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>

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;