log2comp.h

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/*! \file
 * \brief log2comp.h - various base 2 log computation versions
 *
 * Asterisk -- An open source telephony toolkit.
 *
 * \author Alex Volkov <codepro@usa.net>
 *
 * Copyright (c) 2004 - 2005, Digium Inc.
 *
 * This program is free software, distributed under the terms of
 * the GNU General Public License
 *
 * Define WANT_ASM before including this file to use assembly
 *   whenever possible
 */

#if defined(_MSC_VER)
#  define inline __inline
#elif defined(__GNUC__)
#  define inline __inline__
#else
#  define inline
#endif

#if defined(WANT_ASM) && defined(_MSC_VER) && defined(_M_IX86)
/* MS C Inline Asm */
#  pragma warning( disable : 4035 )
static inline int ilog2(int val) { __asm
{
   xor      eax, eax
   dec      eax
   bsr      eax, val
}}
#  pragma warning( default : 4035 )
#elif defined(WANT_ASM) && defined(__GNUC__) && (defined(__i386__) || defined(i386))
/* GNU Inline Asm */
static inline int ilog2(int val)
{
   int a;
   __asm__
   ("\
      xorl  %0, %0      ;\
      decl  %0       ;\
      bsrl  %1, %0      ;\
      "
      : "=&r" (a)
      : "mr" (val)
      : "cc"
   );
   return a;
}
#elif defined(WANT_ASM) && defined(__GNUC__) && defined(__powerpc__)
static inline int ilog2(int val)
{
   int a;
   __asm__ ("cntlzw %0,%1"
       : "=r" (a)
       : "r" (val)
       );
   return 31-a;
}
#else
/* no ASM for this compiler and/or platform */
/* rather slow base 2 log computation
 * Using looped shift.
 */
static inline int ilog2(int val)
{
   int i;
   for (i = -1; val; ++i, val >>= 1)
      ;
   return (i);
}
#endif