linux-kernel - Re: [PATCH] Introduce a boolean "single_bit

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Message-ID: <alpine.LFD.2.00.0906291448510.3864@localhost>
Date:	Mon, 29 Jun 2009 14:50:31 -0400 (EDT)
From:	"Robert P. J. Day" <rpjday@...shcourse.ca>
To:	Petr Tesarik <ptesarik@...e.cz>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Daney <ddaney@...iumnetworks.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Introduce a boolean "single_bit_set" function.

On Mon, 29 Jun 2009, Petr Tesarik wrote:

> Andrew Morton píše v Pá 24. 04. 2009 v 10:46 -0700:
> > On Fri, 24 Apr 2009 06:40:39 -0400 (EDT) "Robert P. J. Day" <rpjday@...shcourse.ca> wrote:
> >
> > >   so it would be a simple matter to define the bit set boolean in
> > > terms of hweight_long(), yes?  so what about, in bitops.h:
> > >
> > >   static inline bool
> > >   exactly_one_bit_set(unsigned long w)
> > >   {
> > > 	return hweight_long(w) == 1;
> > >   }
> > >
> > >   static inline bool
> > >   more_than_one_bit_set(unsigned long w)
> > >   {
> > > 	return hweight_long(w) > 1;
> > >   }
> > >
>
> Andrew, you must be kidding! Are you seriously suggesting to replace
> a simple and instruction with a call to an extern library function
> with 17 instructions (not including the call and ret)?
>
> I'd better check the use of hweight in the kernel to eradicate as
> many calls to it as possible...

  since i originally muttered about this, the rationale behind it was
not for performance (obviously), but for semantic clarification, so
that when you saw the expression "n & (n-1)", it was more obvious
which test you were doing semantically:

1) is n a power of 2?
2) does n represent a single set bit?

nothing ever came of that, but that was the thinking behind it.

rday
--

========================================================================
Robert P. J. Day                               Waterloo, Ontario, CANADA

        Linux Consulting, Training and Annoying Kernel Pedantry.

Web page:                                          http://crashcourse.ca
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