[PATCH 02/11] pack-bitmap.c: avoid unnecessary `offset_to_pack_pos()`

[Taylor Blau <me@ttaylorr.com>]

In `try_partial_reuse()`, for any object which is either an OFS_DELTA
or REF_DELTA, we need to determine the bitmap position for its base.

We handle the case of single- and multi-pack bitmaps separately:

  - For multi-pack bitmaps, we look to see if the MIDX selected the
    base we're looking at to represent that object. If the base object
    was selected from a different pack in the MIDX, we won't reuse the
    delta.

  - For single-pack bitmaps, we convert the base object's offset into
    a pack-relative position (identified here as 'base_pos') by
    calling `offset_to_pack_pos()`.

But we also call `offset_to_pack_pos()` before handling each case
above separately. For the MIDX case, this is unnecessary, since we
don't need to lookup the base object's pack-relative position;
instead, we just care whether the selected copy of that base is from
the same pack we're currently operating on.

For the non-MIDX case, we end up calling offset_to_pack_pos() again,
yielding the same result, making the earlier call wasteful. This
behavior dates back to 519e17ff75 (pack-bitmap: prepare to mark
objects from multiple packs for reuse, 2023-12-14).

Let's correct that by avoiding the unconditional call to
'offset_to_pack_pos()'.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
---
 pack-bitmap.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/pack-bitmap.c b/pack-bitmap.c
index 706ff26a7b1..5c5c26efe0d 100644
--- a/pack-bitmap.c
+++ b/pack-bitmap.c
@@ -2069,7 +2069,6 @@ static int try_partial_reuse(struct bitmap_index *bitmap_git,
 
 	if (type == OBJ_REF_DELTA || type == OBJ_OFS_DELTA) {
 		off_t base_offset;
-		uint32_t base_pos;
 		uint32_t base_bitmap_pos;
 
 		/*
@@ -2085,8 +2084,6 @@ static int try_partial_reuse(struct bitmap_index *bitmap_git,
 		if (!base_offset)
 			return 0;
 
-		offset_to_pack_pos(pack->p, base_offset, &base_pos);
-
 		if (bitmap_is_midx(bitmap_git)) {
 			/*
 			 * Cross-pack deltas are rejected for now, but could
@@ -2105,6 +2102,8 @@ static int try_partial_reuse(struct bitmap_index *bitmap_git,
 				return 0;
 			}
 		} else {
+			uint32_t base_pos;
+
 			if (offset_to_pack_pos(pack->p, base_offset,
 					       &base_pos) < 0)
 				return 0;
-- 
2.47.0.11.g487258bca34