? With gcc 4.5.3 I got "repz
cmpsb" after reverting the patch, just like what Ingo described
(although interestingly it ran a bit faster than current master, glibc
2.11.2 on Atom D510 32 bit). gcc 4.6.3 -O2 (on another machine, 64
bit) produced a call to libc's memcmp instead of "repz cmpsb". I guess
if "repz cmpsb" is what we are against, then we could pass
-fno-builtin-memcmp (potential impact to other parts of git though).

I have glibc 2.13. And looking at the changelog, there were a ton of
ssse-optimized memcmp implementations that went into 2.13 [1], so I suspect
that is what is going on.

-Peff

PS As a side note, you may recall a year or two ago when these went in,
because the initial versions walked backwards, meaning memcpys of
overlapping regions started to fail (as they are allowed to, but many
programs do not properly use memmove).