Build LLVM with MSYS2

It's easier to install LLVM with pacman rather than building it yourself: pacman -S mingw64/mingw-w64-x86_64-llvm.

This assumes you have set up MSYS2.

To build LLVM:

1. Using pacman, install:
   • mingw64/mingw-w64-x86_64-make
   • mingw64/mingw-w64-x86_64-gcc
   • mingw64/mingw-w64-x86_64-cmake
2. Download LLVM source and uncompress to folder $LLVMSRC
3. Create folder $LLVMBUILD and create build.sh containing:

   PATH="/c/Windows/system32:/d/dev/msys64/mingw64/bin"
   
   LLVMSRC="/d/dev/llvm-4.0.0.src"
   LLVMINS="/d/dev/llvm/llvm-4.0"
   
   cmake \
       -G "MinGW Makefiles" \
       -DCMAKE_INSTALL_PREFIX="$LLVMINS" \
       -DCMAKE_BUILD_TYPE="Release" \
       -DLLVM_TARGETS_TO_BUILD="X86" \
       -DLLVM_INCLUDE_TESTS=OFF \
       -DLLVM_ENABLE_CXX1Y=ON \
       -DLLVM_ENABLE_ASSERTIONS=ON \
       -DSPHINX_OUTPUT_HTML=OFF \
       -DSPHINX_OUTPUT_MAN=OFF \
       "$LLVMSRC"
   
   cmake --build .
   cmake --build . --target install

4. Customize the paths in build.sh accordingly
5. Customize the LLVM_* options according to your needs; they're documented here
6. The list of backends for LLVM_TARGETS_TO_BUILD is in $LLVMSRC/CMakeLists.txt, near set(LLVM_ALL_TARGETS. Note: CppBackend was removed in 3.9 :(
7. cd into $LLVMBUILD
8. sh build.sh