unpack-trees: reuse (still valid) cache-tree from src_index

We do n-way merge by walking the source index and n trees at the same
time and add merge results to a new temporary index called o->result.
The merge result for any given path could be either

- keep_entry(): same old index entry in o->src_index is reused
- merged_entry(): either a new entry is added, or an existing one updated
- deleted_entry(): one entry from o->src_index is removed

For some reason [1] we keep making sure that the source index's
cache-tree is still valid if used by o->result: for all those
merged/deleted entries, we invalidate the same path in o->src_index,
so only cache-trees covering the "keep_entry" parts remain good.

Because of this, the cache-tree from o->src_index can be perfectly
reused in o->result. And in fact we already rely on this logic to
reuse untracked cache in edf3b90553 (unpack-trees: preserve index
extensions - 2017-05-08). Move the cache-tree to o->result before
doing cache_tree_update() to reduce hashing cost.

Since cache_tree_update() has risen up as one of the most expensive
parts in unpack_trees() after the last few patches. This does help
reduce unpack_trees() time significantly (on webkit.git):

before       after
--------------------------------------------------------------------
0.080394752  0.051258167 s:  read cache .git/index
0.216010838  0.212106298 s:  preload index
0.008534301  0.280521764 s:  refresh index
0.251992198  0.218160442 s:   traverse_trees
0.377031383  0.374948191 s:   check_updates
0.372768105  0.037040114 s:   cache_tree_update
1.045887251  0.672031609 s:  unpack_trees
0.314983512  0.317456290 s:  write index, changed mask = 2e
0.062572653  0.038382654 s:    traverse_trees
0.000022544  0.000042731 s:    check_updates
0.073795585  0.050930053 s:   unpack_trees
0.073807557  0.051099735 s:  diff-index
1.938191592  1.614241153 s: git command: git checkout -

[1] I'm pretty sure the reason is an oversight in 34110cd4e3 (Make
'unpack_trees()' have a separate source and destination index -
2008-03-06). That patch aims to _not_ update the source index at
all. The invalidation should have been done on o->result in that
patch. But then there was no cache-tree on o->result even then so
it's pointless to do so.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/read-cache.c b/read-cache.c
index 1c9c88c130bec1bbe109b9cfaa225dc26faa9104..5ce40f39b3ce51bf76ffec67727a429f75a0b57f 100644 (file)
--- a/read-cache.c
+++ b/read-cache.c
@@ -2940,6 +2940,8 @@ void move_index_extensions(struct index_state *dst, struct index_state *src)
 {
        dst->untracked = src->untracked;
        src->untracked = NULL;
+       dst->cache_tree = src->cache_tree;
+       src->cache_tree = NULL;
 }
 
 struct cache_entry *dup_cache_entry(const struct cache_entry *ce,

diff --git a/unpack-trees.c b/unpack-trees.c
index dbef6e1b8a2fbd0ed8af9c082c1f1b3e57e8a1f9..aa80b65ee15e221cbb9bf535fe03c0b3d4420b62 100644 (file)
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -1576,6 +1576,7 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 
        ret = check_updates(o) ? (-2) : 0;
        if (o->dst_index) {
+               move_index_extensions(&o->result, o->src_index);
                if (!ret) {
                        if (!o->result.cache_tree)
                                o->result.cache_tree = cache_tree();
@@ -1584,7 +1585,6 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
                                                  WRITE_TREE_SILENT |
                                                  WRITE_TREE_REPAIR);
                }
-               move_index_extensions(&o->result, o->src_index);
                discard_index(o->dst_index);
                *o->dst_index = o->result;
        } else {