lookup_object: remove hashtable_index() and optimize hash_obj()

hashtable_index() appears to be a close duplicate of hash_obj().
Keep only the later and make it usable for all cases.

Also remove the modulus as this is an expensive operation.
The size argument is always a power of 2 anyway, so a simple
mask operation provides the same result.

On a 'git rev-list --all --objects' run this decreased the time spent
in lookup_object from 27.5% to 24.1%.

[jc: with a few comments on "modulus turned into mask" by Peff]

Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/object.c b/object.c
index d8a4b1ffbe68d6e9f720e979168f9d010c9c5a06..5f792cbfd38c0b30d763902ba5f9399992ddab89 100644 (file)
--- a/object.c
+++ b/object.c
@@ -43,16 +43,17 @@ int type_from_string(const char *str)
        die("invalid object type \"%s\"", str);
 }
 
-static unsigned int hash_obj(struct object *obj, unsigned int n)
+static unsigned int hash_obj(const unsigned char *sha1, unsigned int n)
 {
        unsigned int hash;
-       memcpy(&hash, obj->sha1, sizeof(unsigned int));
-       return hash % n;
+       memcpy(&hash, sha1, sizeof(unsigned int));
+       /* Assumes power-of-2 hash sizes in grow_object_hash */
+       return hash & (n - 1);
 }
 
 static void insert_obj_hash(struct object *obj, struct object **hash, unsigned int size)
 {
-       unsigned int j = hash_obj(obj, size);
+       unsigned int j = hash_obj(obj->sha1, size);
 
        while (hash[j]) {
                j++;
@@ -62,13 +63,6 @@ static void insert_obj_hash(struct object *obj, struct object **hash, unsigned i
        hash[j] = obj;
 }
 
-static unsigned int hashtable_index(const unsigned char *sha1)
-{
-       unsigned int i;
-       memcpy(&i, sha1, sizeof(unsigned int));
-       return i % obj_hash_size;
-}
-
 struct object *lookup_object(const unsigned char *sha1)
 {
        unsigned int i, first;
@@ -77,7 +71,7 @@ struct object *lookup_object(const unsigned char *sha1)
        if (!obj_hash)
                return NULL;
 
-       first = i = hashtable_index(sha1);
+       first = i = hash_obj(sha1, obj_hash_size);
        while ((obj = obj_hash[i]) != NULL) {
                if (!hashcmp(sha1, obj->sha1))
                        break;
@@ -101,6 +95,10 @@ struct object *lookup_object(const unsigned char *sha1)
 static void grow_object_hash(void)
 {
        int i;
+       /*
+        * Note that this size must always be power-of-2 to match hash_obj
+        * above.
+        */
        int new_hash_size = obj_hash_size < 32 ? 32 : 2 * obj_hash_size;
        struct object **new_hash;