| Message ID | 20230522205744.2825689-3-dhowells@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | block: Use page pinning | expand |
On Mon 22-05-23 21:57:40, David Howells wrote: > 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 Sure. Feel free to add: Reviewed-by: Jan Kara <jack@suse.cz> Honza > --- > 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 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) >
On Mon, May 22, 2023 at 09:57:40PM +0100, David Howells wrote: > 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>
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)