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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 <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
maint
Taylor Blau 2024-08-27 17:13:36 -04:00 committed by Junio C Hamano
parent 125c32605a
commit db40e3c92b
1 changed files with 7 additions and 4 deletions
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11
pack-bitmap.c
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@ -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