On Tue, Sep 10, 2013 at 06:17:12PM -0400, Nicolas Pitre wrote:

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

Thanks. This duplication has often bugged me when looking at that
hash table, but I just never actually wrote the patch.

> Also remove the modulus as this is an expansive 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%.

Nice. This is a tiny bit subtle, though, as the power-of-2 growth
happens elsewhere, and we may want to tweak it later (the decorate.c
hash, for example, grows by 3/2).

Maybe it's worth squashing in one or both of the comments below as a
warning to anybody who tries to tweak it.

diff --git a/object.c b/object.c
index e2dae22..5f792cb 100644
--- a/object.c
+++ b/object.c
@@ -47,6 +47,7 @@ static unsigned int hash_obj(const unsigned char *sha1, 
unsigned int n)
        unsigned int hash;
        memcpy(&hash, sha1, sizeof(unsigned int));
+       /* Assumes power-of-2 hash sizes in grow_object_hash */
        return hash & (n - 1);
@@ -94,6 +95,10 @@ static void grow_object_hash(void)
 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;
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