describe: Delay looking up commits until searching for an inexact match

Now that struct commit.util is not used until after we've checked that
the argument doesn't exactly match a tag, we can wait until then to
look up the commits for each tag.

This avoids a lot of I/O on --exact-match queries in repositories with
many tags.  For example, 'git describe --exact-match HEAD' becomes
about 12 times faster on a cold cache (3.2s instead of 39s) in a
linux-2.6 repository with 2000 packed tags.  That is a huge win for the
interactivity of the __git_ps1 shell prompt helper when on a detached
HEAD.

Signed-off-by: Anders Kaseorg <andersk@ksplice.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin/describe.c b/builtin/describe.c
index 8149233059d84155cb3d2385e1bc19042f539b6c..a0f52c1b72b2ffd95e781abffb78d7b66ec583c7 100644 (file)
--- a/builtin/describe.c
+++ b/builtin/describe.c
@@ -24,6 +24,7 @@ static int longformat;
 static int abbrev = DEFAULT_ABBREV;
 static int max_candidates = 10;
 static struct hash_table names;
+static int have_util;
 static const char *pattern;
 static int always;
 static const char *dirty;
@@ -62,6 +63,17 @@ static inline struct commit_name *find_commit_name(const unsigned char *peeled)
        return n;
 }
 
+static int set_util(void *chain)
+{
+       struct commit_name *n;
+       for (n = chain; n; n = n->next) {
+               struct commit *c = lookup_commit_reference_gently(n->peeled, 1);
+               if (c)
+                       c->util = n;
+       }
+       return 0;
+}
+
 static int replace_name(struct commit_name *e,
                               int prio,
                               const unsigned char *sha1,
@@ -96,18 +108,16 @@ static int replace_name(struct commit_name *e,
 }
 
 static void add_to_known_names(const char *path,
-                              struct commit *commit,
+                              const unsigned char *peeled,
                               int prio,
                               const unsigned char *sha1)
 {
-       const unsigned char *peeled = commit->object.sha1;
        struct commit_name *e = find_commit_name(peeled);
        struct tag *tag = NULL;
        if (replace_name(e, prio, sha1, &tag)) {
                if (!e) {
                        void **pos;
                        e = xmalloc(sizeof(struct commit_name));
-                       commit->util = e;
                        hashcpy(e->peeled, peeled);
                        pos = insert_hash(hash_sha1(peeled), e, &names);
                        if (pos) {
@@ -128,8 +138,6 @@ static void add_to_known_names(const char *path,
 static int get_name(const char *path, const unsigned char *sha1, int flag, void *cb_data)
 {
        int might_be_tag = !prefixcmp(path, "refs/tags/");
-       struct commit *commit;
-       struct object *object;
        unsigned char peeled[20];
        int is_tag, prio;
 
@@ -137,16 +145,10 @@ static int get_name(const char *path, const unsigned char *sha1, int flag, void
                return 0;
 
        if (!peel_ref(path, peeled) && !is_null_sha1(peeled)) {
-               commit = lookup_commit_reference_gently(peeled, 1);
-               if (!commit)
-                       return 0;
-               is_tag = !!hashcmp(sha1, commit->object.sha1);
+               is_tag = !!hashcmp(sha1, peeled);
        } else {
-               commit = lookup_commit_reference_gently(sha1, 1);
-               object = parse_object(sha1);
-               if (!commit || !object)
-                       return 0;
-               is_tag = object->type == OBJ_TAG;
+               hashcpy(peeled, sha1);
+               is_tag = 0;
        }
 
        /* If --all, then any refs are used.
@@ -169,7 +171,7 @@ static int get_name(const char *path, const unsigned char *sha1, int flag, void
                if (!prio)
                        return 0;
        }
-       add_to_known_names(all ? path + 5 : path + 10, commit, prio, sha1);
+       add_to_known_names(all ? path + 5 : path + 10, peeled, prio, sha1);
        return 0;
 }
 
@@ -286,6 +288,11 @@ static void describe(const char *arg, int last_one)
        if (debug)
                fprintf(stderr, "searching to describe %s\n", arg);
 
+       if (!have_util) {
+               for_each_hash(&names, set_util);
+               have_util = 1;
+       }
+
        list = NULL;
        cmit->object.flags = SEEN;
        commit_list_insert(cmit, &list);