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

From: Taylor Blau <me@ttaylorr.com>
Date: Sep 1, 2024, 16:03 UTC
Message-ID: <e8f5cbd280cc07f68014bd4024d55a740374b349.1725206584.git.me@ttaylorr.com>
In-Reply-To: <cover.1725206584.git.me@ttaylorr.com>

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.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 csum-file.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/csum-file.c b/csum-file.c
index bf82ad8f9f5..cb8c39ecf3a 100644
--- a/csum-file.c
+++ b/csum-file.c
@@ -50,7 +50,7 @@ void hashflush(struct hashfile *f)
 
 	if (offset) {
 		if (!f->skip_hash)
-			the_hash_algo->update_fn(&f->ctx, f->buffer, offset);
+			the_hash_algo->fast_update_fn(&f->ctx, f->buffer, offset);
 		flush(f, f->buffer, offset);
 		f->offset = 0;
 	}
@@ -73,7 +73,7 @@ int finalize_hashfile(struct hashfile *f, unsigned char *result,
 	if (f->skip_hash)
 		hashclr(f->buffer, the_repository->hash_algo);
 	else
-		the_hash_algo->final_fn(f->buffer, &f->ctx);
+		the_hash_algo->fast_final_fn(f->buffer, &f->ctx);
 
 	if (result)
 		hashcpy(result, f->buffer, the_repository->hash_algo);
@@ -128,7 +128,7 @@ void hashwrite(struct hashfile *f, const void *buf, unsigned int count)
 			 * f->offset is necessarily zero.
 			 */
 			if (!f->skip_hash)
-				the_hash_algo->update_fn(&f->ctx, buf, nr);
+				the_hash_algo->fast_update_fn(&f->ctx, buf, nr);
 			flush(f, buf, nr);
 		} else {
 			/*
@@ -174,7 +174,7 @@ static struct hashfile *hashfd_internal(int fd, const char *name,
 	f->name = name;
 	f->do_crc = 0;
 	f->skip_hash = 0;
-	the_hash_algo->init_fn(&f->ctx);
+	the_hash_algo->fast_init_fn(&f->ctx);
 
 	f->buffer_len = buffer_len;
 	f->buffer = xmalloc(buffer_len);
@@ -208,7 +208,7 @@ void hashfile_checkpoint(struct hashfile *f, struct hashfile_checkpoint *checkpo
 {
 	hashflush(f);
 	checkpoint->offset = f->total;
-	the_hash_algo->clone_fn(&checkpoint->ctx, &f->ctx);
+	the_hash_algo->fast_clone_fn(&checkpoint->ctx, &f->ctx);
 }
 
 int hashfile_truncate(struct hashfile *f, struct hashfile_checkpoint *checkpoint)
@@ -219,7 +219,7 @@ int hashfile_truncate(struct hashfile *f, struct hashfile_checkpoint *checkpoint
 	    lseek(f->fd, offset, SEEK_SET) != offset)
 		return -1;
 	f->total = offset;
-	the_hash_algo->clone_fn(&f->ctx, &checkpoint->ctx);
+	the_hash_algo->fast_clone_fn(&f->ctx, &checkpoint->ctx);
 	f->offset = 0; /* hashflush() was called in checkpoint */
 	return 0;
 }
@@ -245,9 +245,9 @@ int hashfile_checksum_valid(const unsigned char *data, size_t total_len)
 	if (total_len < the_hash_algo->rawsz)
 		return 0; /* say "too short"? */
 
-	the_hash_algo->init_fn(&ctx);
-	the_hash_algo->update_fn(&ctx, data, data_len);
-	the_hash_algo->final_fn(got, &ctx);
+	the_hash_algo->fast_init_fn(&ctx);
+	the_hash_algo->fast_update_fn(&ctx, data, data_len);
+	the_hash_algo->fast_final_fn(got, &ctx);
 
 	return hasheq(got, data + data_len, the_repository->hash_algo);
 }

-- 
2.46.0.425.ge8f5cbd280c