FFmpeg源码：ff_ctz / ff_ctz

FFmpeg源码：ff_ctz / ff_ctz_c函数分析

news2025/1/11 16:42:49

一、ff_ctz函数的作用

ff_ctz定义在FFmpeg源码目录的libavutil/intmath.h 下：

#ifndef ff_ctz
#define ff_ctz ff_ctz_c
/**
 * Trailing zero bit count.
 *
 * @param v  input value. If v is 0, the result is undefined.
 * @return   the number of trailing 0-bits
 */
/* We use the De-Bruijn method outlined in:
 * http://supertech.csail.mit.edu/papers/debruijn.pdf. */
static av_always_inline av_const int ff_ctz_c(int v)
{
    //...
}
#endif

可以看到ff_ctz函数等价于ff_ctz_c函数。其作用是：如果形参v的值为0，返回0。否则将形参v转成二进制，返回其最后一个“1”后面“0”的个数。

形参v：输入型参数。需要被计算的值。

返回值：将形参v转成二进制，返回的最后一个“1”后面“0”的个数。

ff_ctz函数一般用在统计H.264 RBSP最后补齐了多少个0位。也就是stop bit后面rbsp_alignment_zero_bit的个数。详情可以参考：《音视频入门基础：H.264专题（3）——EBSP, RBSP和SODB》

二、ff_ctz函数的内部实现

#ifndef ff_ctz
#define ff_ctz ff_ctz_c
/**
 * Trailing zero bit count.
 *
 * @param v  input value. If v is 0, the result is undefined.
 * @return   the number of trailing 0-bits
 */
/* We use the De-Bruijn method outlined in:
 * http://supertech.csail.mit.edu/papers/debruijn.pdf. */
static av_always_inline av_const int ff_ctz_c(int v)
{
    static const uint8_t debruijn_ctz32[32] = {
        0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
        31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
    };
    return debruijn_ctz32[(uint32_t)((v & -v) * 0x077CB531U) >> 27];
}
#endif

三、编写测试例子，测试ff_ctz函数

main.c :

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>


#ifdef __GNUC__
#    define AV_GCC_VERSION_AT_LEAST(x,y) (__GNUC__ > (x) || __GNUC__ == (x) && __GNUC_MINOR__ >= (y))
#    define AV_GCC_VERSION_AT_MOST(x,y)  (__GNUC__ < (x) || __GNUC__ == (x) && __GNUC_MINOR__ <= (y))
#else
#    define AV_GCC_VERSION_AT_LEAST(x,y) 0
#    define AV_GCC_VERSION_AT_MOST(x,y)  0
#endif

#ifndef av_always_inline
#if AV_GCC_VERSION_AT_LEAST(3,1)
#    define av_always_inline __attribute__((always_inline)) inline
#elif defined(_MSC_VER)
#    define av_always_inline __forceinline
#else
#    define av_always_inline inline
#endif
#endif

#if AV_GCC_VERSION_AT_LEAST(2,6) || defined(__clang__)
#    define av_const __attribute__((const))
#else
#    define av_const
#endif


#ifndef ff_ctz
#define ff_ctz ff_ctz_c
/**
 * Trailing zero bit count.
 *
 * @param v  input value. If v is 0, the result is undefined.
 * @return   the number of trailing 0-bits
 */
/* We use the De-Bruijn method outlined in:
 * http://supertech.csail.mit.edu/papers/debruijn.pdf. */
static av_always_inline av_const int ff_ctz_c(int v)
{
    static const uint8_t debruijn_ctz32[32] = {
        0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
        31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9
    };
    return debruijn_ctz32[(uint32_t)((v & -v) * 0x077CB531U) >> 27];
}
#endif


int main()
{
    printf("%d\n", ff_ctz(0));
    printf("%d\n", ff_ctz(80));
    printf("%d\n", ff_ctz(100));
    printf("%d\n", ff_ctz(128));
    printf("%d\n", ff_ctz(255));
    return 0;
}

Linux平台下使用gcc编译（我用的是CentOS 7.5，通过10.2.1版本的gcc编译）。输出为：