Date: Wed, 7 Feb 2007 02:59:16 -0800 From: Luigi Rizzo <rizzo@icir.org> To: Attilio Rao <attilio@freebsd.org> Cc: Stefan Bethke <stb@lassitu.de>, current@freebsd.org Subject: Re: C macro to find the next power of 2 ? Message-ID: <20070207025916.A63989@xorpc.icir.org> In-Reply-To: <3bbf2fe10702070255q58ab6c9fn2b393cfbc75a3fb@mail.gmail.com>; from attilio@freebsd.org on Wed, Feb 07, 2007 at 11:55:27AM %2B0100 References: <20070207004131.A62183@xorpc.icir.org> <15241.SV0BLFJbGwk=.1170843949.squirrel@koef.zs64.net> <20070207024918.D63529@xorpc.icir.org> <3bbf2fe10702070255q58ab6c9fn2b393cfbc75a3fb@mail.gmail.com>
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On Wed, Feb 07, 2007 at 11:55:27AM +0100, Attilio Rao wrote: > 2007/2/7, Luigi Rizzo <rizzo@icir.org>: > > 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; > > } > > You cannot use this unless you don't rewrite as a preprocessing stub. that's why i am using my version and wrote it functionally. luigi
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