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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <ad671496-2461-0b25-48c8-bf474fffe41d@nvidia.com>
References: <ad671496-2461-0b25-48c8-bf474fffe41d@nvidia.com> <20230124170108.1070389-1-dhowells@redhat.com> <20230124170108.1070389-5-dhowells@redhat.com>
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     dhowells@redhat.com, Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Matthew Wilcox <willy@infradead.org>,
        Jens Axboe <axboe@kernel.dk>, "Jan Kara" <jack@suse.cz>,
        Jeff Layton <jlayton@kernel.org>,
        David Hildenbrand <david@redhat.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, "Christoph Hellwig" <hch@lst.de>
Subject: Re: [PATCH v9 4/8] block: Fix bio_flagged() so that gcc can better optimise it
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Date:   Tue, 24 Jan 2023 20:55:11 +0000
Message-ID: <1297525.1674593711@warthog.procyon.org.uk>
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John Hubbard <jhubbard@nvidia.com> wrote:

> I don't know how you noticed that this was even a problem! Neatly
> fixed.

I wanted BIO_PAGE_REFFED/PINNED to translate to FOLL_GET/PIN with no more than
a single AND instruction, assuming they were assigned to the same values (1 &
2), so I checked to see what assembly was produced by:

	gup_flags |= bio_flagged(bio, BIO_PAGE_REFFED) ? FOLL_GET : 0;
	gup_flags |= bio_flagged(bio, BIO_PAGE_PINNED) ? FOLL_PIN : 0;

Complicated though it looks, it should optimise down to something like:

	and $3,%eax

assuming something like REFFED/GET == 0x1 and PINNED/PIN == 0x2.

David