pack-bitmap.c: avoid repeated pack_pos_to_offset() during reuse

When calling `try_partial_reuse()`, the (sole) caller from the function
`reuse_partial_packfile_from_bitmap_1()` has to translate its bit
position to a pack position.

In the MIDX bitmap case, the caller translates from the bit position, to
a position in the MIDX's pseudo-pack order (with `pack_pos_to_midx()`),
then get a pack offset (with `nth_midxed_offset()`) before finally
working backwards to get the pack position in the source pack by calling
`offset_to_pack_pos()`.

In the non-MIDX bitmap case, we can use the bit position as the pack
position directly (see the comment at the beginning of the
`reuse_partial_packfile_from_bitmap_1()` function for why).

In either case, the first thing that `try_partial_reuse()` does after
being called is determine the offset of the object at the given pack
position by calling `pack_pos_to_offset()`. But we already have that
information in the MIDX case!

Avoid re-computing that information by instead passing it in. In the
MIDX case, we already have that information stored. In the non-MIDX
case, the call to `pack_pos_to_offset()` moves from the function
`try_partial_reuse()` to its caller. In total, we'll save one call to
`pack_pos_to_offset()` when processing MIDX bitmaps.

(On my machine, there is a slight speed-up on the order of ~2ms, but it
is within the margin of error over 10 runs, so I think you'd have to
have a truly gigantic repository to confidently measure any significant
improvement here).

Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ttaylorr

pack-bitmap.c

@@ -2055,17 +2055,18 @@ static int try_partial_reuse(struct bitmap_index *bitmap_git,
 			     struct bitmapped_pack *pack,
 			     size_t bitmap_pos,
 			     uint32_t pack_pos,
+			     off_t offset,
 			     struct bitmap *reuse,
 			     struct pack_window **w_curs)
 {
-	off_t offset, delta_obj_offset;
+	off_t delta_obj_offset;
 	enum object_type type;
 	unsigned long size;
 
 	if (pack_pos >= pack->p->num_objects)
 		return -1; /* not actually in the pack */
 
-	offset = delta_obj_offset = pack_pos_to_offset(pack->p, pack_pos);
+	delta_obj_offset = offset;
 	type = unpack_object_header(pack->p, w_curs, &offset, &size);
 	if (type < 0)
 		return -1; /* broken packfile, punt */
@@ -2184,6 +2185,7 @@ static void reuse_partial_packfile_from_bitmap_1(struct bitmap_index *bitmap_git
 		for (offset = 0; offset < BITS_IN_EWORD; offset++) {
 			size_t bit_pos;
 			uint32_t pack_pos;
+			off_t ofs;
 
 			if (word >> offset == 0)
 				break;
@@ -2198,7 +2200,6 @@ static void reuse_partial_packfile_from_bitmap_1(struct bitmap_index *bitmap_git
 
 			if (bitmap_is_midx(bitmap_git)) {
 				uint32_t midx_pos;
-				off_t ofs;
 
 				midx_pos = pack_pos_to_midx(bitmap_git->midx, bit_pos);
 				ofs = nth_midxed_offset(bitmap_git->midx, midx_pos);
@@ -2213,10 +2214,12 @@ static void reuse_partial_packfile_from_bitmap_1(struct bitmap_index *bitmap_git
 					BUG("advanced beyond the end of pack %s (%"PRIuMAX" > %"PRIu32")",
 					    pack_basename(pack->p), (uintmax_t)pack_pos,
 					    pack->p->num_objects);
+
+				ofs = pack_pos_to_offset(pack->p, pack_pos);
 			}
 
 			if (try_partial_reuse(bitmap_git, pack, bit_pos,
-					      pack_pos, reuse, &w_curs) < 0) {
+					      pack_pos, ofs, reuse, &w_curs) < 0) {
 				/*
 				 * try_partial_reuse indicated we couldn't reuse
 				 * any bits, so there is no point in trying more