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    Subject[PATCH] x86: fix description of __fls(): __fls(0) is undefined
    Ricardo M. Correia spotted that the use of __fls() in fls64() did
    not seem to make sense. In fact fls64()'s implementation is fine,
    but the description of __fls() was wrong. Fix that.

    Reported-by: "Ricardo M. Correia" <Ricardo.M.Correia@Sun.COM>
    Signed-off-by: Alexander van Heukelum <>


    On Sat, Jul 05, 2008 at 05:56:37PM +0100, Ricardo M. Correia wrote:
    > Hi,
    > I have a question about fls64() which I hope you or someone else could
    > clarify, please see below.
    > On Sáb, 2008-03-15 at 18:32 +0100, Alexander van Heukelum wrote:
    > > +#elif BITS_PER_LONG == 64
    > > +static inline int fls64(__u64 x)
    > > +{
    > > + if (x == 0)
    > > + return 0;
    > > + return __fls(x) + 1;
    > > +}
    > It seems fls64() is implemented on top of __fls(), however the __fls()
    > implementation on the x86-64 architecture states that the result is
    > undefined if the argument does not have any zero bits.

    You have found a bug. It's not in fls64, though, but a copy/paste
    one in the comment preceding __fls(). __fls() gives an undefined
    result if there are no _set_ bits: only __fls(0) gives an undefined

    The inconsistency is well-spotted, though, thanks.

    Patch is against current -tip.


    > So if I understand correctly, the statement "fls64(~0ULL)" would return
    > an undefined result on x64-64 instead of 64 as one would expect.
    > Wouldn't it make sense to check for ~0ULL in fls64()?
    > Thanks,
    > Ricardo


    diff --git a/include/asm-x86/bitops.h b/include/asm-x86/bitops.h
    index 96b1829..cfb2b64 100644
    --- a/include/asm-x86/bitops.h
    +++ b/include/asm-x86/bitops.h
    @@ -356,7 +356,7 @@ static inline unsigned long ffz(unsigned long word)
    * __fls: find last set bit in word
    * @word: The word to search
    - * Undefined if no zero exists, so code should check against ~0UL first.
    + * Undefined if no set bit exists, so code should check against 0 first.
    static inline unsigned long __fls(unsigned long word)
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      Last update: 2008-07-05 19:59    [W:0.022 / U:71.368 seconds]
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