Optimize the common cases of git-read-tree

This optimizes bind_merge() and oneway_merge() to not unnecessarily
remove and re-add the old index entries when they can just get replaced
by updated ones.

This makes these operations much faster for large trees (where "large"
is in the 50,000+ file range), because we don't unnecessarily move index
entries around in the index array all the time.

Using the "bummer" tree (a test-tree with 100,000 files) we get:

Before:
	[torvalds@woody bummer]$ time git commit -m"Change one file" 50/500
	real    0m9.470s
	user    0m8.729s
	sys     0m0.476s

After:
	[torvalds@woody bummer]$ time git commit -m"Change one file" 50/500
	real    0m1.173s
	user    0m0.720s
	sys     0m0.452s

so for large trees this is easily very noticeable indeed.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/unpack-trees.c b/unpack-trees.c
index 5e457b0bc919c746405a0ec106cdc02d47fd1ff9..d57b91c1b2738f205d7997cc67f25f594ff3e5ed 100644 (file)
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -887,7 +887,6 @@ int bind_merge(struct cache_entry **src,
        struct cache_entry *old = src[0];
        struct cache_entry *a = src[1];
 
-       remove_entry(remove);
        if (o->merge_size != 1)
                return error("Cannot do a bind merge of %d trees\n",
                             o->merge_size);
@@ -912,13 +911,14 @@ int oneway_merge(struct cache_entry **src,
        struct cache_entry *old = src[0];
        struct cache_entry *a = src[1];
 
-       remove_entry(remove);
        if (o->merge_size != 1)
                return error("Cannot do a oneway merge of %d trees",
                             o->merge_size);
 
-       if (!a)
+       if (!a) {
+               remove_entry(remove);
                return deleted_entry(old, old, o);
+       }
        if (old && same(old, a)) {
                if (o->reset) {
                        struct stat st;