read-cache: speed up has_dir_name (part 1)

Teach has_dir_name() to see if the path of the new item
is greater than the last path in the index array before
attempting to search for it.

has_dir_name() is looking for file/directory collisions
in the index and has to consider each sub-directory
prefix in turn.  This can cause multiple binary searches
for each path.

During operations like checkout, merge_working_tree()
populates the new index in sorted order, so we expect
to be able to append in many cases.

This commit is part 1 of 2.  This commit handles the top
of has_dir_name() and the trivial optimization.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/read-cache.c b/read-cache.c
index 836338b3ba7ba094ba60936728424dfb291bf33e..5aa097afb7ded7a5a03074aa8930f9d778157429 100644 (file)
--- a/read-cache.c
+++ b/read-cache.c
@@ -910,6 +910,9 @@ int strcmp_offset(const char *s1, const char *s2, size_t *first_change)
 /*
  * Do we have another file with a pathname that is a proper
  * subset of the name we're trying to add?
+ *
+ * That is, is there another file in the index with a path
+ * that matches a sub-directory in the given entry?
  */
 static int has_dir_name(struct index_state *istate,
                        const struct cache_entry *ce, int pos, int ok_to_replace)
@@ -918,6 +921,48 @@ static int has_dir_name(struct index_state *istate,
        int stage = ce_stage(ce);
        const char *name = ce->name;
        const char *slash = name + ce_namelen(ce);
+       size_t len_eq_last;
+       int cmp_last = 0;
+
+       /*
+        * We are frequently called during an iteration on a sorted
+        * list of pathnames and while building a new index.  Therefore,
+        * there is a high probability that this entry will eventually
+        * be appended to the index, rather than inserted in the middle.
+        * If we can confirm that, we can avoid binary searches on the
+        * components of the pathname.
+        *
+        * Compare the entry's full path with the last path in the index.
+        */
+       if (istate->cache_nr > 0) {
+               cmp_last = strcmp_offset(name,
+                       istate->cache[istate->cache_nr - 1]->name,
+                       &len_eq_last);
+               if (cmp_last > 0) {
+                       if (len_eq_last == 0) {
+                               /*
+                                * The entry sorts AFTER the last one in the
+                                * index and their paths have no common prefix,
+                                * so there cannot be a F/D conflict.
+                                */
+                               return retval;
+                       } else {
+                               /*
+                                * The entry sorts AFTER the last one in the
+                                * index, but has a common prefix.  Fall through
+                                * to the loop below to disect the entry's path
+                                * and see where the difference is.
+                                */
+                       }
+               } else if (cmp_last == 0) {
+                       /*
+                        * The entry exactly matches the last one in the
+                        * index, but because of multiple stage and CE_REMOVE
+                        * items, we fall through and let the regular search
+                        * code handle it.
+                        */
+               }
+       }
 
        for (;;) {
                int len;