Re: [PATCH 4/4] csum-file.c: use fast SHA-1 implementation when available

> Update hashwrite() and friends to use the fast_-variants of hashing
> functions, calling for e.g., "the_hash_algo->fast_update_fn()" instead
> of "the_hash_algo->update_fn()".
> 
> These callers only use the_hash_algo to produce a checksum, which we
> depend on for data integrity, but not for cryptographic purposes, so
> these callers are safe to use the fast (and potentially non-collision
> detecting) SHA-1 implementation.
> 
> To time this, I took a freshly packed copy of linux.git, and ran the
> following with and without the OPENSSL_SHA1_FAST=1 build-knob. Both
> versions were compiled with -O3:
> 
>     $ git for-each-ref --format='%(objectname)' refs/heads refs/tags >in
>     $ valgrind --tool=callgrind ~/src/git/git-pack-objects \
>         --revs --stdout --all-progress --use-bitmap-index <in >/dev/null
> 
> Without OPENSSL_SHA1_FAST=1 (that is, using the collision-detecting
> SHA-1 implementation for both cryptographic and non-cryptographic
> purposes), we spend a significant amount of our instruction count in
> hashwrite():
> 
>     $ callgrind_annotate --inclusive=yes | grep hashwrite | head -n1
>     159,998,868,413 (79.42%)  /home/ttaylorr/src/git/csum-file.c:hashwrite [/home/ttaylorr/src/git/git-pack-objects]
> 
> , and the resulting "clone" takes 19.219 seconds of wall clock time,
> 18.94 seconds of user time and 0.28 seconds of system time.
> 
> Compiling with OPENSSL_SHA1_FAST=1, we spend ~60% fewer instructions in
> hashwrite():
> 
>     $ callgrind_annotate --inclusive=yes | grep hashwrite | head -n1
>      59,164,001,176 (58.79%)  /home/ttaylorr/src/git/csum-file.c:hashwrite [/home/ttaylorr/src/git/git-pack-objects]
> 
> , and generate the resulting "clone" much faster, in only 11.597 seconds
> of wall time, 11.37 seconds of user time, and 0.23 seconds of system
> time, for a ~40% speed-up.