commit: allow associating auxiliary info on-demand

The "indegree" field in the commit object is only used while sorting
a list of commits in topological order, and wasting memory otherwise.

We would prefer to shrink the size of individual commit objects,
which we may have to hold thousands of in-core. We could eject
"indegree" field out from the commit object and represent it as a
dynamic table based on the decoration infrastructure, but the
decoration is meant for sparse annotation and is not a good match.

Instead, let's try a different approach.

- Assign an integer (commit->index) to each commit we keep in-core
(reuse the space of "indegree" field for it);

- When running the topological sort, allocate an array of integers
in bulk (called "slab"), use the commit->index as an index into
this array, and store the "indegree" information there.

This does _not_ reduce the memory footprint of a commit object, but
the commit->index can be used as the index to dynamically associate
commits with other kinds of information as needed.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/commit.c b/commit.c
index 1a41757ee377d804782ccfb4e27c54b607dcf91d..9365e3b34aba1c0d4f1ffe9e5e5eebcce886a42e 100644 (file)
--- a/commit.c
+++ b/commit.c
@@ -14,6 +14,7 @@ static struct commit_extra_header *read_commit_extra_header_lines(const char *bu
 int save_commit_buffer = 1;
 
 const char *commit_type = "commit";
+static int commit_count;
 
 static struct commit *check_commit(struct object *obj,
                                   const unsigned char *sha1,
@@ -58,8 +59,11 @@ struct commit *lookup_commit_or_die(const unsigned char *sha1, const char *ref_n
 struct commit *lookup_commit(const unsigned char *sha1)
 {
        struct object *obj = lookup_object(sha1);
-       if (!obj)
-               return create_object(sha1, OBJ_COMMIT, alloc_commit_node());
+       if (!obj) {
+               struct commit *c = alloc_commit_node();
+               c->index = commit_count++;
+               return create_object(sha1, OBJ_COMMIT, c);
+       }
        if (!obj->type)
                obj->type = OBJ_COMMIT;
        return check_commit(obj, sha1, 0);
@@ -497,6 +501,36 @@ struct commit *pop_commit(struct commit_list **stack)
        return item;
 }
 
+struct commit_slab {
+       int *buf;
+       int alloc;
+};
+
+static void slab_init(struct commit_slab *s)
+{
+       memset(s, 0, sizeof(*s));
+}
+
+static void slab_clear(struct commit_slab *s)
+{
+       free(s->buf);
+       slab_init(s);
+}
+
+static inline int *slab_at(struct commit_slab *s, const struct commit *c)
+{
+       if (s->alloc <= c->index) {
+               int new_alloc = alloc_nr(s->alloc);
+               if (new_alloc <= c->index)
+                       new_alloc = c->index + 1;
+
+               s->buf = xrealloc(s->buf, new_alloc * sizeof(*s->buf));
+               memset(s->buf + s->alloc, 0, new_alloc - s->alloc);
+               s->alloc = new_alloc;
+       }
+       return s->buf + c->index;
+}
+
 /*
  * Performs an in-place topological sort on the list supplied.
  */
@@ -505,15 +539,18 @@ void sort_in_topological_order(struct commit_list ** list, int lifo)
        struct commit_list *next, *orig = *list;
        struct commit_list *work, **insert;
        struct commit_list **pptr;
+       struct commit_slab indegree;
 
        if (!orig)
                return;
        *list = NULL;
 
+       slab_init(&indegree);
+
        /* Mark them and clear the indegree */
        for (next = orig; next; next = next->next) {
                struct commit *commit = next->item;
-               commit->indegree = 1;
+               *slab_at(&indegree, commit) = 1;
        }
 
        /* update the indegree */
@@ -521,9 +558,10 @@ void sort_in_topological_order(struct commit_list ** list, int lifo)
                struct commit_list * parents = next->item->parents;
                while (parents) {
                        struct commit *parent = parents->item;
+                       int *pi = slab_at(&indegree, parent);
 
-                       if (parent->indegree)
-                               parent->indegree++;
+                       if (*pi)
+                               (*pi)++;
                        parents = parents->next;
                }
        }
@@ -540,7 +578,7 @@ void sort_in_topological_order(struct commit_list ** list, int lifo)
        for (next = orig; next; next = next->next) {
                struct commit *commit = next->item;
 
-               if (commit->indegree == 1)
+               if (*slab_at(&indegree, commit) == 1)
                        insert = &commit_list_insert(commit, insert)->next;
        }
 
@@ -561,8 +599,9 @@ void sort_in_topological_order(struct commit_list ** list, int lifo)
                commit = work_item->item;
                for (parents = commit->parents; parents ; parents = parents->next) {
                        struct commit *parent = parents->item;
+                       int *pi = slab_at(&indegree, parent);
 
-                       if (!parent->indegree)
+                       if (!*pi)
                                continue;
 
                        /*
@@ -570,7 +609,7 @@ void sort_in_topological_order(struct commit_list ** list, int lifo)
                         * when all their children have been emitted thereby
                         * guaranteeing topological order.
                         */
-                       if (--parent->indegree == 1) {
+                       if (--(*pi) == 1) {
                                if (!lifo)
                                        commit_list_insert_by_date(parent, &work);
                                else
@@ -581,10 +620,12 @@ void sort_in_topological_order(struct commit_list ** list, int lifo)
                 * work_item is a commit all of whose children
                 * have already been emitted. we can emit it now.
                 */
-               commit->indegree = 0;
+               *slab_at(&indegree, commit) = 0;
                *pptr = work_item;
                pptr = &work_item->next;
        }
+
+       slab_clear(&indegree);
 }
 
 /* merge-base stuff */

diff --git a/commit.h b/commit.h
index 252c7f871c1c90423ae11190ba98cea5b94422de..70e749d69ffec1818575b327e08dde6808ad44f1 100644 (file)
--- a/commit.h
+++ b/commit.h
@@ -14,7 +14,7 @@ struct commit_list {
 struct commit {
        struct object object;
        void *util;
-       unsigned int indegree;
+       unsigned int index;
        unsigned long date;
        struct commit_list *parents;
        struct tree *tree;