unpack-trees: fix accidentally quadratic behavior

While unpacking trees (e.g. during git checkout), when we hit a cache
entry that's past and outside our path, we cut off iteration.

This provides about a 45% speedup on git checkout between master and
master^20000 on Twitter's monorepo.  Speedup in general will depend on
repostitory structure, number of changes, and packfile packing
decisions.

Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/unpack-trees.c b/unpack-trees.c
index 5f541c28c6aadda347c85018a6591ac6b2622820..9f55cc28b9dd41231644053b49875671f5a505fa 100644 (file)
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -695,8 +695,19 @@ static int find_cache_pos(struct traverse_info *info,
                                ++o->cache_bottom;
                        continue;
                }
-               if (!ce_in_traverse_path(ce, info))
+               if (!ce_in_traverse_path(ce, info)) {
+                       /*
+                        * Check if we can skip future cache checks
+                        * (because we're already past all possible
+                        * entries in the traverse path).
+                        */
+                       if (info->traverse_path) {
+                               if (strncmp(ce->name, info->traverse_path,
+                                           info->pathlen) > 0)
+                                       break;
+                       }
                        continue;
+               }
                ce_name = ce->name + pfxlen;
                ce_slash = strchr(ce_name, '/');
                if (ce_slash)