read-cache: speed up add_index_entry during checkout

Teach add_index_entry_with_check() 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.

During checkout, merge_working_tree() populates the new
index in sorted order, so this change will save a binary
lookups per file.  This preserves the original behavior
but simply checks the last element before starting the
search.

This helps performance on very large repositories.

Signed-off-by: Jeff Hostetler <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jeffhostetler

read-cache.c

@@ -1021,7 +1021,16 @@ static int add_index_entry_with_check(struct index_state *istate, struct cache_e
 
 	if (!(option & ADD_CACHE_KEEP_CACHE_TREE))
 		cache_tree_invalidate_path(istate, ce->name);
-	pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce), ce_stage(ce));
+
+	/*
+	 * If this entry's path sorts after the last entry in the index,
+	 * we can avoid searching for it.
+	 */
+	if (istate->cache_nr > 0 &&
+		strcmp(ce->name, istate->cache[istate->cache_nr - 1]->name) > 0)
+		pos = -istate->cache_nr - 1;
+	else
+		pos = index_name_stage_pos(istate, ce->name, ce_namelen(ce), ce_stage(ce));
 
 	/* existing match? Just replace it. */
 	if (pos >= 0) {