diff options
| author | David Howells | 2023-05-22 21:57:40 +0100 |
|---|---|---|
| committer | Jens Axboe | 2023-05-24 08:42:44 -0600 |
| commit | 09e8c253415b8eb9ca29a2131d2ebf17743534c5 (patch) | |
| tree | 7c481e23f004a7c27018a2e5bce174774ce0114d | |
| parent | a450f49708ea2ccabd1c5d2fe8a702ca5ef77941 (diff) | |
block: Fix bio_flagged() so that gcc can better optimise it
Fix bio_flagged() so that multiple instances of it, such as:
if (bio_flagged(bio, BIO_PAGE_REFFED) ||
bio_flagged(bio, BIO_PAGE_PINNED))
can be combined by the gcc optimiser into a single test in assembly
(arguably, this is a compiler optimisation issue[1]).
The missed optimisation stems from bio_flagged() comparing the result of
the bitwise-AND to zero. This results in an out-of-line bio_release_page()
being compiled to 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>
However, the test is superfluous as the return type is bool. Removing it
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.
Also, the MOVZBL instruction looks unnecessary[2] - I think it's just
're-booling' the mark_dirty parameter.
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: John Hubbard <jhubbard@nvidia.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]
Link: https://lore.kernel.org/r/167391056756.2311931.356007731815807265.stgit@warthog.procyon.org.uk/ # v6
Reviewed-by: Christian Brauner <brauner@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230522205744.2825689-3-dhowells@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
| -rw-r--r-- | include/linux/bio.h | 2 |
1 files changed, 1 insertions, 1 deletions
diff --git a/include/linux/bio.h b/include/linux/bio.h index b3e7529ff55e..7f53be035cf0 100644 --- a/include/linux/bio.h +++ b/include/linux/bio.h @@ -229,7 +229,7 @@ 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) |