在Ubuntu 16中编译开启NEON优化的Opencv时，遇到libpng编译是使用汇编代码的错误，完整错误见文章末尾。通过查询发现解决方案是安装跨平台编译器，安装代码如下：
sudo apt-get install gcc-arm-linux-gnueabi
sudo apt-get install gcc-arm-linux-gnueabihf
sudo apt-get install g++-arm-linux-gnueabihf
在参考1中说的是用OCV源码中的toolchain_file，但是我用这个编译出来的会导致各种undefin reference，还是要用NDK中的toolchai_file才行
参考1
参考2
[ 17%] Building C object 3rdparty/libpng/CMakeFiles/libpng.dir/arm/arm_init.c.o
[ 17%] Building ASM object 3rdparty/libpng/CMakeFiles/libpng.dir/arm/filter_neon.S.o
<instantiation>:18:9: error: unknown directive
.func png_read_filter_row_sub4_neon
^
/home/willhua/opencv/opencv-3.4.1/3rdparty/libpng/arm/filter_neon.S:65:1: note: while in macro instantiation
func png_read_filter_row_sub4_neon, export=1
^
<instantiation>:2:9: error: unknown directive
.endfunc
^
/home/willhua/opencv/opencv-3.4.1/3rdparty/libpng/arm/filter_neon.S:79:1: note: while in macro instantiation
endfunc
^
<instantiation>:18:9: error: unknown directive
.func png_read_filter_row_sub3_neon
^
/home/willhua/opencv/opencv-3.4.1/3rdparty/libpng/arm/filter_neon.S:81:1: note: while in macro instantiation
func png_read_filter_row_sub3_neon, export=1
^
<instantiation>:2:9: error: unknown directive
.endfunc
^
/home/willhua/opencv/opencv-3.4.1/3rdparty/libpng/arm/filter_neon.S:105:1: note: while in macro instantiation
endfunc
^
<instantiation>:18:9: error: unknown directive
.func png_read_filter_row_up_neon
^
/home/willhua/opencv/opencv-3.4.1/3rdparty/libpng/arm/filter_neon.S:107:1: note: while in macro instantiation
func png_read_filter_row_up_neon, export=1
^
<instantiation>:2:9: error: unknown directive
.endfunc
^
/home/willhua/opencv/opencv-3.4.1/3rdparty/libpng/arm/filter_neon.S:118:1: note: while in macro instantiation
endfunc
^
<instantiation>:18:9: error: unknown directive
.func png_read_filter_row_avg4_neon
^
/home/willhua/opencv/opencv-3.4.1/3rdparty/libpng/arm/filter_neon.S:120:1: note: while in macro instantiation
func png_read_filter_row_avg4_neon, export=1
^
<instantiation>:2:9: error: unknown directive
.endfunc
^
/home/willhua/opencv/opencv-3.4.1/3rdparty/libpng/arm/filter_neon.S:139:1: note: while in macro instantiation
endfunc
^
<instantiation>:18:9: error: unknown directive
.func png_read_filter_row_avg3_neon
^
/home/willhua/opencv/opencv-3.4.1/3rdparty/libpng/arm/filter_neon.S:141:1: note: while in macro instantiation
func png_read_filter_row_avg3_neon, export=1
^
<instantiation>:2:9: error: unknown directive
.endfunc
^
/home/willhua/opencv/opencv-3.4.1/3rdparty/libpng/arm/filter_neon.S:174:1: note: while in macro instantiation
endfunc
^
<instantiation>:18:9: error: unknown directive
.func png_read_filter_row_paeth4_neon
^
/home/willhua/opencv/opencv-3.4.1/3rdparty/libpng/arm/filter_neon.S:192:1: note: while in macro instantiation
func png_read_filter_row_paeth4_neon, export=1
^
<instantiation>:2:9: error: unknown directive
.endfunc
^
/home/willhua/opencv/opencv-3.4.1/3rdparty/libpng/arm/filter_neon.S:213:1: note: while in macro instantiation
endfunc
^
<instantiation>:18:9: error: unknown directive
.func png_read_filter_row_paeth3_neon
^
/home/willhua/opencv/opencv-3.4.1/3rdparty/libpng/arm/filter_neon.S:215:1: note: while in macro instantiation
func png_read_filter_row_paeth3_neon, export=1
^
<instantiation>:2:9: error: unknown directive
.endfunc
^
/home/willhua/opencv/opencv-3.4.1/3rdparty/libpng/arm/filter_neon.S:250:1: note: while in macro instantiation
endfunc
^
3rdparty/libpng/CMakeFiles/libpng.dir/build.make:446: recipe for target \'3rdparty/libpng/CMakeFiles/libpng.dir/arm/filter_neon.S.o\' failed
make[2]: *** [3rdparty/libpng/CMakeFiles/libpng.dir/arm/filter_neon.S.o] Error 1
CMakeFiles/Makefile2:655: recipe for target \'3rdparty/libpng/CMakeFiles/libpng.dir/all\' failed
make[1]: *** [3rdparty/libpng/CMakeFiles/libpng.dir/all] Error 2
Makefile:162: recipe for target \'all\' failed
make: *** [all] Error 2