iOS 上的 OpenMP 支持

遇到的问题

最近在编译 iOS 下的 rapidsnark 发现运行起来对比 Android 性能低了不少

检查了 CMakeLists.txt 中的如下片段，发现在 iOS 上就没有启用 openmp 支持

if(USE_OPENMP)
    find_package(OpenMP)

    if(OpenMP_CXX_FOUND)
        if(TARGET_PLATFORM MATCHES "android")
            message("OpenMP is used")

        elseif(CMAKE_HOST_SYSTEM_NAME STREQUAL "Linux")
            message("OpenMP is used")

        else()
            set(OpenMP_CXX_FOUND FALSE)
            message("OpenMP is not used")

        endif()
    endif()
endif()

网上的说法

按网上的说法，因为 Apple 推荐用 GCD 的方式处理并行需求，所以对 openmp 在 iOS 上的支持不足。 用brew install libomp, 也只能安装 macOS 的 openmp library。

用 xcode 自带的 clang 运行clang -fopenmp -E -dM - < /dev/null 会提示clang: error: unsupported option '-fopenmp'

OpenMP 的工作方式

template <typename Field>
void FFT<Field>::reversePermutation(Element *a, u_int64_t n) {
    int domainPow = log2(n);
    #pragma omp parallel for
    for (u_int64_t i=0; i<n; i++) {
        Element tmp;
        u_int64_t r = BR(i, domainPow);
        if (i>r) {
            f.copy(tmp, a[i]);
            f.copy(a[i], a[r]);
            f.copy(a[r], tmp);
        }
    }
}

注意其中的#pragma omp parallel for, 这和 GCD 或者多线程/进程开发的方式有些不同，看起来不需要开发者显示告诉编译器并行优化逻辑，而是让开启了 openmp 支持的编译器自行生成并行优化代码。

iOS 上 openmp 库的编译方式

自行编译

build_oms_ios.sh

#!/bin/bash
usage() { echo "Usage: $0 {-d | --disableSimulator}" 1>&2; exit 1; }

 while [ "${1:-}" != "" ]; do
     case "$1" in
         "-d" | "--disableSimulator")
             disableSimulator=true
             ;;
         "-h" | "--help")
             usage
             exit
             ;;
     esac
     shift
 done

 OMP_VER=14.0.6
 DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
 SRC_DIR="${DIR}/openmp-${OMP_VER}.src"

 # Download OpenMP
 (cd ${DIR} && curl -OL "https://github.com/llvm/llvm-project/releases/download/llvmorg-${OMP_VER}/openmp-${OMP_VER}.src.tar.xz")
 (cd ${DIR} && tar -xf ${DIR}/openmp-${OMP_VER}.src.tar.xz openmp-${OMP_VER}.src)

 # Build iOS
 mkdir -p ${SRC_DIR}/build
 cmake -B ${SRC_DIR}/build -S ${SRC_DIR} \
     -DCMAKE_TOOLCHAIN_FILE=${DIR}/ios.toolchain.cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=${SRC_DIR}/build/install \
     -DIOS_PLATFORM=OS -DENABLE_BITCODE=1 -DENABLE_ARC=0 -DENABLE_VISIBILITY=0 -DIOS_ARCH="arm64;arm64e" \
     -DPERL_EXECUTABLE=$(which perl) \
     -DLIBOMP_ENABLE_SHARED=OFF -DLIBOMP_OMPT_SUPPORT=OFF -DLIBOMP_USE_HWLOC=OFF
 cmake --build ${SRC_DIR}/build -j 3
 cmake --build ${SRC_DIR}/build --target install
 FRAMEWORK_DIR="${SRC_DIR}/build/install/OpenMP.framework"
 mkdir -p "${FRAMEWORK_DIR}/Versions/A/Headers"
 ln -sfh A "${FRAMEWORK_DIR}/Versions/Current"
 ln -sfh Versions/Current/Headers "${FRAMEWORK_DIR}/Headers"
 ln -sfh "Versions/Current/libomp.a" \
              "${FRAMEWORK_DIR}/OpenMP"
 cp ${SRC_DIR}/build/install/lib/libomp.a "${FRAMEWORK_DIR}/Versions/A/libomp.a"
 cp ${SRC_DIR}/build/install/include/* "${FRAMEWORK_DIR}/Versions/A/Headers"


 # Build Simulator
 if [ -z "$disableSimulator" ]
 then
     mkdir -p ${SRC_DIR}/build-simulator
     cmake -B ${SRC_DIR}/build-simulator -S ${SRC_DIR} \
         -DCMAKE_TOOLCHAIN_FILE=${DIR}/ios.toolchain.cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=${SRC_DIR}/build-simulator/install \
         -DIOS_PLATFORM=SIMULATOR -DENABLE_BITCODE=1 -DENABLE_ARC=0 -DENABLE_VISIBILITY=0 -DIOS_ARCH="x86_64;arm64" \
         -DPERL_EXECUTABLE=$(which perl) \
         -DLIBOMP_ENABLE_SHARED=OFF -DLIBOMP_OMPT_SUPPORT=OFF -DLIBOMP_USE_HWLOC=OFF
     cmake --build ${SRC_DIR}/build-simulator -j 3
     cmake --build ${SRC_DIR}/build-simulator --target install
     FRAMEWORK_SIM_DIR="${SRC_DIR}/build-simulator/install/OpenMP.framework"
     mkdir -p "${FRAMEWORK_SIM_DIR}/Versions/A/Headers"
     ln -sfh A "${FRAMEWORK_SIM_DIR}/Versions/Current"
     ln -sfh Versions/Current/Headers "${FRAMEWORK_SIM_DIR}/Headers"
     ln -sfh "Versions/Current/libomp.a" \
                 "${FRAMEWORK_SIM_DIR}/OpenMP"
     cp ${SRC_DIR}/build-simulator/install/lib/libomp.a "${FRAMEWORK_SIM_DIR}/Versions/A/libomp.a"
     cp ${SRC_DIR}/build-simulator/install/include/* "${FRAMEWORK_SIM_DIR}/Versions/A/Headers"

     ARG_SIM="-framework $FRAMEWORK_SIM_DIR "
 fi

 # Create XCFramework
 rm -rf "${DIR}/OpenMP.xcframework"
 xcodebuild -create-xcframework -framework $FRAMEWORK_DIR $ARG_SIM -output "${DIR}/OpenMP.xcframework"

 # Clean up
 rm -rf "${DIR}/openmp-${OMP_VER}"*

ios.toolchain.cmake

 # This file is part of the ios-cmake project. It was retrieved from
 # https://github.com/cristeab/ios-cmake.git, which is a fork of
 # https://code.google.com/p/ios-cmake/. Which in turn is based off of
 # the Platform/Darwin.cmake and Platform/UnixPaths.cmake files which
 # are included with CMake 2.8.4
 #
 # The ios-cmake project is licensed under the new BSD license.
 #
 # Copyright (c) 2014, Bogdan Cristea and LTE Engineering Software,
 # Kitware, Inc., Insight Software Consortium.  All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions
 # are met:
 # 1. Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #
 # 2. Redistributions in binary form must reproduce the above copyright
 # notice, this list of conditions and the following disclaimer in the
 # documentation and/or other materials provided with the distribution.
 #
 # 3. Neither the name of the copyright holder nor the names of its
 # contributors may be used to endorse or promote products derived from
 # this software without specific prior written permission.
 #
 # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 # "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 # LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 # FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 # COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 # INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 # BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 # LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 # CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 # LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
 # ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 # POSSIBILITY OF SUCH DAMAGE.
 #
 # This file is based off of the Platform/Darwin.cmake and
 # Platform/UnixPaths.cmake files which are included with CMake 2.8.4
 # It has been altered for iOS development.
 #
 # Updated by Alex Stewart (alexs.mac@gmail.com)
 #
 # *****************************************************************************
 #      Now maintained by Alexander Widerberg (widerbergaren [at] gmail.com)
 #                      under the BSD-3-Clause license
 #                   https://github.com/leetal/ios-cmake
 # *****************************************************************************
 #
 #                           INFORMATION / HELP
 #
 # The following variables control the behaviour of this toolchain:
 #
 # IOS_PLATFORM: OS (default) or SIMULATOR or SIMULATOR64 or TVOS or SIMULATOR_TVOS or WATCHOS or SIMULATOR_WATCHOS
 #    OS = Build for iPhoneOS.
 #    OS64 = Build for arm64 arm64e iPhoneOS.
 #    SIMULATOR = Build for x86 i386 iPhone Simulator.
 #    SIMULATOR64 = Build for x86_64 iPhone Simulator.
 #    TVOS = Build for AppleTVOS.
 #    SIMULATOR_TVOS = Build for x86_64 AppleTV Simulator.
 #    WATCHOS = Build for armv7k arm64_32 for WatchOS.
 #    SIMULATOR_WATCHOS = Build for x86_64 for Watch Simulator.
 # CMAKE_OSX_SYSROOT: Path to the iOS SDK to use.  By default this is
 #    automatically determined from IOS_PLATFORM and xcodebuild, but
 #    can also be manually specified (although this should not be required).
 # CMAKE_IOS_DEVELOPER_ROOT: Path to the Developer directory for the iOS platform
 #    being compiled for.  By default this is automatically determined from
 #    CMAKE_OSX_SYSROOT, but can also be manually specified (although this should
 #    not be required).
 # ENABLE_BITCODE: (1|0) Enables or disables bitcode support. Default 1 (true)
 # ENABLE_ARC: (1|0) Enables or disables ARC support. Default 1 (true, ARC enabled by default)
 # ENABLE_VISIBILITY: (1|0) Enables or disables symbol visibility support. Default 0 (false, visibility hidden by default)
 # IOS_ARCH: (armv7 armv7s armv7k arm64 arm64e arm64_32 i386 x86_64) If specified, will override the default architectures for the given IOS_PLATFORM
 #    OS = armv7 armv7s arm64 arm64e (if applicable)
 #    OS64 = arm64 arm64e (if applicable)
 #    SIMULATOR = i386 x86_64
 #    SIMULATOR64 = x86_64
 #    TVOS = arm64
 #    SIMULATOR_TVOS = x86_64 (i386 has since long been deprecated)
 #    WATCHOS = armv7k arm64_32 (if applicable)
 #    SIMULATOR_WATCHOS = x86_64 (i386 has since long been deprecated)
 #
 # This toolchain defines the following variables for use externally:
 #
 # XCODE_VERSION: Version number (not including Build version) of Xcode detected.
 # IOS_SDK_VERSION: Version of iOS SDK being used.
 # CMAKE_OSX_ARCHITECTURES: Architectures being compiled for (generated from
 #    IOS_PLATFORM).
 #
 # This toolchain defines the following macros for use externally:
 #
 # set_xcode_property (TARGET XCODE_PROPERTY XCODE_VALUE XCODE_VARIANT)
 #   A convenience macro for setting xcode specific properties on targets.
 #   Available variants are: All, Release, RelWithDebInfo, Debug, MinSizeRel
 #   example: set_xcode_property (myioslib IPHONEOS_DEPLOYMENT_TARGET "3.1" "all").
 #
 # find_host_package (PROGRAM ARGS)
 #   A macro used to find executable programs on the host system, not within the
 #   iOS environment.  Thanks to the android-cmake project for providing the
 #   command.

 # Fix for PThread library not in path
 set(CMAKE_THREAD_LIBS_INIT "-lpthread")
 set(CMAKE_HAVE_THREADS_LIBRARY 1)
 set(CMAKE_USE_WIN32_THREADS_INIT 0)
 set(CMAKE_USE_PTHREADS_INIT 1)

 # Get the Xcode version being used.
 execute_process(COMMAND xcodebuild -version
   OUTPUT_VARIABLE XCODE_VERSION
   ERROR_QUIET
   OUTPUT_STRIP_TRAILING_WHITESPACE)
 string(REGEX MATCH "Xcode [0-9\\.]+" XCODE_VERSION "${XCODE_VERSION}")
 string(REGEX REPLACE "Xcode ([0-9\\.]+)" "\\1" XCODE_VERSION "${XCODE_VERSION}")
 message(STATUS "Building with Xcode version: ${XCODE_VERSION}")
 # Default to building for iPhoneOS if not specified otherwise, and we cannot
 # determine the platform from the CMAKE_OSX_ARCHITECTURES variable. The use
 # of CMAKE_OSX_ARCHITECTURES is such that try_compile() projects can correctly
 # determine the value of IOS_PLATFORM from the root project, as
 # CMAKE_OSX_ARCHITECTURES is propagated to them by CMake.
 if (NOT DEFINED IOS_PLATFORM)
   if (CMAKE_OSX_ARCHITECTURES)
     if (CMAKE_OSX_ARCHITECTURES MATCHES ".*arm.*")
       set(IOS_PLATFORM "OS")
     elseif (CMAKE_OSX_ARCHITECTURES MATCHES "i386")
       set(IOS_PLATFORM "SIMULATOR")
     elseif (CMAKE_OSX_ARCHITECTURES MATCHES "x86_64")
       set(IOS_PLATFORM "SIMULATOR64")
     elseif (CMAKE_OSX_ARCHITECTURES MATCHES "armv7k")
       set(IOS_PLATFORM "WATCHOS")
     endif()
   endif()
   if (NOT IOS_PLATFORM)
     set(IOS_PLATFORM "OS")
   endif()
 endif()
 set(IOS_PLATFORM ${IOS_PLATFORM} CACHE STRING
   "Type of iOS platform for which to build.")
 # Determine the platform name and architectures for use in xcodebuild commands
 # from the specified IOS_PLATFORM name.
 if (IOS_PLATFORM STREQUAL "OS")
   set(XCODE_IOS_PLATFORM iphoneos)
   if(NOT IOS_ARCH)
     if (XCODE_VERSION VERSION_GREATER 10.0)
       set(IOS_ARCH armv7 armv7s arm64 arm64e)
     else()
       set(IOS_ARCH armv7 armv7s arm64)
     endif()
   endif()
  elseif (IOS_PLATFORM STREQUAL "OS64")
   set(XCODE_IOS_PLATFORM iphoneos)
   if(NOT IOS_ARCH)
     if (XCODE_VERSION VERSION_GREATER 10.0)
       set(IOS_ARCH arm64 arm64e)
     else()
       set(IOS_ARCH arm64)
     endif()
   endif()
 elseif (IOS_PLATFORM STREQUAL "SIMULATOR")
   set(XCODE_IOS_PLATFORM iphonesimulator)
   if(NOT IOS_ARCH)
     set(IOS_ARCH i386 x86_64)
   endif()
 elseif(IOS_PLATFORM STREQUAL "SIMULATOR64")
   set(XCODE_IOS_PLATFORM iphonesimulator)
   if(NOT IOS_ARCH)
     set(IOS_ARCH x86_64)
   endif()
 elseif (IOS_PLATFORM STREQUAL "TVOS")
   set(XCODE_IOS_PLATFORM appletvos)
   if(NOT IOS_ARCH)
     set(IOS_ARCH arm64)
   endif()
 elseif (IOS_PLATFORM STREQUAL "SIMULATOR_TVOS")
   set(XCODE_IOS_PLATFORM appletvsimulator)
   if(NOT IOS_ARCH)
     set(IOS_ARCH x86_64)
   endif()
 elseif (IOS_PLATFORM STREQUAL "WATCHOS")
   set(XCODE_IOS_PLATFORM watchos)
   if(NOT IOS_ARCH)
     if (XCODE_VERSION VERSION_GREATER 10.0)
       set(IOS_ARCH armv7k arm64_32)
     else()
       set(IOS_ARCH armv7k)
     endif()
   endif()
 elseif (IOS_PLATFORM STREQUAL "SIMULATOR_WATCHOS")
   set(XCODE_IOS_PLATFORM watchsimulator)
   if(NOT IOS_ARCH)
     set(IOS_ARCH x86_64)
   endif()
 else()
   message(FATAL_ERROR "Invalid IOS_PLATFORM: ${IOS_PLATFORM}")
 endif()
 message(STATUS "Configuring iOS build for platform: ${IOS_PLATFORM}, "
   "architecture(s): ${IOS_ARCH}")
 # If user did not specify the SDK root to use, then query xcodebuild for it.
 execute_process(COMMAND xcodebuild -version -sdk ${XCODE_IOS_PLATFORM} Path
     OUTPUT_VARIABLE CMAKE_OSX_SYSROOT_INT
      OUTPUT_QUIET ERROR_QUIET
     OUTPUT_STRIP_TRAILING_WHITESPACE)
 # If user did not specify the SDK root to use, then query xcodebuild for it.
 if (NOT DEFINED CMAKE_OSX_SYSROOT OR (NOT CMAKE_OSX_SYSROOT STREQUAL CMAKE_OSX_SYSROOT_INT))
   execute_process(COMMAND xcodebuild -version -sdk ${XCODE_IOS_PLATFORM} Path
     OUTPUT_VARIABLE CMAKE_OSX_SYSROOT
     ERROR_QUIET
     OUTPUT_STRIP_TRAILING_WHITESPACE)
 endif()
 if (NOT EXISTS ${CMAKE_OSX_SYSROOT})
   message(SEND_ERROR "Please make sure that Xcode is installed and that the toolchain"
   "is pointing to the correct path. Please run:"
   "sudo xcode-select -s /Applications/Xcode.app/Contents/Developer"
   "and see if that fixes the problem for you.")
   message(FATAL_ERROR "Invalid CMAKE_OSX_SYSROOT: ${CMAKE_OSX_SYSROOT} "
   "does not exist.")
 elseif(DEFINED CMAKE_OSX_SYSROOT)
   message(STATUS "Using manually set SDK path: ${CMAKE_OSX_SYSROOT} for platform: ${IOS_PLATFORM}")
 else()
    message(STATUS "Using SDK: ${CMAKE_OSX_SYSROOT} for platform: ${IOS_PLATFORM}")
 endif()
 # Specify minimum version of deployment target.
 if (NOT DEFINED IOS_DEPLOYMENT_TARGET)
   if (IOS_PLATFORM STREQUAL "WATCHOS" OR IOS_PLATFORM STREQUAL "SIMULATOR_WATCHOS")
     # Unless specified, SDK version 2.0 is used by default as minimum target version (watchOS).
     set(IOS_DEPLOYMENT_TARGET "2.0"
             CACHE STRING "Minimum iOS version to build for." )
   else()
     # Unless specified, SDK version 9.0 is used by default as minimum target version (iOS, tvOS).
     set(IOS_DEPLOYMENT_TARGET "9.0"
               CACHE STRING "Minimum iOS version to build for." )
   endif()
   message(STATUS "Using the default min-version since IOS_DEPLOYMENT_TARGET not provided!")
 endif()
 # Use bitcode or not
 if (NOT DEFINED ENABLE_BITCODE AND NOT IOS_ARCH MATCHES "((^|, )(i386|x86_64))+")
   # Unless specified, enable bitcode support by default
   set(ENABLE_BITCODE TRUE CACHE BOOL "Whether or not to enable bitcode")
   message(STATUS "Enabling bitcode support by default. ENABLE_BITCODE not provided!")
 endif()
 if (NOT DEFINED ENABLE_BITCODE)
   message(STATUS "Disabling bitcode support by default on simulators. ENABLE_BITCODE not provided for override!")
 endif()
 # Use ARC or not
 if (NOT DEFINED ENABLE_ARC)
   # Unless specified, enable ARC support by default
   set(ENABLE_ARC TRUE CACHE BOOL "Whether or not to enable ARC")
   message(STATUS "Enabling ARC support by default. ENABLE_ARC not provided!")
 endif()
 # Use hidden visibility or not
 if (NOT DEFINED ENABLE_VISIBILITY)
   # Unless specified, disable symbols visibility by default
   set(ENABLE_VISIBILITY FALSE CACHE BOOL "Whether or not to hide symbols (-fvisibility=hidden)")
   message(STATUS "Hiding symbols visibility by default. ENABLE_VISIBILITY not provided!")
 endif()
 # Get the SDK version information.
 execute_process(COMMAND xcodebuild -sdk ${CMAKE_OSX_SYSROOT} -version SDKVersion
   OUTPUT_VARIABLE IOS_SDK_VERSION
   ERROR_QUIET
   OUTPUT_STRIP_TRAILING_WHITESPACE)
 # Find the Developer root for the specific iOS platform being compiled for
 # from CMAKE_OSX_SYSROOT.  Should be ../../ from SDK specified in
 # CMAKE_OSX_SYSROOT.  There does not appear to be a direct way to obtain
 # this information from xcrun or xcodebuild.
 if (NOT CMAKE_IOS_DEVELOPER_ROOT)
   get_filename_component(IOS_PLATFORM_SDK_DIR ${CMAKE_OSX_SYSROOT} PATH)
   get_filename_component(CMAKE_IOS_DEVELOPER_ROOT ${IOS_PLATFORM_SDK_DIR} PATH)
 endif()
 if (NOT EXISTS ${CMAKE_IOS_DEVELOPER_ROOT})
   message(FATAL_ERROR "Invalid CMAKE_IOS_DEVELOPER_ROOT: "
     "${CMAKE_IOS_DEVELOPER_ROOT} does not exist.")
 endif()
 # Find the C & C++ compilers for the specified SDK.
 if (NOT CMAKE_C_COMPILER)
   execute_process(COMMAND xcrun -sdk ${CMAKE_OSX_SYSROOT} -find clang
     OUTPUT_VARIABLE CMAKE_C_COMPILER
     ERROR_QUIET
     OUTPUT_STRIP_TRAILING_WHITESPACE)
   message(STATUS "Using C compiler: ${CMAKE_C_COMPILER}")
 endif()
 if (NOT CMAKE_CXX_COMPILER)
   execute_process(COMMAND xcrun -sdk ${CMAKE_OSX_SYSROOT} -find clang++
     OUTPUT_VARIABLE CMAKE_CXX_COMPILER
     ERROR_QUIET
     OUTPUT_STRIP_TRAILING_WHITESPACE)
   message(STATUS "Using CXX compiler: ${CMAKE_CXX_COMPILER}")
 endif()
 # Find (Apple's) libtool.
 execute_process(COMMAND xcrun -sdk ${CMAKE_OSX_SYSROOT} -find libtool
   OUTPUT_VARIABLE IOS_LIBTOOL
   ERROR_QUIET
   OUTPUT_STRIP_TRAILING_WHITESPACE)
 message(STATUS "Using libtool: ${IOS_LIBTOOL}")
 # Configure libtool to be used instead of ar + ranlib to build static libraries.
 # This is required on Xcode 7+, but should also work on previous versions of
 # Xcode.
 set(CMAKE_C_CREATE_STATIC_LIBRARY
   "${IOS_LIBTOOL} -static -o <TARGET> <LINK_FLAGS> <OBJECTS> ")
 set(CMAKE_CXX_CREATE_STATIC_LIBRARY
   "${IOS_LIBTOOL} -static -o <TARGET> <LINK_FLAGS> <OBJECTS> ")
 # Get the version of Darwin (OS X) of the host.
 execute_process(COMMAND uname -r
   OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_VERSION
   ERROR_QUIET
   OUTPUT_STRIP_TRAILING_WHITESPACE)
 # Standard settings.
 set(CMAKE_SYSTEM_NAME Darwin CACHE INTERNAL "")
 set(CMAKE_SYSTEM_VERSION ${IOS_SDK_VERSION} CACHE INTERNAL "")
 set(UNIX TRUE CACHE BOOL "")
 set(APPLE TRUE CACHE BOOL "")
 set(IOS TRUE CACHE BOOL "")
 set(CMAKE_AR ar CACHE FILEPATH "" FORCE)
 set(CMAKE_RANLIB ranlib CACHE FILEPATH "" FORCE)
 # Force unset of OS X-specific deployment target (otherwise autopopulated),
 # required as of cmake 2.8.10.
 set(CMAKE_OSX_DEPLOYMENT_TARGET "" CACHE STRING
   "Must be empty for iOS builds." FORCE)
 # Set the architectures for which to build.
 set(CMAKE_OSX_ARCHITECTURES ${IOS_ARCH} CACHE STRING "Build architecture for iOS")
 # Change the type of target generated for try_compile() so it'll work when cross-compiling
 set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
 # All iOS/Darwin specific settings - some may be redundant.
 set(CMAKE_SHARED_LIBRARY_PREFIX "lib")
 set(CMAKE_SHARED_LIBRARY_SUFFIX ".dylib")
 set(CMAKE_SHARED_MODULE_PREFIX "lib")
 set(CMAKE_SHARED_MODULE_SUFFIX ".so")
 set(CMAKE_C_COMPILER_ABI ELF)
 set(CMAKE_CXX_COMPILER_ABI ELF)
 set(CMAKE_C_HAS_ISYSROOT 1)
 set(CMAKE_CXX_HAS_ISYSROOT 1)
 set(CMAKE_MODULE_EXISTS 1)
 set(CMAKE_DL_LIBS "")
 set(CMAKE_C_OSX_COMPATIBILITY_VERSION_FLAG "-compatibility_version ")
 set(CMAKE_C_OSX_CURRENT_VERSION_FLAG "-current_version ")
 set(CMAKE_CXX_OSX_COMPATIBILITY_VERSION_FLAG "${CMAKE_C_OSX_COMPATIBILITY_VERSION_FLAG}")
 set(CMAKE_CXX_OSX_CURRENT_VERSION_FLAG "${CMAKE_C_OSX_CURRENT_VERSION_FLAG}")

 if(IOS_ARCH MATCHES "((^|, )(arm64|arm64e|x86_64))+")
   set(CMAKE_C_SIZEOF_DATA_PTR 8)
   set(CMAKE_CXX_SIZEOF_DATA_PTR 8)
   message(STATUS "Using a data_ptr size of 8")
 else()
   set(CMAKE_C_SIZEOF_DATA_PTR 4)
   set(CMAKE_CXX_SIZEOF_DATA_PTR 4)
   message(STATUS "Using a data_ptr size of 4")
 endif()

 message(STATUS "Building for minimum iOS version: ${IOS_DEPLOYMENT_TARGET}"
                " (SDK version: ${IOS_SDK_VERSION})")
 # Note that only Xcode 7+ supports the newer more specific:
 # -m${XCODE_IOS_PLATFORM}-version-min flags, older versions of Xcode use:
 # -m(ios/ios-simulator)-version-min instead.
 if (IOS_PLATFORM STREQUAL "OS" OR IOS_PLATFORM STREQUAL "OS64")
   if (XCODE_VERSION VERSION_LESS 7.0)
     set(XCODE_IOS_PLATFORM_VERSION_FLAGS
       "-mios-version-min=${IOS_DEPLOYMENT_TARGET}")
   else()
     # Xcode 7.0+ uses flags we can build directly from XCODE_IOS_PLATFORM.
     set(XCODE_IOS_PLATFORM_VERSION_FLAGS
       "-m${XCODE_IOS_PLATFORM}-version-min=${IOS_DEPLOYMENT_TARGET}")
   endif()
 elseif (IOS_PLATFORM STREQUAL "TVOS")
   set(XCODE_IOS_PLATFORM_VERSION_FLAGS
     "-mtvos-version-min=${IOS_DEPLOYMENT_TARGET}")
 elseif (IOS_PLATFORM STREQUAL "SIMULATOR_TVOS")
   set(XCODE_IOS_PLATFORM_VERSION_FLAGS
     "-mtvos-simulator-version-min=${IOS_DEPLOYMENT_TARGET}")
 elseif (IOS_PLATFORM STREQUAL "WATCHOS")
   set(XCODE_IOS_PLATFORM_VERSION_FLAGS
     "-mwatchos-version-min=${IOS_DEPLOYMENT_TARGET}")
 elseif (IOS_PLATFORM STREQUAL "SIMULATOR_WATCHOS")
   set(XCODE_IOS_PLATFORM_VERSION_FLAGS
     "-mwatchos-simulator-version-min=${IOS_DEPLOYMENT_TARGET}")
 else()
   # SIMULATOR or SIMULATOR64 both use -mios-simulator-version-min.
   set(XCODE_IOS_PLATFORM_VERSION_FLAGS
     "-mios-simulator-version-min=${IOS_DEPLOYMENT_TARGET}")
 endif()
 message(STATUS "Version flags set to: ${XCODE_IOS_PLATFORM_VERSION_FLAGS}")

 if (ENABLE_BITCODE)
   set(BITCODE "-fembed-bitcode")
   set(HEADER_PAD "")
   message(STATUS "Enabling bitcode support.")
 else()
   set(BITCODE "")
   set(HEADER_PAD "-headerpad_max_install_names")
   message(STATUS "Disabling bitcode support.")
 endif()

 if (ENABLE_ARC)
   set(FOBJC_ARC "-fobjc-arc")
   message(STATUS "Enabling ARC support.")
 else()
   set(FOBJC_ARC "-fno-objc-arc")
   message(STATUS "Disabling ARC support.")
 endif()

 if (NOT ENABLE_VISIBILITY)
   set(VISIBILITY "-fvisibility=hidden")
   message(STATUS "Hiding symbols (-fvisibility=hidden).")
 else()
   set(VISIBILITY "")
 endif()

 set(CMAKE_C_FLAGS
 "${XCODE_IOS_PLATFORM_VERSION_FLAGS} ${BITCODE} -fobjc-abi-version=2 ${FOBJC_ARC} ${CMAKE_C_FLAGS}")
 # Hidden visibilty is required for C++ on iOS.
 set(CMAKE_CXX_FLAGS
 "${XCODE_IOS_PLATFORM_VERSION_FLAGS} ${BITCODE} ${VISIBILITY} -fvisibility-inlines-hidden -fobjc-abi-version=2 ${FOBJC_ARC} ${CMAKE_CXX_FLAGS}")
 set(CMAKE_CXX_FLAGS_MINSIZEREL "${CMAKE_CXX_FLAGS} -DNDEBUG -Os -ffast-math ${BITCODE} ${CMAKE_CXX_FLAGS_MINSIZEREL}")
 set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS} -DNDEBUG -O2 -g -ffast-math ${BITCODE} ${CMAKE_CXX_FLAGS_RELWITHDEBINFO}")
 set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS} -DNDEBUG -O3 -ffast-math ${BITCODE} ${CMAKE_CXX_FLAGS_RELEASE}")
 set(CMAKE_C_LINK_FLAGS "${XCODE_IOS_PLATFORM_VERSION_FLAGS} -Wl,-search_paths_first ${CMAKE_C_LINK_FLAGS}")
 set(CMAKE_CXX_LINK_FLAGS "${XCODE_IOS_PLATFORM_VERSION_FLAGS}  -Wl,-search_paths_first ${CMAKE_CXX_LINK_FLAGS}")

 # In order to ensure that the updated compiler flags are used in try_compile()
 # tests, we have to forcibly set them in the CMake cache, not merely set them
 # in the local scope.
 list(APPEND VARS_TO_FORCE_IN_CACHE
   CMAKE_C_FLAGS
   CMAKE_CXX_FLAGS
   CMAKE_CXX_FLAGS_RELWITHDEBINFO
   CMAKE_CXX_FLAGS_MINSIZEREL
   CMAKE_CXX_FLAGS_RELEASE
   CMAKE_C_LINK_FLAGS
   CMAKE_CXX_LINK_FLAGS)
 foreach(VAR_TO_FORCE ${VARS_TO_FORCE_IN_CACHE})
   set(${VAR_TO_FORCE} "${${VAR_TO_FORCE}}" CACHE STRING "")
 endforeach()

 set(CMAKE_PLATFORM_HAS_INSTALLNAME 1)
 set (CMAKE_SHARED_LINKER_FLAGS "-rpath @executable_path/Frameworks -rpath @loader_path/Frameworks")
 set(CMAKE_SHARED_LIBRARY_CREATE_C_FLAGS "-dynamiclib ${HEADER_PAD}")
 set(CMAKE_SHARED_MODULE_CREATE_C_FLAGS "-bundle ${HEADER_PAD}")
 set(CMAKE_SHARED_MODULE_LOADER_C_FLAG "-Wl,-bundle_loader,")
 set(CMAKE_SHARED_MODULE_LOADER_CXX_FLAG "-Wl,-bundle_loader,")
 set(CMAKE_FIND_LIBRARY_SUFFIXES ".dylib" ".so" ".a")

 # Hack: if a new cmake (which uses CMAKE_INSTALL_NAME_TOOL) runs on an old
 # build tree (where install_name_tool was hardcoded) and where
 # CMAKE_INSTALL_NAME_TOOL isn't in the cache and still cmake didn't fail in
 # CMakeFindBinUtils.cmake (because it isn't rerun) hardcode
 # CMAKE_INSTALL_NAME_TOOL here to install_name_tool, so it behaves as it did
 # before, Alex.
 if (NOT DEFINED CMAKE_INSTALL_NAME_TOOL)
   find_program(CMAKE_INSTALL_NAME_TOOL install_name_tool)
 endif (NOT DEFINED CMAKE_INSTALL_NAME_TOOL)

 # Set the find root to the iOS developer roots and to user defined paths.
 set(CMAKE_FIND_ROOT_PATH ${CMAKE_IOS_DEVELOPER_ROOT} ${CMAKE_OSX_SYSROOT}
   ${CMAKE_PREFIX_PATH} CACHE STRING  "iOS find search path root" FORCE)
 # Default to searching for frameworks first.
 set(CMAKE_FIND_FRAMEWORK FIRST)
 # Set up the default search directories for frameworks.
 set(CMAKE_SYSTEM_FRAMEWORK_PATH
   ${CMAKE_OSX_SYSROOT}/System/Library/Frameworks
   ${CMAKE_OSX_SYSROOT}/System/Library/PrivateFrameworks
   ${CMAKE_OSX_SYSROOT}/Developer/Library/Frameworks)
 # Only search the specified iOS SDK, not the remainder of the host filesystem.
 set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM ONLY)
 set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
 set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
 # This little macro lets you set any XCode specific property.
 macro(set_xcode_property TARGET XCODE_PROPERTY XCODE_VALUE XCODE_RELVERSION)
   set(XCODE_RELVERSION_I "${XCODE_RELVERSION}")
   if (XCODE_RELVERSION_I STREQUAL "All")
     set_property(TARGET ${TARGET} PROPERTY
     XCODE_ATTRIBUTE_${XCODE_PROPERTY} "${XCODE_VALUE}")
   else()
     set_property(TARGET ${TARGET} PROPERTY
     XCODE_ATTRIBUTE_${XCODE_PROPERTY}[variant=${XCODE_RELVERSION_I}] "${XCODE_VALUE}")
   endif()
 endmacro(set_xcode_property)
 # This macro lets you find executable programs on the host system.
 macro(find_host_package)
   set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
   set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY NEVER)
   set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE NEVER)
   set(IOS FALSE)
   find_package(${ARGN})
   set(IOS TRUE)
   set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM ONLY)
   set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
   set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)
 endmacro(find_host_package)

github 上的链接

https://github.com/DeveloperMindset-com/openmp-mobile

编译器支持

参见https://mac.r-project.org/openmp/

简单点说 就是在 xcode 的 Other C Flags / Other C++ Flags 中加入 如下内容

-Xpreprocessor -fopenmp

具体操作时需要加入两行 ‘-Xpreprocessor’ ‘-fopenmp’

而不是一行’-Xpreprocessor -fopenmp’

可以用这个命令确认

clang -Xpreprocessor -fopenmp -E -dM - < /dev/null

可以看到输出包含

#define _OPENMP 201811