[PATCH v5 9/9] bio: Fix bio_flagged() so that it can be combined

David Howells
2023-01-11T14:34:37Z

Fix bio_flagged() so that multiple instances of it can be combined by the
compiler into a single test (arguably, this is a compiler optimisation
issue[1]).

The problem is that it compares the result of the bitwise-AND to zero.
This results in an out-of-line bio_release_page() that looks something
like:

<+0>: mov 0x14(%rdi),%eax
<+3>: test $0x1,%al
<+5>: jne 0xffffffff816dac53 <bio_release_pages+11>
<+7>: test $0x2,%al
<+9>: je 0xffffffff816dac5c <bio_release_pages+20>
<+11>: movzbl %sil,%esi
<+15>: jmp 0xffffffff816daba1 <__bio_release_pages>
<+20>: jmp 0xffffffff81d0b800 <__x86_return_thunk>

Removing the test (it's superfluous as the return type is bool - the
compiler will reduce the return to 0 or 1 as needed) results in:

<+0>: testb $0x3,0x14(%rdi)
<+4>: je 0xffffffff816e4af4 <bio_release_pages+15>
<+6>: movzbl %sil,%esi
<+10>: jmp 0xffffffff816dab7c <__bio_release_pages>
<+15>: jmp 0xffffffff81d0b7c0 <__x86_return_thunk>

instead.

The MOVZBL instruction also looks unnecessary[2] - I think it's just
're-booling' the mark_dirty.

Fixes: b7c44ed9d2fc ("block: manipulate bio->bi_flags through helpers")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: linux-block@vger.kernel.org
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108370 [1]
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108371 [2]
---

include/linux/bio.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/bio.h b/include/linux/bio.h
index 1c6f051f6ff2..2e6109b0fca8 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -227,7 +227,7 @@ static inline void bio_cnt_set(struct bio *bio, unsigned int count)

static inline bool bio_flagged(struct bio *bio, unsigned int bit)
{
- return (bio->bi_flags & (1U << bit)) != 0;
+ return bio->bi_flags & (1U << bit);
}

static inline void bio_set_flag(struct bio *bio, unsigned int bit)