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 <[email protected]>
 *
 * 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