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 > > _______________________________________________ > freebsd-current@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-current > To unsubscribe, send any mail to "freebsd-current-unsubscribe@freebsd.org"
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