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    SubjectRe: [PATCH] x86 rwsem optimization extreme
    On 02/17/2010 02:10 PM, Linus Torvalds wrote:
    >
    >
    > On Wed, 17 Feb 2010, Zachary Amsden wrote:
    >>
    >> The x86 instruction set provides the ability to add an additional
    >> bit into addition or subtraction by using the carry flag.
    >> It also provides instructions to directly set or clear the
    >> carry flag. By forcibly setting the carry flag, we can then
    >> represent one particular 64-bit constant, namely
    >>
    >> 0xffffffff + 1 = 0x100000000
    >>
    >> using only 32-bit values. In particular we can optimize the rwsem
    >> write lock release by noting it is of exactly this form.
    >
    > Don't do this.
    >
    > Just shift the constants down by two, and suddenly you don't need any
    > clever tricks, because all the constants fit in 32 bits anyway,
    > regardless of sign issues.
    >

    Why bother at all? I thought it mattered when I saw __downgrade_write()
    as an inline, but in fact it is only ever used inside the
    downgrade_write() out-of-line function, so we're talking about saving
    *five bytes* across the whole kernel in the best case. I vote for
    leaving it the way it is, and get the very slight extra readability.
    There is no point in moving bits around, either.

    -hpa


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