Re: proposal: some pointer and arithmetic utilities

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

   Date: Thu, 21 Mar 2013 18:42:16 +0000
   From: Taylor R Campbell <riastradh%NetBSD.org@localhost>

   /* typeof-free roundup2/rounddown2 */
   #define      roundup2(x, m)  (((x) + ((m) - 1)) & ~((m) - 1 + ((x) - (x))))
   #define      rounddown2(x,m) ((x) & ~((m) - 1 + ((x) - (x))))

   /* multiple-evaluation-free roundup2/rounddown2 */
   #define      roundup2(x, m)  ((((x) - 1) | ((m) - 1)) + 1)
   #define      rounddown2(x,m) ((x) &~ ((typeof(x))((m) - 1)))

Another possibility for rounddown2 which avoids multiple evaluation
and typeof, but requires uintmax_t and wants a little help from the
compiler, is

#define rounddown2(x,m) ((x) & ~(uintmax_t)((m) - 1))

or

#define rounddown2(x,m) ((x) & -(uintmax_t)(m))

to make it a little terser.