From 09e8c253415b8eb9ca29a2131d2ebf17743534c5 Mon Sep 17 00:00:00 2001 From: David Howells Date: Mon, 22 May 2023 21:57:40 +0100 Subject: [PATCH] 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 <+7>: test $0x2,%al <+9>: je 0xffffffff816dac5c <+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 <+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 Reviewed-by: Christoph Hellwig Reviewed-by: John Hubbard cc: Jens Axboe 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 Reviewed-by: Jan Kara Link: https://lore.kernel.org/r/20230522205744.2825689-3-dhowells@redhat.com Signed-off-by: Jens Axboe --- 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 b3e7529..7f53be0 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) -- 2.7.4