The R Project SVN R

///////////////////////////////////////////////////////////////////////////////
//
/// \file       bsr.h
/// \brief      Bit scan reverse
//
//  Author:     Lasse Collin
//
//  This file has been put into the public domain.
//  You can do whatever you want with this file.
//
///////////////////////////////////////////////////////////////////////////////

#ifndef LZMA_BSR_H
#define LZMA_BSR_H

// NOTE: Both input and output variables for lzma_bsr must be uint32_t.

#if defined(__GNUC__) && (defined (HAVE_ASM_X86) || defined(HAVE_ASM_X86_64))
#   define lzma_bsr(dest, n) \
        __asm__("bsrl %1, %0" : "=r" (dest) : "rm" (n))

#else
#   define lzma_bsr(dest, n) dest = lzma_bsr_helper(n)

static inline uint32_t
lzma_bsr_helper(uint32_t n)
{
    assert(n != 0);

    uint32_t i = 31;

    if ((n & UINT32_C(0xFFFF0000)) == 0) {
        n <<= 16;
        i = 15;
    }

    if ((n & UINT32_C(0xFF000000)) == 0) {
        n <<= 8;
        i -= 8;
    }

    if ((n & UINT32_C(0xF0000000)) == 0) {
        n <<= 4;
        i -= 4;
    }

    if ((n & UINT32_C(0xC0000000)) == 0) {
        n <<= 2;
        i -= 2;
    }

    if ((n & UINT32_C(0x80000000)) == 0)
        --i;

    return i;
}

#endif

#endif