老规矩，编译环境：Win10 1903，WSL ubuntu，ndk r20b，FFmpeg 4.2.2，首先分享下编译脚本：
#!/bin/bash
make clean
export NDK=/home/kres/android-ndk-r20b
TOOLCHAIN=$NDK/toolchains/llvm/prebuilt/linux-x86_64/
API=21
function build_android
{
echo "Compiling FFmpeg for $CPU"
./configure \
--prefix=$PREFIX \
--enable-neon \
--enable-hwaccels \
--enable-gpl \
--enable-postproc \
--enable-shared \
--enable-jni \
--enable-mediacodec \
--disable-decoders \
--enable-decoder=h264_mediacodec \
--enable-decoder=vp9 \
--enable-decoder=h264 \
--enable-decoder=mpeg4 \
--enable-decoder=aac \
--enable-decoder=aac_latm \
--enable-decoder=mjpeg \
--enable-decoder=png \
--enable-decoder=mpeg4_mediacodec \
--disable-encoders \
--enable-encoder=vp9_vaapi --enable-encoder=h264_nvenc --enable-encoder=h264_v4l2m2m --enable-encoder=hevc_nvenc \
--disable-demuxers \
--enable-demuxer=rtsp --enable-demuxer=rtp --enable-demuxer=flv --enable-demuxer=h264 \
--disable-muxers \
--enable-muxer=rtsp --enable-muxer=rtp --enable-muxer=flv --enable-muxer=h264 \
--disable-parsers \
--enable-parser=mpeg4video --enable-parser=aac --enable-parser=h264 --enable-parser=vp9 \
--disable-protocols \
--enable-protocol=rtmp --enable-protocol=rtp --enable-protocol=tcp --enable-protocol=udp \
--disable-bsfs \
--disable-indevs --enable-indev=v4l2 \
--disable-outdevs \
--disable-filters \
--disable-postproc \
--disable-static \
--disable-doc \
--disable-ffmpeg \
--disable-ffplay \
--disable-ffprobe \
--enable-avdevice \
--disable-doc \
--disable-symver \
--cross-prefix=$CROSS_PREFIX \
--target-os=android \
--arch=$ARCH \
--cpu=$CPU \
--cc=$CC \
--cxx=$CXX \
--enable-cross-compile \
--sysroot=$SYSROOT \
--extra-cflags="-Os -fpic $OPTIMIZE_CFLAGS" \
--extra-ldflags="$ADDI_LDFLAGS" \
$ADDITIONAL_CONFIGURE_FLAG
make clean
make -j6
make install
echo "The Compilation of FFmpeg for $CPU is completed"
}
#armv8-a
ARCH=arm64
CPU=armv8-a
CC=$TOOLCHAIN/bin/aarch64-linux-android$API-clang
CXX=$TOOLCHAIN/bin/aarch64-linux-android$API-clang++
SYSROOT=$NDK/toolchains/llvm/prebuilt/linux-x86_64/sysroot
CROSS_PREFIX=$TOOLCHAIN/bin/aarch64-linux-android-
PREFIX=$(pwd)/android/$CPU
OPTIMIZE_CFLAGS="-march=$CPU"
build_android
#armv7-a
ARCH=arm
CPU=armv7-a
CC=$TOOLCHAIN/bin/armv7a-linux-androideabi$API-clang
CXX=$TOOLCHAIN/bin/armv7a-linux-androideabi$API-clang++
SYSROOT=$NDK/toolchains/llvm/prebuilt/linux-x86_64/sysroot
CROSS_PREFIX=$TOOLCHAIN/bin/arm-linux-androideabi-
PREFIX=$(pwd)/android/$CPU
OPTIMIZE_CFLAGS="-mfloat-abi=softfp -mfpu=vfp -marm -march=$CPU "
build_android
#x86
ARCH=x86
CPU=x86
CC=$TOOLCHAIN/bin/i686-linux-android$API-clang
CXX=$TOOLCHAIN/bin/i686-linux-android$API-clang++
SYSROOT=$NDK/toolchains/llvm/prebuilt/linux-x86_64/sysroot
CROSS_PREFIX=$TOOLCHAIN/bin/i686-linux-android-
PREFIX=$(pwd)/android/$CPU
OPTIMIZE_CFLAGS="-march=i686 -mtune=intel -mssse3 -mfpmath=sse -m32"
build_android
#x86_64
ARCH=x86_64
CPU=x86-64
CC=$TOOLCHAIN/bin/x86_64-linux-android$API-clang
CXX=$TOOLCHAIN/bin/x86_64-linux-android$API-clang++
SYSROOT=$NDK/toolchains/llvm/prebuilt/linux-x86_64/sysroot
CROSS_PREFIX=$TOOLCHAIN/bin/x86_64-linux-android-
PREFIX=$(pwd)/android/$CPU
OPTIMIZE_CFLAGS="-march=$CPU -msse4.2 -mpopcnt -m64 -mtune=intel"
build_android
其中，在编译到x86 32位版时，会遇到“error: inline assembly requires more registers than available”的问题：
libavcodec/x86/cabac.h:193:9: error: inline assembly requires more registers than available
BRANCHLESS_GET_CABAC("%0", "%q0", "(%4)", "%1", "%w1",
^
libavcodec/x86/cabac.h:143:9: note: expanded from macro 'BRANCHLESS_GET_CABAC'
"movzbl "statep" , "ret" \n\t"\
^
libavcodec/x86/cabac.h:193:9: error: inline assembly requires more registers than available
libavcodec/x86/cabac.h:143:9: note: expanded from macro 'BRANCHLESS_GET_CABAC'
"movzbl "statep" , "ret" \n\t"\
^
libavcodec/x86/cabac.h:193:9: error: inline assembly requires more registers than available
libavcodec/x86/cabac.h:143:9: note: expanded from macro 'BRANCHLESS_GET_CABAC'
"movzbl "statep" , "ret" \n\t"\
^
libavcodec/x86/cabac.h:193:9: error: inline assembly requires more registers than available
libavcodec/x86/cabac.h:143:9: note: expanded from macro 'BRANCHLESS_GET_CABAC'
"movzbl "statep" , "ret" \n\t"\
^
libavcodec/x86/cabac.h:193:9: error: inline assembly requires more registers than available
libavcodec/x86/cabac.h:143:9: note: expanded from macro 'BRANCHLESS_GET_CABAC'
"movzbl "statep" , "ret" \n\t"\
^
libavcodec/x86/cabac.h:193:9: error: inline assembly requires more registers than available
libavcodec/x86/cabac.h:143:9: note: expanded from macro 'BRANCHLESS_GET_CABAC'
"movzbl "statep" , "ret" \n\t"\
^
libavcodec/x86/cabac.h:193:9: error: inline assembly requires more registers than available
libavcodec/x86/cabac.h:143:9CC libavcodec/h264_refs.o
: note: expanded from macro 'BRANCHLESS_GET_CABAC'
"movzbl "statep" , "ret" \n\t"\
各种修改编译脚本设置都无法解决，虽然ndk的32位abi感觉可能也不太会用到吧，但是强迫症发作还是想把它也编出来，最后查到这个： https://trac.ffmpeg.org/ticket/7796 ，研读后发现“ libavutil/x86/asm.h:75”中的
#define HAVE_7REGS (ARCH_X86_64 || (HAVE_EBX_AVAILABLE && HAVE_EBP_AVAILABLE))
修改为：
#define HAVE_7REGS (ARCH_X86_64)
就可以编译通过了，先记下，后面发现副作用以及其它更新时再做补充。
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