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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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