Messages in this thread | | | From | David Howells <> | Subject | Re: [PATCH v9 4/8] block: Fix bio_flagged() so that gcc can better optimise it | Date | Tue, 24 Jan 2023 20:55:11 +0000 |
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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
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