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