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How to build OpenBLAS for Android
In addition to the Android NDK, you will need both Perl and a C compiler on the build host as these are currently required by the OpenBLAS build environment.
see below for building with clang as required by recent ndks
The prebuilt Android NDK toolchains do not include Fortran, hence parts like LAPACK cannot be built. You can still build OpenBLAS without it. For instructions on how to build OpenBLAS with Fortran, see the next section.
To use easily the prebuilt toolchains, follow building a standalone toolchain for your desired architecture.
This would be arm-linux-androideabi-gcc-4.9 for ARMV7 and aarch64-linux-android-gcc-4.9 for ARMV8.
You can build OpenBLAS (0.2.19 and earlier) with:
# Add the toolchain to your path
export PATH=/path/to/standalone-toolchain/bin:$PATH
# Build without Fortran for ARMV7
make TARGET=ARMV7 HOSTCC=gcc CC=arm-linux-androideabi-gcc NOFORTRAN=1 libs
# Build without Fortran for ARMV8
make TARGET=ARMV8 BINARY=64 HOSTCC=gcc CC=aarch64-linux-android-gcc NOFORTRAN=1 libs
Since we are cross-compiling, we make the libs recipe, not all. Otherwise you will get errors when trying to link/run tests as versions up to and including 0.2.19 cannot build a shared library for Android.
From 0.2.20 on, you can just leave off the "libs" to get a full build, and you may want to use the softfp ABI instead of the deprecated hardfp one on ARMV7 so you would use
# Add the toolchain to your path
export PATH=/path/to/standalone-toolchain/bin:$PATH
# Build without Fortran for ARMV7
make TARGET=ARMV7 ARM_SOFTFP_ABI=1 HOSTCC=gcc CC=arm-linux-androideabi-gcc NOFORTRAN=1
# Build without Fortran for ARMV8
make TARGET=ARMV8 BINARY=64 HOSTCC=gcc CC=aarch64-linux-android-gcc NOFORTRAN=1
If you get an error about stdio.h not being found, you need to specify your sysroot in the CFLAGS argument to make like
CFLAGS=--sysroot=$NDK/platforms/android-16/arch-arm
When you are done, install OpenBLAS into the desired directory. Be sure to also use all command line options
here that you specified for building, otherwise errors may occur as it tries to install things you did not build:
make PREFIX=/path/to/install-dir TARGET=... install
Instructions on how to build the GNU toolchains with Fortran can be found here. The Releases section provides prebuilt versions, use the standalone one.
You can build OpenBLAS with:
# Add the toolchain to your path
export PATH=/path/to/standalone-toolchain-with-fortran/bin:$PATH
# Build with Fortran for ARMV7
make TARGET=ARMV7 HOSTCC=gcc CC=arm-linux-androideabi-gcc FC=arm-linux-androideabi-gfortran libs
# Build with LAPACK for ARMV8
make TARGET=ARMV8 BINARY=64 HOSTCC=gcc CC=aarch64-linux-android-gcc FC=aarch64-linux-android-gfortran libs
As mentioned above you can leave off the libs argument here when building 0.2.20 and later, and you may want to add ARM_SOFTFP_ABI=1 when building for ARMV7.
If you are using ndk-build, you need to set the ABI to hard floating points in your Application.mk:
APP_ABI := armeabi-v7a-hard
This will set the appropriate flags for you. If you are not using ndk-build, you will want to add the following flags:
TARGET_CFLAGS += -mhard-float -D_NDK_MATH_NO_SOFTFP=1
TARGET_LDFLAGS += -Wl,--no-warn-mismatch -lm_hard
From 0.2.20 on, it is also possible to build for the softfp ABI by specifying ARM_SOFTFP_ABI=1 during the build. In that case, also make sure that all your dependencies are compiled with -mfloat-abi=softfp as well, as mixing "hard" and "soft" floating point ABIs in a program will make it crash.
Most recent Android NDKs stopped supporting gcc, so you would need to use clang to compile OpenBLAS. clang is supported from OpenBLAS 0.2.20 version onwards. See below sections on how to build with clang for ARMV7 and ARMV8 targets. Note that you still need to build a standalone toolchain first, following building a standalone toolchain for your desired architecture. If you want to use static linking, you need to choose an API level below 23 currently due to NDK bug 272 (https://github.com/android-ndk/ndk/issues/272 , the libc.a lacks a definition of stderr) that will probably be fixed in r17 of the NDK.
# Set path to ndk-bundle
export NDK_BUNDLE_DIR=/path/to/ndk-bundle
# Set the PATH to contain paths to clang and arm-linux-androideabi-* utilities
export PATH=${NDK_BUNDLE_DIR}/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/bin:${NDK_BUNDLE_DIR}/toolchains/llvm/prebuilt/linux-x86_64/bin:$PATH
# Set LDFLAGS so that the linker finds the appropriate libgcc
export LDFLAGS="-L${NDK_BUNDLE_DIR}/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/lib/gcc/arm-linux-androideabi/4.9.x"
# Set the clang cross compile flags
export CLANG_FLAGS="-target arm-linux-androideabi -marm -mfpu=vfp -mfloat-abi=softfp --sysroot ${NDK_BUNDLE_DIR}/platforms/android-23/arch-arm -gcc-toolchain ${NDK_BUNDLE_DIR}/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/"
#OpenBLAS Compile
make TARGET=ARMV7 ONLY_CBLAS=1 AR=ar CC="clang ${CLANG_FLAGS}" HOSTCC=gcc ARM_SOFTFP_ABI=1 -j4
On a Mac, it may also be necessary to give the complete path to the ar utility in the make command above, like so:
AR=${NDK_BUNDLE_DIR}/toolchains/arm-linux-androideabi-4.9/prebuilt/darwin-x86_64/bin/arm-linux-androideabi-gcc-ar
otherwise you may get a linker error complaining about a "malformed archive header name at 8" when the native OSX ar command was invoked instead.
# Set path to ndk-bundle
export NDK_BUNDLE_DIR=/path/to/ndk-bundle/
# Export PATH to contain directories of clang and aarch64-linux-android-* utilities
export PATH=${NDK_BUNDLE_DIR}/toolchains/aarch64-linux-android-4.9/prebuilt/linux-x86_64/bin/:${NDK_BUNDLE_DIR}/toolchains/llvm/prebuilt/linux-x86_64/bin:$PATH
# Setup LDFLAGS so that loader can find libgcc and pass -lm for sqrt
export LDFLAGS="-L${NDK_BUNDLE_DIR}/toolchains/aarch64-linux-android-4.9/prebuilt/linux-x86_64/lib/gcc/aarch64-linux-android/4.9.x -lm"
# Setup the clang cross compile options
export CLANG_FLAGS="-target aarch64-linux-android --sysroot ${NDK_BUNDLE_DIR}/platforms/android-23/arch-arm64 -gcc-toolchain ${NDK_BUNDLE_DIR}/toolchains/aarch64-linux-android-4.9/prebuilt/linux-x86_64/"
# Compile
make TARGET=ARMV8 ONLY_CBLAS=1 AR=ar CC="clang ${CLANG_FLAGS}" HOSTCC=gcc -j4
Note: Using TARGET=CORTEXA57 in place of ARMV8 will pick up better optimized routines. Implementations for CORTEXA57 target is compatible with all other armv8 targets.