pack-objects: refer to delta objects by index instead of pointer

These delta pointers always point to elements in the objects[] array
in packing_data struct. We can only hold maximum 4G of those objects
because the array size in nr_objects is uint32_t. We could use
uint32_t indexes to address these elements instead of pointers. On
64-bit architecture (8 bytes per pointer) this would save 4 bytes per
pointer.

Convert these delta pointers to indexes. Since we need to handle NULL
pointers as well, the index is shifted by one [1].

[1] This means we can only index 2^32-2 objects even though nr_objects
    could contain 2^32-1 objects. It should not be a problem in
    practice because when we grow objects[], nr_alloc would probably
    blow up long before nr_objects hits the wall.

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

1 files changed, 62 insertions, 5 deletions

diff --git a/pack-objects.h b/pack-objects.h
index 7bedd5af81..e962dce3c0 100644
--- a/pack-objects.h
+++ b/pack-objects.h
@@ -70,11 +70,11 @@ struct object_entry {
 	unsigned long size;	/* uncompressed size */
 	unsigned in_pack_idx:OE_IN_PACK_BITS;	/* already in pack */
 	off_t in_pack_offset;
-	struct object_entry *delta;	/* delta base object */
-	struct object_entry *delta_child; /* deltified objects who bases me */
-	struct object_entry *delta_sibling; /* other deltified objects who
-					     * uses the same base as me
-					     */
+	uint32_t delta_idx;	/* delta base object */
+	uint32_t delta_child_idx; /* deltified objects who bases me */
+	uint32_t delta_sibling_idx; /* other deltified objects who
+				     * uses the same base as me
+				     */
 	void *delta_data;	/* cached delta (uncompressed) */
 	unsigned long delta_size;	/* delta data size (uncompressed) */
 	unsigned long z_delta_size;	/* delta data size (compressed) */
@@ -194,4 +194,61 @@ static inline void oe_set_in_pack(struct packing_data *pack,
 		pack->in_pack[e - pack->objects] = p;
 }
 
+static inline struct object_entry *oe_delta(
+		const struct packing_data *pack,
+		const struct object_entry *e)
+{
+	if (e->delta_idx)
+		return &pack->objects[e->delta_idx - 1];
+	return NULL;
+}
+
+static inline void oe_set_delta(struct packing_data *pack,
+				struct object_entry *e,
+				struct object_entry *delta)
+{
+	if (delta)
+		e->delta_idx = (delta - pack->objects) + 1;
+	else
+		e->delta_idx = 0;
+}
+
+static inline struct object_entry *oe_delta_child(
+		const struct packing_data *pack,
+		const struct object_entry *e)
+{
+	if (e->delta_child_idx)
+		return &pack->objects[e->delta_child_idx - 1];
+	return NULL;
+}
+
+static inline void oe_set_delta_child(struct packing_data *pack,
+				      struct object_entry *e,
+				      struct object_entry *delta)
+{
+	if (delta)
+		e->delta_child_idx = (delta - pack->objects) + 1;
+	else
+		e->delta_child_idx = 0;
+}
+
+static inline struct object_entry *oe_delta_sibling(
+		const struct packing_data *pack,
+		const struct object_entry *e)
+{
+	if (e->delta_sibling_idx)
+		return &pack->objects[e->delta_sibling_idx - 1];
+	return NULL;
+}
+
+static inline void oe_set_delta_sibling(struct packing_data *pack,
+					struct object_entry *e,
+					struct object_entry *delta)
+{
+	if (delta)
+		e->delta_sibling_idx = (delta - pack->objects) + 1;
+	else
+		e->delta_sibling_idx = 0;
+}
+
 #endif