strmap: enable faster clearing and reusing of strmaps

When strmaps are used heavily, such as is done by my new merge-ort
algorithm, and strmaps need to be cleared but then re-used (because of
e.g. picking multiple commits to cherry-pick, or due to a recursive
merge having several different merges while recursing), free-ing and
reallocating map->table repeatedly can add up in time, especially since
it will likely be reallocated to a much smaller size but the previous
merge provides a good guide to the right size to use for the next merge.

Introduce strmap_partial_clear() to take advantage of this type of
situation; it will act similar to strmap_clear() except that
map->table's entries are zeroed instead of map->table being free'd.
Making use of this function reduced the cost of
clear_or_reinit_internal_opts() by about 20% in mert-ort, and dropped
the overall runtime of my rebase testcase by just under 2%.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/strmap.c b/strmap.c
index 829f1bc0956ef974f7b5a09e879259b8f7f3fadf..c410c5241a840a449fae617d757f3af556bb4c8b 100644 (file)
--- a/strmap.c
+++ b/strmap.c
@@ -64,6 +64,12 @@ void strmap_clear(struct strmap *map, int free_values)
        hashmap_clear(&map->map);
 }
 
+void strmap_partial_clear(struct strmap *map, int free_values)
+{
+       strmap_free_entries_(map, free_values);
+       hashmap_partial_clear(&map->map);
+}
+
 void *strmap_put(struct strmap *map, const char *str, void *data)
 {
        struct strmap_entry *entry = find_strmap_entry(map, str);

diff --git a/strmap.h b/strmap.h
index f74bc582e4d0d6486868445982aff631aa629f9c..c14fcee148e85b52ce414e5f0c6142f21b217e16 100644 (file)
--- a/strmap.h
+++ b/strmap.h
@@ -42,6 +42,12 @@ void strmap_init_with_options(struct strmap *map,
  */
 void strmap_clear(struct strmap *map, int free_values);
 
+/*
+ * Similar to strmap_clear() but leaves map->map->table allocated and
+ * pre-sized so that subsequent uses won't need as many rehashings.
+ */
+void strmap_partial_clear(struct strmap *map, int free_values);
+
 /*
  * Insert "str" into the map, pointing to "data".
  *