[Win] Update ClangCl for OpenMP

[ax3l]

Tool information

• Tool name: ClangCl with omp.dll
• Add: omp.dll
• Desired version: same as Clang from LLVM
• Approximate size: ~1MB?
• If this is an add request:
  • Brief description of tool: OpenMP support in LLVM for Clang on Windows
  • URL for tool's homepage: https://openmp.llvm.org/
  • Provide a basic test case to validate the tool's functionality: https://computing.llnl.gov/tutorials/openMP/exercise.html or https://github.com/conda-forge/openmp-feedstock/blob/master/recipe/omp_hello.c

Rationale: MSVC only supports OpenMP 2.0, which is very ancient (2002 standard).
LLVM/Clang + LLVM/OpenMP is a great toolchain for modern OpenMP support on Windows.

Area for Triage:
Windows Compiler Support (Clang) - Support libraries

Question, Bug, or Feature?:
Feature request

Virtual environments affected

• Ubuntu 16.04
• Ubuntu 18.04
• Ubuntu 20.04
• macOS 10.15
• macOS 11.0
• Windows Server 2016 R2
• Windows Server 2019

Can this tool be installed during the build?
Yes

Tool installation time in runtime
Should not add overly much on top of an LLVM/Clang build.

Example from conda-forge: https://github.com/conda-forge/openmp-feedstock/blob/master/recipe/bld-llvm-openmp.bat

Are you willing to submit a PR?
I can try with guidance, but my Windows skills are very rusty.

[dsame]

@ax3l
Just to clarify the requirements. The official site says:

With the release of Clang 3.8.0, OpenMP 3.1 support is enabled in Clang by default, and the OpenMP runtime is therefore built as a normal part of the Clang build

Does it relate to your statement "MSVC only supports OpenMP 2.0, which is very ancient (2002 standard)." and thus we should upgrade openmp only?

[ax3l]

@dsame thank you for the hint!

Can you tell me what else I need in the virtual environment besides -T "ClangCl" to find this OpenMP install with CMake?
I am trying this, but cannot pick up the libs with find_package(OpenMP REQUIRED): BLAST-WarpX/warpx#1541

-- The C compiler identification is Clang 10.0.0 with MSVC-like command-line
-- The CXX compiler identification is Clang 10.0.0 with MSVC-like command-line
...
CMake Error at C:/Program Files/CMake/share/cmake-3.18/Modules/FindPackageHandleStandardArgs.cmake:165 (message):
  Could NOT find OpenMP_CXX (missing: OpenMP_CXX_FLAGS OpenMP_CXX_LIB_NAMES)

[ax3l]

I think I found a way by using the Ninja generator 🎉

  build_win_clang:
    name: Clang w/ OMP
    runs-on: windows-2019
    steps:
    - uses: actions/checkout@v2
    - uses: seanmiddleditch/gha-setup-ninja@master
    - name: Build & Install
      shell: cmd
      run: |
        call "C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\vc\Auxiliary\build\vcvarsall.bat" x64
        cmake -S . -B build                 ^
              -G "Ninja"                    ^
              -DCMAKE_BUILD_TYPE=Release    ^ 
              -DCMAKE_C_COMPILER=clang-cl   ^
              -DCMAKE_CXX_COMPILER=clang-cl
        cmake --build build --config Release --parallel 2

[ax3l]

@dsame Is there a recommended shortcut/environment variable for the location of vcvarsall.bat?

[dsame]

@ax3l actually now. The path to these bats is documented in VS site

The closest variable you can use are:

ProgramFiles=C:\Program Files
ProgramFiles(x86)=C:\Program Files (x86)