关于Android下FFmpeg配合libx264 NEON默认开启的验证

起因

近期工作涉及到在android上做视频编码, 除了利用系统自带的MediaCodec做硬编码外, 同时需要软编码以保证适配性. 由于编码对CPU的消耗比较大, 自然就想到要开启arm NEON的优化. 而之前接触WebRtc代码的时候发现, 其中的信号处理函数在初始化时, 会动态判断当前的CPU版本来决定加载普通指令版本还是加载NEON指令的版本. 而从x264的代码来看, 发现默认针对arm编译的动态库下面是有neon版本的函数的! 于是猜想现在的的x264是可以自动判断cpu来开启neon的, 不需要额外配置, 事实证明我的猜想是对的. 下面是验证过程, 欢迎讨论和指正.

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配置信息

环境变量:

CROSS_PRIFIX=/YOUR_PATH/android-ndk-r13b/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/bin/arm-linux-androideabi-
SYS_ROOT=/YOUR_PATH/android-ndk-r13b/platforms/android-14/arch-arm

ffmpeg: 2.7.7

export PKG_CONFIG_PATH=/YOUR_PATH/x264/lqp_install/lib/pkgconfig
配置pkg_config是为了让ffmpeg找到x264的头文件, 编译时要用到

./configure –disable-doc –sysroot=${SYS_ROOT} –cross-prefix=${CROSS_PRIFIX} –prefix=`pwd`/lqp_install –pkg-config=pkg-config –target-os=linux –arch=arm –disable-avdevice –disable-all –enable-static –enable-libx264 –enable-decoder=h264 –enable-encoder=libx264 –enable-swscale –enable-avutil –enable-avcodec –enable-gpl –extra-cflags=’-fvisibility=hidden’ –extra-cxxflags=’-fvisibility=hidden’
注意没有明确配置NEON

x264: r2744M b97ae06

./configure –sysroot=${SYS_ROOT} –cross-prefix=${CROSS_PRIFIX} –prefix=`pwd`/lqp_install –host=arm-linux –disable-cli –enable-shared
同样的, 没有配置NEON

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验证过程

在ffmpeg/libavcodec/libx264.c中可以看到编码器初始化函数:

static av_cold int X264_init(AVCodecContext *avctx)
{
    X264Context *x4 = avctx->priv_data;
    ...
    //lqp comment: 注意这里调用了x264_param_default, 是x264库提供的初始化函数
    x264_param_default(&x4->params); 
    ...
 }

跟进:

void x264_param_default( x264_param_t *param )
{
    /* */
    memset( param, 0, sizeof( x264_param_t ) );

    /* CPU autodetect */
    param->cpu = x264_cpu_detect();

    //lqp comment: add print start
    if (!printed) {
        int curPos = strlen(x264InitBuffer);
        snprintf(x264InitBuffer + curPos, sizeof(x264InitBuffer) - curPos, "x264_param_default-> param->cpu: %d, ", param->cpu);

        printed = 1;
    }
    //lqp comment: add print end

上面的x264初始化过程中, 我添加了打印获取到的flag代码, 同时可以发现这么一行注释:

/* CPU autodetect */

人家注释里都明白说了CPU自动检测了…, 但是为了验证完整, 还是继续做下去吧~~

上面获取的flag会产生什么影响呢? 简单来说, 就是后面在初始化编码器的时候, 会根据flag来判断CPU特性, 然后调用NEON的处理函数. 举个例子:

x264/common/dct.c:

/****************************************************************************
 * x264_dct_init:
 ****************************************************************************/
void x264_dct_init( int cpu, x264_dct_function_t *dctf )
{
    dctf->sub4x4_dct    = sub4x4_dct;
    dctf->add4x4_idct   = add4x4_idct;

    dctf->sub8x8_dct    = sub8x8_dct;
    dctf->sub8x8_dct_dc = sub8x8_dct_dc;
    dctf->add8x8_idct   = add8x8_idct;
    dctf->add8x8_idct_dc = add8x8_idct_dc;

    dctf->sub8x16_dct_dc = sub8x16_dct_dc;

    dctf->sub16x16_dct  = sub16x16_dct;
    dctf->add16x16_idct = add16x16_idct;
    dctf->add16x16_idct_dc = add16x16_idct_dc;

    dctf->sub8x8_dct8   = sub8x8_dct8;
    dctf->add8x8_idct8  = add8x8_idct8;

    dctf->sub16x16_dct8  = sub16x16_dct8;
    dctf->add16x16_idct8 = add16x16_idct8;

    dctf->dct4x4dc  = dct4x4dc;
    dctf->idct4x4dc = idct4x4dc;

    dctf->dct2x4dc = dct2x4dc;

    //lqp comment: print start
    static int initPrinted;
    int pos = strlen(x264InitBuffer);
    if (!initPrinted) {
        pos += snprintf(x264InitBuffer + pos, sizeof(x264InitBuffer) - pos, "x264_dct_init begin: cpu:%d, detected: %d", cpu, x264_cpu_detect());
    }
    //lqp comment: print end
    ...

    #if HAVE_ARMV6 || ARCH_AARCH64
    if( cpu&X264_CPU_NEON )
    {
        //lqp comment: print start
        if (!initPrinted) {
            pos += snprintf(x264InitBuffer + pos, sizeof(x264InitBuffer) - pos, ", neon selected!!!");
            initPrinted = 1;
        }
        //lqp comment: print end
        dctf->sub4x4_dct    = x264_sub4x4_dct_neon;
        dctf->sub8x8_dct    = x264_sub8x8_dct_neon;
        dctf->sub16x16_dct  = x264_sub16x16_dct_neon;
        dctf->add8x8_idct_dc = x264_add8x8_idct_dc_neon;
        dctf->add16x16_idct_dc = x264_add16x16_idct_dc_neon;
        dctf->sub8x8_dct_dc = x264_sub8x8_dct_dc_neon;
        dctf->dct4x4dc      = x264_dct4x4dc_neon;
        dctf->idct4x4dc     = x264_idct4x4dc_neon;

        dctf->add4x4_idct   = x264_add4x4_idct_neon;
        dctf->add8x8_idct   = x264_add8x8_idct_neon;
        dctf->add16x16_idct = x264_add16x16_idct_neon;

        dctf->sub8x8_dct8   = x264_sub8x8_dct8_neon;
        dctf->sub16x16_dct8 = x264_sub16x16_dct8_neon;

        dctf->add8x8_idct8  = x264_add8x8_idct8_neon;
        dctf->add16x16_idct8= x264_add16x16_idct8_neon;
        dctf->sub8x16_dct_dc= x264_sub8x16_dct_dc_neon;
    }
#endif
    ...
}

上面这个是编码器在初始化的时候会调用到的一个模块处理函数, 细心观察可以发现,里面的一些函数会根据cpu的flag把其中的处理函数指向NEON版本, 同时我在里面做了打印, 如果运行的时候跑到了这里, 说明用的确实是NEON版本的函数.

让我们怀疑更近一步, NEON函数在编译后是不是确实是NEON指令? 就拿

dctf->sub4x4_dct    = x264_sub4x4_dct_neon;

这个来开刀看看吧

于是, 打开反汇编神器IDA Pro一看:

嗯, 这下确定了, 的确是看起来很奇怪的NEON的指令. 那么只待后面运行打印出来的信息了.
于是在jni调用的地方加了这么一行打印:

SCALE_JNI_LOG(LOG_TAG, "!!!!!!x264InitBuffer: %s", x264InitBuffer);

直接上结果:

04-28 13:35:58.802: I/scale-jni(10189): !!!!!!x264InitBuffer: x264_param_default-> param->cpu: 3, x264_dct_init begin: cpu:3, detected: 3, neon selected!!!

那么看到了, 的确是用的NEON版本! 进一步解释一下, cpu flag等于3表示什么意思?
看看flag相关的宏定义:

/* ARM and AArch64 */
#define X264_CPU_ARMV6           0x0000001
#define X264_CPU_NEON            0x0000002  /* ARM NEON */

3表示X264_CPU_ARMV6且X264_CPU_NEON

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证毕

为什么会写这篇博客

在验证之前, 在网上找了很多文章, 但是都是通过很复杂的配置编译选项来做, 但是心中一直有疑问, 既然可以自动判断CPU了, 那自然可以直接按NEON版本的进行编译, 在遇到不支持的机型时, 退化到普通版本的函数就行了. 我想x264和ffmpeg应该不会这么不智能吧?希望能帮助到一些人.