Add the support of the cross-compiling for iOS devices

[Xreki]

Xcode is used as the toolchain for cross-compiling for iOS devices. Make sure Xcode is installed on your Mac.

Use the following CMake command

cmake -DCMAKE_INSTALL_PREFIX=/home/liuyiqun/paddle \
      -DCMAKE_SYSTEM_NAME=iOS \
      -DIOS_PLATFORM=OS \
      -DCMAKE_OSX_ARCHITECTURES="arm64" \
      -DWITH_C_API=ON \
      -DUSE_EIGEN_FOR_BLAS=ON \
      -DWITH_TESTING=OFF \
      -DWITH_SWIG_PY=OFF \
      -DWITH_STYLE_CHECK=OFF \
      -DCMAKE_BUILD_TYPE=Release \
      ..
make
make install

Set -DIOS_USE_VECLIB_FOR_BLAS=ON to use veclib, such as

cmake -DCMAKE_INSTALL_PREFIX=/home/liuyiqun/paddle \
      -DCMAKE_SYSTEM_NAME=iOS \
      -DIOS_PLATFORM=OS \
      -DCMAKE_OSX_ARCHITECTURES="arm64" \
      -DIOS_USE_VECLIB_FOR_BLAS=ON \
      -DWITH_C_API=ON \
      -DWITH_TESTING=OFF \
      -DWITH_SWIG_PY=OFF \
      -DWITH_STYLE_CHECK=OFF \
      -DCMAKE_BUILD_TYPE=Release \
      ..

When neither IOS_USE_VECLIB_FOR_BLAS nor USE_EIGEN_FOR_BLAS is set , OpenBLAS will be used. You can change CMAKE_OSX_ARCHITECTURES to "armv7", but note OpenBLAS can not be built for armv7.

TODO:

• Support multiple architectures through a single make process
• Fix the linking problem

[NHZlX]

look forward to the support for armv7 and simulator

[hedaoyuan]

赞！

[hedaoyuan]

这个ANDROID判断需要提到外面，否则后面那个elseif分支有问题。

[Xreki]

对的，merge的时候没注意。Done。

[hedaoyuan]

目前看起来好像没有必要用arm_soft_fp_abi分支，这个分支的所有commit已经都合入develop了。

This branch is 242 commits behind develop.

[Xreki]

这个分支的修改内容似乎并没有合入master或者develop分支，从两个分支的Makefile.arm看的出来：

• Makefile.arm on arm_soft_fp_abi

ifeq ($(CORE), ARMV7)
ifeq ($(OSNAME), Android)
ifeq ($(ARM_SOFTFP_ABI), 1)
CCOMMON_OPT += -mfpu=neon -march=armv7-a
FCOMMON_OPT += -mfpu=neon -march=armv7-a
else
CCOMMON_OPT += -mfpu=neon -march=armv7-a -Wl,--no-warn-mismatch
FCOMMON_OPT += -mfpu=neon -march=armv7-a -Wl,--no-warn-mismatch
endif
else
CCOMMON_OPT += -mfpu=vfpv3 -march=armv7-a
FCOMMON_OPT += -mfpu=vfpv3 -march=armv7-a
endif
endif

• Makefile.arm on develop

[hedaoyuan]

arm_soft_fp_abi分支只显示This branch is 242 commits behind develop.说明它已经merge到develop了，否则应该会显示commits ahead of develop这类的。至于，develop的Makefile.arm与arm_soft_fp_abi的不一样可能是develop后来又修改了。
ps，我用develop的编译跑了一下，好像没发现比arm_soft_fp_abi慢。

[Xreki]

我验证一下，下一个PR修复吧。