osb/cmake/TargetArch.cmake

# Based on the Qt 5 processor detection code, so should be very accurate
# https://qt.gitorious.org/qt/qtbase/blobs/master/src/corelib/global/qprocessordetection.h
# Currently handles arm (v5, v6, v7), x86 (32/64), ia64, and ppc (32/64)

# Regarding POWER/PowerPC, just as is noted in the Qt source,
# "There are many more known variants/revisions that we do not handle/detect."

set(archdetect_c_code "
#if defined(__arm__) || defined(__TARGET_ARCH_ARM)
    #if defined(__ARM_ARCH_7__) \\
        || defined(__ARM_ARCH_7A__) \\
        || defined(__ARM_ARCH_7R__) \\
        || defined(__ARM_ARCH_7M__) \\
        || (defined(__TARGET_ARCH_ARM) && __TARGET_ARCH_ARM-0 >= 7)
        #error cmake_ARCH armv7
    #elif defined(__ARM_ARCH_6__) \\
        || defined(__ARM_ARCH_6J__) \\
        || defined(__ARM_ARCH_6T2__) \\
        || defined(__ARM_ARCH_6Z__) \\
        || defined(__ARM_ARCH_6K__) \\
        || defined(__ARM_ARCH_6ZK__) \\
        || defined(__ARM_ARCH_6M__) \\
        || (defined(__TARGET_ARCH_ARM) && __TARGET_ARCH_ARM-0 >= 6)
        #error cmake_ARCH armv6
    #elif defined(__ARM_ARCH_5TEJ__) \\
        || (defined(__TARGET_ARCH_ARM) && __TARGET_ARCH_ARM-0 >= 5)
        #error cmake_ARCH armv5
    #else
        #error cmake_ARCH arm
    #endif
#elif defined(__arm64__)
    #error cmake_ARCH arm64
#elif defined(__i386) || defined(__i386__) || defined(_M_IX86)
    #error cmake_ARCH i386
#elif defined(__x86_64) || defined(__x86_64__) || defined(__amd64) || defined(_M_X64)
    #error cmake_ARCH x86_64
#elif defined(__ia64) || defined(__ia64__) || defined(_M_IA64)
    #error cmake_ARCH ia64
#endif

#error cmake_ARCH unknown
")

function(target_architecture output_var)
    if(CMAKE_SYSTEM_NAME STREQUAL "Darwin" AND CMAKE_OSX_ARCHITECTURES)
        # On macOS, we use CMAKE_OSX_ARCHITECTURES *if* it was set

        # Apple currently only supports x86_64 and arm64 architectures to target Intel-based
        # and Silicon-based Macs. Building as a universal binary will make each machine
        # use the proper native code to run the resulting executable.
        foreach(osx_arch ${CMAKE_OSX_ARCHITECTURES})
            if("${osx_arch}" STREQUAL "x86_64")
                set(osx_arch_x86_64 TRUE)
            elseif("${osx_arch}" STREQUAL "arm64")
                set(osx_arch_arm64 TRUE)
            else()
                message(FATAL_ERROR "Invalid OS X arch name: ${osx_arch}")
            endif()
        endforeach()

        if(osx_arch_x86_64)
            list(APPEND ARCH x86_64)
        endif()

        if(osx_arch_arm64)
            list(APPEND ARCH arm64)
        endif()
    else()
        file(WRITE "${CMAKE_BINARY_DIR}/arch.c" "${archdetect_c_code}")

        enable_language(C)

        # Detect the architecture in a rather creative way...
        # This compiles a small C program which is a series of ifdefs that selects a
        # particular #error preprocessor directive whose message string contains the
        # target architecture. The program will always fail to compile (both because
        # file is not a valid C program, and obviously because of the presence of the
        # #error preprocessor directives... but by exploiting the preprocessor in this
        # way, we can detect the correct target architecture even when cross-compiling,
        # since the program itself never needs to be run (only the compiler/preprocessor)
        try_run(
            run_result_unused
            compile_result_unused
            "${CMAKE_BINARY_DIR}"
            "${CMAKE_BINARY_DIR}/arch.c"
            COMPILE_OUTPUT_VARIABLE ARCH
            CMAKE_FLAGS CMAKE_OSX_ARCHITECTURES=${CMAKE_OSX_ARCHITECTURES}
        )

        # Parse the architecture name from the compiler output
        string(REGEX MATCH "cmake_ARCH ([a-zA-Z0-9_]+)" ARCH "${ARCH}")

        # Get rid of the value marker leaving just the architecture name
        string(REPLACE "cmake_ARCH " "" ARCH "${ARCH}")

        # If we are compiling with an unknown architecture this variable should
        # already be set to "unknown" but in the case that it's empty (i.e. due
        # to a typo in the code), then set it to unknown
        if (NOT ARCH)
            set(ARCH unknown)
        endif()
    endif()

    set(${output_var} "${ARCH}" PARENT_SCOPE)
endfunction()