In order to build flang on a Windows on Arm machine, you will first need to setup your machine for LLVM development as described here: How to setup Windows on Arm for llvm development
You will have to add flang and mlir to the list of enabled projects.
At the moment, due to a bug that needs further investigation, clang is failing to find the builtins, and adding the path to the builtin library as described in the https://clang.llvm.org/docs/UsersManual.html#finding-clang-runtime-libraries doesn’t seem to be working either (bug report pending, since the LLVM bugzilla is currently locked due to the upstream migration to Github issues). However, we can work around this issue by adding the builtin library directly on every link command (-DCMAKE_EXE_LINKER_FLAGS=path/to/clang_rt.builtins-aarch64.lib).
You should end up with a script that looks something like this:
REM You need to modify the paths below: set build_dir=path\to\where\you\want\the\build set clang_root=path\to\where\clang\is\installed set clang_version=x.0.y (should match what’s in %clang_root%) REM Some helper variables. REM Setting CMAKE_CL_SHOWINCLUDES_PREFIX to work around PR27226. set cmake_flags=^ -DCMAKE_BUILD_TYPE=Release ^ -DLLVM_ENABLE_ASSERTIONS=ON ^ -DLLVM_INSTALL_TOOLCHAIN_ONLY=ON ^ -DLLVM_BUILD_LLVM_C_DYLIB=ON ^ -DCMAKE_INSTALL_UCRT_LIBRARIES=ON ^ -DCMAKE_CL_SHOWINCLUDES_PREFIX="Note: including file: " ^ -DLLVM_DEFAULT_TARGET_TRIPLE=aarch64-windows-msvc ^ -DLLVM_HOST_TRIPLE=aarch64-windows-msvc ^ -DLLVM_TARGET_ARCH=AArch64 ^ -DCLANG_DEFAULT_LINKER=lld cd %build_dir% set clang_path=%clang_root%\bin\clang-cl.exe set builtins_path=%clang_root%\lib\clang\%clang_version%\lib\windows set builtins_lib=clang_rt.builtins-aarch64.lib set CC=%clang_path% set CXX=%clang_path% cmake -GNinja %cmake_flags% ^ -DLLVM_ENABLE_PROJECTS="clang;flang;mlir" ^ -DLLVM_TARGETS_TO_BUILD="AArch64" ^ -DCMAKE_C_FLAGS="-fms-compatibility-version=19.14" ^ -DCMAKE_CXX_FLAGS="-fms-compatibility-version=19.14" ^ -DCMAKE_EXE_LINKER_FLAGS="%builtins_path%/%builtins_lib%" ^ ..\llvm-project\llvm || exit /b ninja all || ninja all || ninja all || exit /b
Depending on which revision of flang you’re building, you should now have one or both of f18.exe or flang-new.exe in %build_dir%/bin.