[PATCH 4/7] btrfs: accessors: compile-time fast path for u8

David Sterba
2025-07-01T17:24:13Z

Reading/writing 1 byte (u8) is a special case compared to the others as
it's always contained in the folio we find, so the split memcpy will
never be needed. Turn it to a compile-time check that the memcpy part
can be optimized out.

The stack usage is reduced:

btrfs_set_8 -16 (32 -> 16)
btrfs_get_8 -16 (24 -> 8)

Code size reduction:

text data bss dec hex filename
1454951 115665 16088 1586704 183610 pre/btrfs.ko
1454691 115665 16088 1586444 18350c post/btrfs.ko

DELTA: -260

Signed-off-by: David Sterba <dsterba@suse.com>
---
fs/btrfs/accessors.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/accessors.c b/fs/btrfs/accessors.c
index a7b6b2d7bde224..547e9f8fb87d61 100644
--- a/fs/btrfs/accessors.c
+++ b/fs/btrfs/accessors.c
@@ -55,7 +55,8 @@ u##bits btrfs_get_##bits(const struct extent_buffer *eb, \
report_setget_bounds(eb, ptr, off, sizeof(u##bits)); \
return 0; \
} \
- if (INLINE_EXTENT_BUFFER_PAGES == 1 || likely(sizeof(u##bits) <= part)) \
+ if (INLINE_EXTENT_BUFFER_PAGES == 1 || sizeof(u##bits) == 1 || \
+ likely(sizeof(u##bits) <= part)) \
return get_unaligned_le##bits(kaddr + oil); \
\
memcpy(lebytes, kaddr + oil, part); \
@@ -78,7 +79,7 @@ void btrfs_set_##bits(const struct extent_buffer *eb, void *ptr, \
report_setget_bounds(eb, ptr, off, sizeof(u##bits)); \
return; \
} \
- if (INLINE_EXTENT_BUFFER_PAGES == 1 || \
+ if (INLINE_EXTENT_BUFFER_PAGES == 1 || sizeof(u##bits) == 1 || \
likely(sizeof(u##bits) <= part)) { \
put_unaligned_le##bits(val, kaddr + oil); \
return; \
--
2.49.0