linux-kernel - Re: [PATCH] x86: merge the simple bitops and move them to bitops.h

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Date:	Fri, 14 Mar 2008 23:18:45 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Alexander van Heukelum <heukelum@...tmail.fm>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Alexander van Heukelum <heukelum@...lshack.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: merge the simple bitops and move them to bitops.h

> #else
> static inline int fls64(__u64 x)
> {
>         if (x == 0)
>                 return 0;
>         return __fls(x) + 1;

That would require a polymorphic macro __fls that adapts to 32bit and 64bit
arguments.  Not good C style.

> This is the only reason that this define exists. With another
> name it would be fine. HWEIGHT_USE_MULTIPLIER?

AFAIK it only exists because some ancient sparc chips had incredibly
slow multipliers.

> And my feeling is that this is exactly the reason why this is
> not a good version for a generic implementation in bitops.h. But
> I don't care much.

I bet most different approaches who might be slightly
faster for larger bit strings would make the one bit 
case slower.

-Andi

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