codecs/log2comp.h

   1 /*! \file
   2  * \brief log2comp.h - various base 2 log computation versions
   3  *
   4  * Asterisk -- A telephony toolkit for Linux.
   5  *
   6  * \author Alex Volkov <codepro@usa.net>
   7  *
   8  * Copyright (c) 2004 - 2005, Digium Inc.
   9  *
  10  * This program is free software, distributed under the terms of
  11  * the GNU General Public License
  12  *
  13  * Define WANT_ASM before including this file to use assembly
  14  *   whenever possible
  15  */
  16
  17 #if defined(_MSC_VER)
  18 #       define inline __inline
  19 #elif defined(__GNUC__)
  20 #       define inline __inline__
  21 #else
  22 #       define inline
  23 #endif
  24
  25 #if defined(WANT_ASM) && defined(_MSC_VER) && defined(_M_IX86)
  26 /* MS C Inline Asm */
  27 #       pragma warning( disable : 4035 )
  28 static inline int ilog2(int val) { __asm
  29 {
  30         xor             eax, eax
  31         dec             eax
  32         bsr             eax, val
  33 }}
  34 #       pragma warning( default : 4035 )
  35 #elif defined(WANT_ASM) && defined(__GNUC__) && (defined(__i386__) || defined(i386))
  36 /* GNU Inline Asm */
  37 static inline int ilog2(int val)
  38 {
  39         int a;
  40         __asm__
  41         ("\
  42                 xorl    %0, %0          ;\
  43                 decl    %0                      ;\
  44                 bsrl    %1, %0          ;\
  45                 "
  46                 : "=&r" (a)
  47                 : "mr" (val)
  48                 : "cc"
  49         );
  50         return a;
  51 }
  52 #elif defined(WANT_ASM) && defined(__GNUC__) && defined(__powerpc__)
  53 static inline int ilog2(int val)
  54 {
  55         int a;
  56         __asm__ ("cntlzw %0,%1"
  57                  : "=r" (a)
  58                  : "r" (val)
  59                  );
  60         return 31-a;
  61 }
  62 #else
  63 /* no ASM for this compiler and/or platform */
  64 /* rather slow base 2 log computation
  65  * Using looped shift.
  66  */
  67 static inline int ilog2(int val)
  68 {
  69         int i;
  70         for (i = -1; val; ++i, val >>= 1)
  71                 ;
  72         return (i);
  73 }
  74 #endif