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t=1684845462; bh=IrFR6hSXFwTPIqL0va6YERj1ltruCOkzI1N56fCgPFM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=BbcPp5Wk39tfRgN0F4hqRWr2z1n0SIKLjtFrqEbV6rdwWzWWBf41JaluXdPwngS7n YkP2M+MNrre1IqYdowqD2gvwspBlw+78DyFX72q66G+F/EgL87TSfBL03dsoLrD1qM au4VSvKklwQat8ncTYQIWu6YEu3YE2e8269d+DGcLRwFi/TP2lU7v9EbyBbjcrqMjT TfL44g4byc+ONKKhrrNPF7Del3TtLGY10AAIErkDGWFzW9t8m2z9qEjENI9kADmq1S RJahiUXw1uO7b7wkD3a2j0nrda5NEi0cA233jlDNrLclS7KunYJMBDqfAzEFmnY2a2 OI0JXdTdk+PsQ== Date: Tue, 23 May 2023 14:37:35 +0200 From: Christian Brauner To: David Howells Cc: Jens Axboe , Al Viro , Christoph Hellwig , Matthew Wilcox , Jan Kara , Jeff Layton , David Hildenbrand , Jason Gunthorpe , Logan Gunthorpe , Hillf Danton , 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: <20230523-kommst-gewechselt-e7b94e891a12@brauner> References: <20230522205744.2825689-1-dhowells@redhat.com> <20230522205744.2825689-3-dhowells@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20230522205744.2825689-3-dhowells@redhat.com> X-Rspamd-Queue-Id: 794FC80018 X-Rspam-User: X-Rspamd-Server: rspam02 X-Stat-Signature: c6di7bhj883ifdygzjcuj7zqn6yrt3d4 X-HE-Tag: 1684845464-690898 X-HE-Meta: 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 h5YjgOXn 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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, 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 > <+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