Date: Wed, 7 Feb 2007 02:49:18 -0800 From: Luigi Rizzo <rizzo@icir.org> To: Stefan Bethke <stb@lassitu.de> Cc: current@freebsd.org Subject: Re: C macro to find the next power of 2 ? Message-ID: <20070207024918.D63529@xorpc.icir.org> In-Reply-To: <15241.SV0BLFJbGwk=.1170843949.squirrel@koef.zs64.net>; from stb@lassitu.de on Wed, Feb 07, 2007 at 11:25:49AM %2B0100 References: <20070207004131.A62183@xorpc.icir.org> <15241.SV0BLFJbGwk=.1170843949.squirrel@koef.zs64.net>
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On Wed, Feb 07, 2007 at 11:25:49AM +0100, Stefan Bethke wrote:
> On Wed, February 7, 2007 09:41, Luigi Rizzo wrote:
> > My problem is that some hardware wants data structures aligned to
> > the next power of 2 with respect to their size, and the code
> > in question (the ehci driver in case you care) has hardwired
> > constants for this, and possibly wrong ones.
> >
> > It would be nice if one could write
> >
> > struct foo_desc {
> > ...
> > };
> >
> > #define FOO_ALIGN next_power_of_2(sizeof(struct foo_desc))
> >
> > instead of having to count fields and make guesses on the size
> > of pointers and so on.
>
> _Hacker's Delight_ contains many cool tricks, including multiple solutions
> for this, IIRC. I'll have a look tonight when I'm back home.
funny it looks like the same code that i posted,
except that mine is functional and theirs procedural:
my version:
#define b2(x) ( (x) | ( (x) >> 1) )
#define b4(x) ( b2(x) | ( b2(x) >> 2) )
#define b8(x) ( b4(x) | ( b4(x) >> 4) )
#define b16(x) ( b8(x) | ( b8(x) >> 8) )
#define b32(x) (b16(x) | (b16(x) >>16) )
#define next_power_of_2(x) (b32(x-1) + 1)
Hacker's Delight version
unsigned clp2(unsigned x) {
x = x - 1;
x = x | (x >> 1);
x = x | (x >> 2);
x = x | (x >> 4);
x = x | (x >> 8);
x = x | (x >>16);
return x + 1;
}
thanks for the pointer.
luigi
>
> Cheers,
> Stefan
>
> --
> Stefan Bethke <stb@lassitu.de> +49 170 3460140
>
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