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SubjectRe: [PATCH v9 4/8] block: Fix bio_flagged() so that gcc can better optimise it
Date
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

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