How to build OpenBLAS for iPhone iOS (ARMv8)

How to build OpenBLAS for iPhone/iOS

As none of the current developers uses iOS, it is unfortunately still unclear if the following instructions are sufficient to build a fully working OpenBLAS for this platform. Apple provides its own implementation of BLAS/LAPACK under the name Accelerate so you may want to just use that unless you are specifically interested in OpenBLAS.

Go to the directory where you unpacked OpenBLAS.
In file c_check look for the section that looks like

     $cross_suffix = "";

     if (dirname($compiler_name) ne ".") {
         $cross_suffix .= dirname($compiler_name) . "/";
     }

     if (basename($compiler_name) =~ /([^\s]*-)(.*)/) {
         $cross_suffix .= $1;
     }

and add the following line after it, adjusting the path as necessary for your Xcode installation:

    $cross_suffix = "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/";

You need this because we will provide CC with arguments and Perl function for extracting directory doesn't work well.

Next, create a file with the following content:

     TOOLCHAIN_PATH=/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
     SYSROOT_PATH=/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS10.2.sdk
     make TARGET=ARMV8 BINARY=64 HOSTCC=clang CC="$TOOLCHAIN_PATH/clang -isysroot $SYSROOT_PATH -arch arm64" NOFORTRAN=1 libs

(again adjust the TOOLCHAIN_PATH and SYSROOT_PATH as necessary for your installation, e.g. change the version number of the SDK to what you have installed) and execute this file to build the library.

Note that ARMV8 is a generic implementation that should work on most arm64 hardware, for best performance try compiling with TARGET=CORTEXA57 or equivalent cpu-specific entry from TargetList.txt as appropriate for your hardware. Unfortunately it is unclear at the moment whether the Xcode assembler will accept the dialect used in the assembly files. According to issue #1531, you will probably have to edit the file kernel/arm64/KERNEL.ARMV8 and replace the line SGEMMKERNEL= sgemm_kernel_4x4.S with SGEMMKERNEL=../generic/gemmkernel_2x2.c to work around one such incompatibility.(This is done automatically starting from version 0.3.4)