com32/include/ilog2.h

   1 /* ----------------------------------------------------------------------- *
   2  *
   3  *   Copyright 2010 Intel Corporation; author: H. Peter Anvin
   4  *
   5  *   Permission is hereby granted, free of charge, to any person
   6  *   obtaining a copy of this software and associated documentation
   7  *   files (the "Software"), to deal in the Software without
   8  *   restriction, including without limitation the rights to use,
   9  *   copy, modify, merge, publish, distribute, sublicense, and/or
  10  *   sell copies of the Software, and to permit persons to whom
  11  *   the Software is furnished to do so, subject to the following
  12  *   conditions:
  13  *
  14  *   The above copyright notice and this permission notice shall
  15  *   be included in all copies or substantial portions of the Software.
  16  *
  17  *   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
  18  *   EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
  19  *   OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
  20  *   NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
  21  *   HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
  22  *   WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
  23  *   FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
  24  *   OTHER DEALINGS IN THE SOFTWARE.
  25  *
  26  * ----------------------------------------------------------------------- */
  27
  28 #ifndef _ILOG2_H
  29 #define _ILOG2_H
  30
  31 /*
  32  * Computes floor(log2(x)) -- undefined for x = 0.
  33  */
  34
  35 #include <klibc/compiler.h>
  36 #include <stdint.h>
  37
  38 static inline __constfunc uint32_t ilog2(uint32_t __v)
  39 {
  40 # if __GNUC__ >= 4
  41     return __builtin_clz(__v) ^ 31;
  42 # else
  43     asm("bsrl %1,%0" : "=r" (__v) : "rm" (__v));
  44     return __v;
  45 # endif
  46 }
  47
  48 #endif /* _ILOG2_H */