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Cross-compiling Swift for Windows with clang
This document describes how to cross compile Swift for Windows on a non-Windows host. For more context on the status of Swift on Windows in general, see Getting Started with Swift on Windows
1. Set up Visual Studio environment variables
Building for Windows requires that the Visual Studio environment variables are setup similar to the values on Windows. Currently, the runtime has been tested to build against the Windows 10 SDK at revision 10.10.586.
# Visual Studio 2015 does not have VCToolsInstallDir, use VCINSTALLDIR's value
export UCRTVersion=10.0.10586.0
export UniversalCRTSdkDir=".../Windows Kits/10"
export VCToolsInstallDir=".../Microsoft Visual Studio/2017/Community"
2. Set up the visualc and ucrt modules
The ucrt.modulemap located at
swift/stdlib/public/Platform/ucrt.modulemap needs to be copied into
${UniversalCRTSdkDir}/Include/${UCRTVersion}/ucrt as module.modulemap. The
visualc.modulemap located at swift/stdlib/public/Platform/visualc.modulemap
needs to be copied into ${VCToolsInstallDir}/include as module.modulemap
3. Configure the runtime to be built with the just built clang
Ensure that we use the tools from the just built LLVM and clang tools to
build the Windows SDK. You will need to pass a few extra options to cmake via
the build-script invocation to achieve this. You will need to expand out the
path where llvm-ar and llvm-ranlib are built. These are needed to correctly
build the static libraries. Note that cross-compiling will require the use of
lld. Ensure that lld-link.exe is available to clang via your path.
macOS
--extra-cmake-options=-DSWIFT_BUILD_RUNTIME_WITH_HOST_COMPILER=FALSE,\
-DCMAKE_AR=<path to llvm-ar>,\
-DCMAKE_RANLIB=<path to llvm-ranlib>,\
-DSWIFT_SDKS='OSX;WINDOWS'
Linux
For Linux, you will need to build the Linux SDK instead of the macOS SDK by
replacing the cmake option with -DSWIFT_SDKS='LINUX;WINDOWS'.
4. Build the Swift runtime and standard library with ninja
From the build directory, you can build the Swift runtime and standard library
for Windows using ninja swiftCore-windows-x86_64.