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From: "brian m. carlson" <sandals@crustytoothpaste.net>
To: git@vger.kernel.org
Cc: "Derrick Stolee" <stolee@gmail.com>,
	"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
	"Duy Nguyen" <pclouds@gmail.com>,
	"SZEDER Gábor" <szeder.dev@gmail.com>
Subject: [PATCH v4 04/12] cache: make hashcmp and hasheq work with larger hashes
Date: Thu, 25 Oct 2018 02:39:57 +0000
Message-ID: <20181025024005.154208-5-sandals@crustytoothpaste.net> (raw)
In-Reply-To: <20181025024005.154208-1-sandals@crustytoothpaste.net>

In 183a638b7d ("hashcmp: assert constant hash size", 2018-08-23), we
modified hashcmp to assert that the hash size was always 20 to help it
optimize and inline calls to memcmp.  In a future series, we replaced
many calls to hashcmp and oidcmp with calls to hasheq and oideq to
improve inlining further.

However, we want to support hash algorithms other than SHA-1, namely
SHA-256.  When doing so, we must handle the case where these values are
32 bytes long as well as 20.  Adjust hashcmp to handle two cases:
20-byte matches, and maximum-size matches.  Therefore, when we include
SHA-256, we'll automatically handle it properly, while at the same time
teaching the compiler that there are only two possible options to
consider.  This will allow the compiler to write the most efficient
possible code.

Copy similar code into hasheq and perform an identical transformation.
At least with GCC 8.2.0, making hasheq defer to hashcmp when there are
two branches prevents the compiler from inlining the comparison, while
the code in this patch is inlined properly.  Add a comment to avoid an
accidental performance regression from well-intentioned refactoring.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
---
 cache.h | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/cache.h b/cache.h
index 51580c4b77..bab8e8964f 100644
--- a/cache.h
+++ b/cache.h
@@ -1027,16 +1027,12 @@ extern const struct object_id null_oid;
 static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
 {
 	/*
-	 * This is a temporary optimization hack. By asserting the size here,
-	 * we let the compiler know that it's always going to be 20, which lets
-	 * it turn this fixed-size memcmp into a few inline instructions.
-	 *
-	 * This will need to be extended or ripped out when we learn about
-	 * hashes of different sizes.
+	 * Teach the compiler that there are only two possibilities of hash size
+	 * here, so that it can optimize for this case as much as possible.
 	 */
-	if (the_hash_algo->rawsz != 20)
-		BUG("hash size not yet supported by hashcmp");
-	return memcmp(sha1, sha2, the_hash_algo->rawsz);
+	if (the_hash_algo->rawsz == GIT_MAX_RAWSZ)
+		return memcmp(sha1, sha2, GIT_MAX_RAWSZ);
+	return memcmp(sha1, sha2, GIT_SHA1_RAWSZ);
 }
 
 static inline int oidcmp(const struct object_id *oid1, const struct object_id *oid2)
@@ -1046,7 +1042,13 @@ static inline int oidcmp(const struct object_id *oid1, const struct object_id *o
 
 static inline int hasheq(const unsigned char *sha1, const unsigned char *sha2)
 {
-	return !hashcmp(sha1, sha2);
+	/*
+	 * We write this here instead of deferring to hashcmp so that the
+	 * compiler can properly inline it and avoid calling memcmp.
+	 */
+	if (the_hash_algo->rawsz == GIT_MAX_RAWSZ)
+		return !memcmp(sha1, sha2, GIT_MAX_RAWSZ);
+	return !memcmp(sha1, sha2, GIT_SHA1_RAWSZ);
 }
 
 static inline int oideq(const struct object_id *oid1, const struct object_id *oid2)

  parent reply index

Thread overview: 58+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-25  2:39 [PATCH v4 00/12] Base SHA-256 implementation brian m. carlson
2018-10-25  2:39 ` [PATCH v4 01/12] sha1-file: rename algorithm to "sha1" brian m. carlson
2018-10-25  2:39 ` [PATCH v4 02/12] sha1-file: provide functions to look up hash algorithms brian m. carlson
2018-10-25  2:39 ` [PATCH v4 03/12] hex: introduce functions to print arbitrary hashes brian m. carlson
2018-10-25  2:39 ` brian m. carlson [this message]
2018-10-25  2:39 ` [PATCH v4 05/12] t: add basic tests for our SHA-1 implementation brian m. carlson
2018-10-25  2:39 ` [PATCH v4 06/12] t: make the sha1 test-tool helper generic brian m. carlson
2018-10-25  2:40 ` [PATCH v4 07/12] sha1-file: add a constant for hash block size brian m. carlson
2018-10-25  2:40 ` [PATCH v4 08/12] t/helper: add a test helper to compute hash speed brian m. carlson
2018-10-25  2:40 ` [PATCH v4 09/12] commit-graph: convert to using the_hash_algo brian m. carlson
2018-10-25  2:40 ` [PATCH v4 10/12] Add a base implementation of SHA-256 support brian m. carlson
2018-10-25  3:02   ` Carlo Arenas
2018-10-28 15:52     ` brian m. carlson
2018-10-29  0:39       ` Junio C Hamano
2018-10-31 22:55         ` brian m. carlson
2018-11-01  5:29           ` Junio C Hamano
2018-10-27  9:03   ` Christian Couder
2018-10-25  2:40 ` [PATCH v4 11/12] sha256: add an SHA-256 implementation using libgcrypt brian m. carlson
2018-10-25  2:40 ` [PATCH v4 12/12] hash: add an SHA-256 implementation using OpenSSL brian m. carlson
2018-11-04 23:44 ` [PATCH v5 00/12] Base SHA-256 implementation brian m. carlson
2018-11-04 23:44   ` [PATCH v5 01/12] sha1-file: rename algorithm to "sha1" brian m. carlson
2018-11-05  7:21     ` Ævar Arnfjörð Bjarmason
2018-11-04 23:44   ` [PATCH v5 02/12] sha1-file: provide functions to look up hash algorithms brian m. carlson
2018-11-13 18:42     ` Derrick Stolee
2018-11-13 18:45       ` Duy Nguyen
2018-11-14  1:01         ` brian m. carlson
2018-11-14  0:11       ` Ramsay Jones
2018-11-14  0:42         ` Ramsay Jones
2018-11-14  0:51           ` Jeff King
2018-11-14  2:11         ` brian m. carlson
2018-11-14  3:53           ` Ramsay Jones
2018-11-04 23:44   ` [PATCH v5 03/12] hex: introduce functions to print arbitrary hashes brian m. carlson
2018-11-04 23:44   ` [PATCH v5 04/12] cache: make hashcmp and hasheq work with larger hashes brian m. carlson
2018-11-04 23:44   ` [PATCH v5 05/12] t: add basic tests for our SHA-1 implementation brian m. carlson
2018-11-04 23:44   ` [PATCH v5 06/12] t: make the sha1 test-tool helper generic brian m. carlson
2018-11-04 23:44   ` [PATCH v5 07/12] sha1-file: add a constant for hash block size brian m. carlson
2018-11-04 23:44   ` [PATCH v5 08/12] t/helper: add a test helper to compute hash speed brian m. carlson
2018-11-04 23:44   ` [PATCH v5 09/12] commit-graph: convert to using the_hash_algo brian m. carlson
2018-11-04 23:44   ` [PATCH v5 10/12] Add a base implementation of SHA-256 support brian m. carlson
2018-11-05 11:39     ` Ævar Arnfjörð Bjarmason
2018-11-07  1:30       ` brian m. carlson
2018-11-10 15:52         ` Ævar Arnfjörð Bjarmason
2018-11-04 23:44   ` [PATCH v5 11/12] sha256: add an SHA-256 implementation using libgcrypt brian m. carlson
2018-11-04 23:44   ` [PATCH v5 12/12] hash: add an SHA-256 implementation using OpenSSL brian m. carlson
2018-11-05  2:45   ` [PATCH v5 00/12] Base SHA-256 implementation Junio C Hamano
2018-11-14  4:09   ` [PATCH v6 " brian m. carlson
2018-11-14  4:09     ` [PATCH v6 01/12] sha1-file: rename algorithm to "sha1" brian m. carlson
2018-11-14  4:09     ` [PATCH v6 02/12] sha1-file: provide functions to look up hash algorithms brian m. carlson
2018-11-14  4:09     ` [PATCH v6 03/12] hex: introduce functions to print arbitrary hashes brian m. carlson
2018-11-14  4:09     ` [PATCH v6 04/12] cache: make hashcmp and hasheq work with larger hashes brian m. carlson
2018-11-14  4:09     ` [PATCH v6 05/12] t: add basic tests for our SHA-1 implementation brian m. carlson
2018-11-14  4:09     ` [PATCH v6 06/12] t: make the sha1 test-tool helper generic brian m. carlson
2018-11-14  4:09     ` [PATCH v6 07/12] sha1-file: add a constant for hash block size brian m. carlson
2018-11-14  4:09     ` [PATCH v6 08/12] t/helper: add a test helper to compute hash speed brian m. carlson
2018-11-14  4:09     ` [PATCH v6 09/12] commit-graph: convert to using the_hash_algo brian m. carlson
2018-11-14  4:09     ` [PATCH v6 10/12] Add a base implementation of SHA-256 support brian m. carlson
2018-11-14  4:09     ` [PATCH v6 11/12] sha256: add an SHA-256 implementation using libgcrypt brian m. carlson
2018-11-14  4:09     ` [PATCH v6 12/12] hash: add an SHA-256 implementation using OpenSSL brian m. carlson

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