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    SubjectRe: [PATCH] include/log2.h: Fix rounddown_pow_of_two(1)
    On Mon, Dec 12, 2011 at 03:50:11PM -0800, Linus Torvalds wrote:
    > On Mon, Dec 12, 2011 at 10:02 AM, Dmitry Torokhov <dtor@vmware.com> wrote:
    > > From: Andrei Warkentin <andreiw@vmware.com>
    > >
    > > 1 is a power of two, therefore rounddown_pow_of_two(1) should return 1.
    > > It does in case the argument is a variable but in case it's a constant
    > > it behaves incorrectly and returns 0. Probably nobody ever did it so
    > > this was never noticed, however drivers/net/vmxnet3 with latest GCC does
    > > and breaks on unicpu systems.
    > >
    > > This is similar to Rolf's patch to roundup_pow_of_two(1).
    >
    > Umm. I already applied this patch, but then I started looking at it
    > more, and asked myself:
    >
    > - Why is that "n == 1" test there AT ALL?
    >
    > Afaik, that whole test is just plain stupid. It seems to have been
    > copied from the "roundup()" case (where it exists due to the "-1/+1"
    > hackery that breaks ilog2()) without any thought about the actual math
    > of the function at all.
    >
    > I think the *real* fix is to just remove that incorrect line, no?

    Yes, you are right, special-casing for 1 is not necessary in rounddown
    case.

    Thanks,
    Dmitry



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