linux-kernel - Re: [PATCH v9 4/8] block: Fix bio_flagged() so that gcc can better optimise it

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Message-ID: <1297525.1674593711@warthog.procyon.org.uk>
Date:   Tue, 24 Jan 2023 20:55:11 +0000
From:   David Howells <dhowells@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     dhowells@...hat.com, Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, "Jan Kara" <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, "Christoph Hellwig" <hch@....de>
Subject: Re: [PATCH v9 4/8] block: Fix bio_flagged() so that gcc can better optimise it

John Hubbard <jhubbard@...dia.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