linux-kernel - [PATCH] x86: fix description of __fls(): _

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Message-ID: <20080705175345.GA7698@mailshack.com>
Date:	Sat, 5 Jul 2008 19:53:46 +0200
From:	Alexander van Heukelum <heukelum@...lshack.com>
To:	"Ricardo M. Correia" <Ricardo.M.Correia@....COM>,
	Ingo Molnar <mingo@...e.hu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>, heukelum@...tmail.fm
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@....COM>
Signed-off-by: Alexander van Heukelum <heukelum@...tmail.fm>

---

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
result.

The inconsistency is well-spotted, though, thanks.

Patch is against current -tip.

Greetings,
    Alexander

> 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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