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

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Date:	Fri, 24 Apr 2009 09:51:21 -0400 (EDT)
From:	"Robert P. J. Day" <rpjday@...shcourse.ca>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	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 Thu, 23 Apr 2009, Andrew Morton wrote:

> On Thu, 23 Apr 2009 12:57:11 -0700
> David Daney <ddaney@...iumnetworks.com> wrote:
>
> > > +static inline __attribute__((const))
> > > +bool single_bit_set(unsigned long n)
> > > +{
> > > +        return (n != 0 && ((n & (n - 1)) == 0));
> > > +}
> > > +
> > > +
> >
> >
> > It would be nice to be able to override this per architecture.
> >
> > For example a more efficient implementation on CPUs that have a
> > population count instruction (__builtin_popcountl()) might be:
> >
> > static inline __attribute__((const))
> > bool singe_bit_set(unsigned long n)
> > {
> > 	return __builtin_popcountl(n) == 1;
> > }
>
> Already done, via hweight_long().

  i'm guessing that taking advantage of helper functions like that
might simplify code like, say, this from fs/nfs/internal.h:

/*
 * Determine the actual block size (and log2 thereof)
 */
static inline
unsigned long nfs_block_bits(unsigned long bsize, unsigned char *nrbitsp)
{
        /* make sure blocksize is a power of two */
        if ((bsize & (bsize - 1)) || nrbitsp) {
                unsigned char   nrbits;

                for (nrbits = 31; nrbits && !(bsize & (1 << nrbits)); nrbits--)
                        ;
                bsize = 1 << nrbits;
                if (nrbitsp)
                        *nrbitsp = nrbits;
        }

        return bsize;
}


  surely, there's a simpler way to write that.

rday
--

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

        Linux Consulting, Training and Annoying Kernel Pedantry.

Web page:                                          http://crashcourse.ca
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