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Tue, 23 May 2023 08:07:20 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id 9BWLJjh0bGQ0MQAAMHmgww (envelope-from ); Tue, 23 May 2023 08:07:20 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id 43C4FA075D; Tue, 23 May 2023 10:07:20 +0200 (CEST) Date: Tue, 23 May 2023 10:07:20 +0200 From: Jan Kara To: David Howells Cc: Jens Axboe , Al Viro , Christoph Hellwig , Matthew Wilcox , Jan Kara , Jeff Layton , David Hildenbrand , Jason Gunthorpe , Logan Gunthorpe , Hillf Danton , Christian Brauner , Linus Torvalds , linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, Christoph Hellwig , John Hubbard Subject: Re: [PATCH v21 2/6] block: Fix bio_flagged() so that gcc can better optimise it Message-ID: <20230523080720.3eovz5wbwmpckhsm@quack3> References: <20230522205744.2825689-1-dhowells@redhat.com> <20230522205744.2825689-3-dhowells@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230522205744.2825689-3-dhowells@redhat.com> X-Rspamd-Queue-Id: 0CDBF18000D X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: 35cjjt6rcwbc979ngespmu3pj1dasco5 X-HE-Tag: 1684829241-794266 X-HE-Meta: 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 eZ07Hhn7 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 > <+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 Sure. Feel free to add: Reviewed-by: Jan Kara 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) > -- Jan Kara SUSE Labs, CR