Date: Sat, 05 Jul 2008 17:56:37 +0100
From: "Ricardo M. Correia" <Ricardo.M.Correia@Sun.COM>
Subject: Re: [3/3] Use __fls for fls64 on 64-bit archs
In-reply-to: <20080315173236.GC21659@mailshack.com>
To: Alexander van Heukelum <heukelum@mailshack.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
       linux-arch <linux-arch@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
       Andi Kleen <andi@firstfloor.org>, LKML <linux-kernel@vger.kernel.org>,
       heukelum@fastmail.fm
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(Sorry, sending this again as I screwed up the previous mail).

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.
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


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