Merge pull request git-for-windows#429 from vdye/unpack-trees/fast-cache-traversal

Improve `next_cache_entry` cache traversal performance

Author: vdye

t/perf/p2000-sparse-operations.sh

@@ -113,6 +113,7 @@ test_perf_on_all git commit -a -m A
 test_perf_on_all git checkout -f -
 test_perf_on_all git reset
 test_perf_on_all git reset --hard
+test_perf_on_all git reset -- does-not-exist
 test_perf_on_all git read-tree -mu HEAD
 test_perf_on_all git checkout-index -f --all
 test_perf_on_all git update-index --add --remove

unpack-trees.c

@@ -665,17 +665,24 @@ static void mark_ce_used_same_name(struct cache_entry *ce,
 	}
 }
 
-static struct cache_entry *next_cache_entry(struct unpack_trees_options *o)
+static struct cache_entry *next_cache_entry(struct unpack_trees_options *o, int *hint)
 {
 	const struct index_state *index = o->src_index;
 	int pos = o->cache_bottom;
 
+	if (*hint > pos)
+		pos = *hint;
+
 	while (pos < index->cache_nr) {
 		struct cache_entry *ce = index->cache[pos];
-		if (!(ce->ce_flags & CE_UNPACKED))
+		if (!(ce->ce_flags & CE_UNPACKED)) {
+			*hint = pos + 1;
 			return ce;
+		}
 		pos++;
 	}
+
+	*hint = pos;
 	return NULL;
 }
 
@@ -1386,12 +1393,13 @@ static int unpack_callback(int n, unsigned long mask, unsigned long dirmask, str
 
 	/* Are we supposed to look at the index too? */
 	if (o->merge) {
+		int hint = -1;
 		while (1) {
 			int cmp;
 			struct cache_entry *ce;
 
 			if (o->diff_index_cached)
-				ce = next_cache_entry(o);
+				ce = next_cache_entry(o, &hint);
 			else
 				ce = find_cache_entry(info, p);
 
@@ -1720,7 +1728,7 @@ static int verify_absent(const struct cache_entry *,
 int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options *o)
 {
 	struct repository *repo = the_repository;
-	int i, ret;
+	int i, hint, ret;
 	static struct cache_entry *dfc;
 	struct pattern_list pl;
 	int free_pattern_list = 0;
@@ -1852,13 +1860,15 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 		info.pathspec = o->pathspec;
 
 		if (o->prefix) {
+			hint = -1;
+
 			/*
 			 * Unpack existing index entries that sort before the
 			 * prefix the tree is spliced into.  Note that o->merge
 			 * is always true in this case.
 			 */
 			while (1) {
-				struct cache_entry *ce = next_cache_entry(o);
+				struct cache_entry *ce = next_cache_entry(o, &hint);
 				if (!ce)
 					break;
 				if (ce_in_traverse_path(ce, &info))
@@ -1879,8 +1889,9 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 
 	/* Any left-over entries in the index? */
 	if (o->merge) {
+		hint = -1;
 		while (1) {
-			struct cache_entry *ce = next_cache_entry(o);
+			struct cache_entry *ce = next_cache_entry(o, &hint);
 			if (!ce)
 				break;
 			if (unpack_index_entry(ce, o) < 0)