hash: set, copy, and use algo field in struct object_id

Now that struct object_id has an algorithm field, we should populate it.
This will allow us to handle object IDs in any supported algorithm and
distinguish between them.  Ensure that the field is written whenever we
write an object ID by storing it explicitly every time we write an
object.  Set values for the empty blob and tree values as well.

In addition, use the algorithm field to compare object IDs.  Note that
because we zero-initialize struct object_id in many places throughout
the codebase, we default to the default algorithm in cases where the
algorithm field is zero rather than explicitly initialize all of those
locations.

This leads to a branch on every comparison, but the alternative is to
compare the entire buffer each time and padding the buffer for SHA-1.
That alternative ranges up to 3.9% worse than this approach on the perf
t0001, t1450, and t1451.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 34 insertions, 6 deletions

diff --git a/hash.h b/hash.h
index c8f03d8..0e85e44 100644
--- a/hash.h
+++ b/hash.h
@@ -192,36 +192,56 @@ static inline int hash_algo_by_ptr(const struct git_hash_algo *p)
 
 extern const struct object_id null_oid;
 
-static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
+static inline int hashcmp_algop(const unsigned char *sha1, const unsigned char *sha2, const struct git_hash_algo *algop)
 {
 	/*
 	 * 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 == GIT_MAX_RAWSZ)
+	if (algop->rawsz == GIT_MAX_RAWSZ)
 		return memcmp(sha1, sha2, GIT_MAX_RAWSZ);
 	return memcmp(sha1, sha2, GIT_SHA1_RAWSZ);
 }
 
+static inline int hashcmp(const unsigned char *sha1, const unsigned char *sha2)
+{
+	return hashcmp_algop(sha1, sha2, the_hash_algo);
+}
+
 static inline int oidcmp(const struct object_id *oid1, const struct object_id *oid2)
 {
-	return hashcmp(oid1->hash, oid2->hash);
+	const struct git_hash_algo *algop;
+	if (!oid1->algo)
+		algop = the_hash_algo;
+	else
+		algop = &hash_algos[oid1->algo];
+	return hashcmp_algop(oid1->hash, oid2->hash, algop);
 }
 
-static inline int hasheq(const unsigned char *sha1, const unsigned char *sha2)
+static inline int hasheq_algop(const unsigned char *sha1, const unsigned char *sha2, const struct git_hash_algo *algop)
 {
 	/*
 	 * 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)
+	if (algop->rawsz == GIT_MAX_RAWSZ)
 		return !memcmp(sha1, sha2, GIT_MAX_RAWSZ);
 	return !memcmp(sha1, sha2, GIT_SHA1_RAWSZ);
 }
 
+static inline int hasheq(const unsigned char *sha1, const unsigned char *sha2)
+{
+	return hasheq_algop(sha1, sha2, the_hash_algo);
+}
+
 static inline int oideq(const struct object_id *oid1, const struct object_id *oid2)
 {
-	return hasheq(oid1->hash, oid2->hash);
+	const struct git_hash_algo *algop;
+	if (!oid1->algo)
+		algop = the_hash_algo;
+	else
+		algop = &hash_algos[oid1->algo];
+	return hasheq_algop(oid1->hash, oid2->hash, algop);
 }
 
 static inline int is_null_oid(const struct object_id *oid)
@@ -237,6 +257,7 @@ static inline void hashcpy(unsigned char *sha_dst, const unsigned char *sha_src)
 static inline void oidcpy(struct object_id *dst, const struct object_id *src)
 {
 	memcpy(dst->hash, src->hash, GIT_MAX_RAWSZ);
+	dst->algo = src->algo;
 }
 
 static inline struct object_id *oiddup(const struct object_id *src)
@@ -254,11 +275,13 @@ static inline void hashclr(unsigned char *hash)
 static inline void oidclr(struct object_id *oid)
 {
 	memset(oid->hash, 0, GIT_MAX_RAWSZ);
+	oid->algo = hash_algo_by_ptr(the_hash_algo);
 }
 
 static inline void oidread(struct object_id *oid, const unsigned char *hash)
 {
 	memcpy(oid->hash, hash, the_hash_algo->rawsz);
+	oid->algo = hash_algo_by_ptr(the_hash_algo);
 }
 
 static inline int is_empty_blob_sha1(const unsigned char *sha1)
@@ -281,6 +304,11 @@ static inline int is_empty_tree_oid(const struct object_id *oid)
 	return oideq(oid, the_hash_algo->empty_tree);
 }
 
+static inline void oid_set_algo(struct object_id *oid, const struct git_hash_algo *algop)
+{
+	oid->algo = hash_algo_by_ptr(algop);
+}
+
 const char *empty_tree_oid_hex(void);
 const char *empty_blob_oid_hex(void);