index-pack: eliminate recursion in find_unresolved_deltas

Current find_unresolved_deltas() links all bases together in a form of
tree, using struct base_data, with prev_base pointer to point to
parent node. Then it traverses down from parent to children in
recursive manner with all base_data allocated on stack.

To eliminate recursion, we simply need to put all on heap
(parse_pack_objects and fix_unresolved_deltas). After that, it's
simple non-recursive depth-first traversal loop. Each node also
maintains its own state (ofs and ref indices) to iterate over all
children nodes.

So we process one node:

 - if it returns a new (child) node (a parent base), we link it to our
   tree, then process the new node.

 - if it returns nothing, the node is done, free it. We go back to
   parent node and resume whatever it's doing.

and do it until we have no nodes to process.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index af7dc37a441d12bc4283ac730524e07a582d014f..38ff03a5cdab4eb6107b471daa156dfd57753be7 100644 (file)
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -34,6 +34,8 @@ struct base_data {
        struct object_entry *obj;
        void *data;
        unsigned long size;
+       int ref_first, ref_last;
+       int ofs_first, ofs_last;
 };
 
 /*
@@ -221,6 +223,15 @@ static NORETURN void bad_object(unsigned long offset, const char *format, ...)
        die("pack has bad object at offset %lu: %s", offset, buf);
 }
 
+static struct base_data *alloc_base_data(void)
+{
+       struct base_data *base = xmalloc(sizeof(struct base_data));
+       memset(base, 0, sizeof(*base));
+       base->ref_last = -1;
+       base->ofs_last = -1;
+       return base;
+}
+
 static void free_base_data(struct base_data *c)
 {
        if (c->data) {
@@ -553,58 +564,76 @@ static void resolve_delta(struct object_entry *delta_obj,
        nr_resolved_deltas++;
 }
 
-static void find_unresolved_deltas(struct base_data *base,
-                                  struct base_data *prev_base)
+static struct base_data *find_unresolved_deltas_1(struct base_data *base,
+                                                 struct base_data *prev_base)
 {
-       int i, ref_first, ref_last, ofs_first, ofs_last;
-
-       /*
-        * This is a recursive function. Those brackets should help reducing
-        * stack usage by limiting the scope of the delta_base union.
-        */
-       {
+       if (base->ref_last == -1 && base->ofs_last == -1) {
                union delta_base base_spec;
 
                hashcpy(base_spec.sha1, base->obj->idx.sha1);
                find_delta_children(&base_spec,
-                                   &ref_first, &ref_last, OBJ_REF_DELTA);
+                                   &base->ref_first, &base->ref_last, OBJ_REF_DELTA);
 
                memset(&base_spec, 0, sizeof(base_spec));
                base_spec.offset = base->obj->idx.offset;
                find_delta_children(&base_spec,
-                                   &ofs_first, &ofs_last, OBJ_OFS_DELTA);
-       }
+                                   &base->ofs_first, &base->ofs_last, OBJ_OFS_DELTA);
 
-       if (ref_last == -1 && ofs_last == -1) {
-               free(base->data);
-               return;
-       }
+               if (base->ref_last == -1 && base->ofs_last == -1) {
+                       free(base->data);
+                       return NULL;
+               }
 
-       link_base_data(prev_base, base);
+               link_base_data(prev_base, base);
+       }
 
-       for (i = ref_first; i <= ref_last; i++) {
-               struct object_entry *child = objects + deltas[i].obj_no;
-               struct base_data result;
+       if (base->ref_first <= base->ref_last) {
+               struct object_entry *child = objects + deltas[base->ref_first].obj_no;
+               struct base_data *result = alloc_base_data();
 
                assert(child->real_type == OBJ_REF_DELTA);
-               resolve_delta(child, base, &result);
-               if (i == ref_last && ofs_last == -1)
+               resolve_delta(child, base, result);
+               if (base->ref_first == base->ref_last && base->ofs_last == -1)
                        free_base_data(base);
-               find_unresolved_deltas(&result, base);
+
+               base->ref_first++;
+               return result;
        }
 
-       for (i = ofs_first; i <= ofs_last; i++) {
-               struct object_entry *child = objects + deltas[i].obj_no;
-               struct base_data result;
+       if (base->ofs_first <= base->ofs_last) {
+               struct object_entry *child = objects + deltas[base->ofs_first].obj_no;
+               struct base_data *result = alloc_base_data();
 
                assert(child->real_type == OBJ_OFS_DELTA);
-               resolve_delta(child, base, &result);
-               if (i == ofs_last)
+               resolve_delta(child, base, result);
+               if (base->ofs_first == base->ofs_last)
                        free_base_data(base);
-               find_unresolved_deltas(&result, base);
+
+               base->ofs_first++;
+               return result;
        }
 
        unlink_base_data(base);
+       return NULL;
+}
+
+static void find_unresolved_deltas(struct base_data *base)
+{
+       struct base_data *new_base, *prev_base = NULL;
+       for (;;) {
+               new_base = find_unresolved_deltas_1(base, prev_base);
+
+               if (new_base) {
+                       prev_base = base;
+                       base = new_base;
+               } else {
+                       free(base);
+                       base = prev_base;
+                       if (!base)
+                               return;
+                       prev_base = base->base;
+               }
+       }
 }
 
 static int compare_delta_entry(const void *a, const void *b)
@@ -684,13 +713,13 @@ static void parse_pack_objects(unsigned char *sha1)
                progress = start_progress("Resolving deltas", nr_deltas);
        for (i = 0; i < nr_objects; i++) {
                struct object_entry *obj = &objects[i];
-               struct base_data base_obj;
+               struct base_data *base_obj = alloc_base_data();
 
                if (is_delta_type(obj->type))
                        continue;
-               base_obj.obj = obj;
-               base_obj.data = NULL;
-               find_unresolved_deltas(&base_obj, NULL);
+               base_obj->obj = obj;
+               base_obj->data = NULL;
+               find_unresolved_deltas(base_obj);
                display_progress(progress, nr_resolved_deltas);
        }
 }
@@ -783,20 +812,20 @@ static void fix_unresolved_deltas(struct sha1file *f, int nr_unresolved)
        for (i = 0; i < n; i++) {
                struct delta_entry *d = sorted_by_pos[i];
                enum object_type type;
-               struct base_data base_obj;
+               struct base_data *base_obj = alloc_base_data();
 
                if (objects[d->obj_no].real_type != OBJ_REF_DELTA)
                        continue;
-               base_obj.data = read_sha1_file(d->base.sha1, &type, &base_obj.size);
-               if (!base_obj.data)
+               base_obj->data = read_sha1_file(d->base.sha1, &type, &base_obj->size);
+               if (!base_obj->data)
                        continue;
 
-               if (check_sha1_signature(d->base.sha1, base_obj.data,
-                               base_obj.size, typename(type)))
+               if (check_sha1_signature(d->base.sha1, base_obj->data,
+                               base_obj->size, typename(type)))
                        die("local object %s is corrupt", sha1_to_hex(d->base.sha1));
-               base_obj.obj = append_obj_to_pack(f, d->base.sha1,
-                                       base_obj.data, base_obj.size, type);
-               find_unresolved_deltas(&base_obj, NULL);
+               base_obj->obj = append_obj_to_pack(f, d->base.sha1,
+                                       base_obj->data, base_obj->size, type);
+               find_unresolved_deltas(base_obj);
                display_progress(progress, nr_resolved_deltas);
        }
        free(sorted_by_pos);