unpack-trees: improve performance of next_cache_entry

To find the first non-unpacked cache entry, `next_cache_entry` iterates
through index, starting at `cache_bottom`. The performance of this in full
indexes is helped by `cache_bottom` advancing with each invocation of
`mark_ce_used` (called by `unpack_index_entry`). However, the presence of
sparse directories can prevent the `cache_bottom` from advancing in a sparse
index case, effectively forcing `next_cache_entry` to search from the
beginning of the index each time it is called.

The `cache_bottom` must be preserved for the sparse index (see 17a1bb570b
(unpack-trees: preserve cache_bottom, 2021-07-14)). Therefore, to retain the
benefit `cache_bottom` provides in non-sparse index cases, a separate `hint`
position indicates the first position `next_cache_entry` should search,
updated each execution with a new position.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/unpack-trees.c b/unpack-trees.c
index 8ea0a542da8d9f92efcb30964bc8b14c534f834f..b94733de6bedcf20dc6c26532ab99a7b4288c123 100644 (file)
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -645,17 +645,24 @@ 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;
 }
 
@@ -1365,12 +1372,13 @@ 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);
 
@@ -1690,7 +1698,7 @@ 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;
@@ -1763,13 +1771,15 @@ 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))
@@ -1790,8 +1800,9 @@ 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)