Add llvm-openmp as recommended OpenMP runtime for Windows

[matthewfeickert]

• Add recommendation of having llvm-openmp be a host requirement for Windows builds that need an OpenMP runtime and note that for this to work the /openmp:llvm option must be added.
  • c.f. https://learn.microsoft.com/en-us/cpp/build/reference/openmp-enable-openmp-2-0-support?view=msvc-170
• c.f. #general > ✔ Help with Windows, CMake, Ninja, and overlinking @ 💬

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[matthewfeickert]

Relevant build preview page: https://deploy-preview-2804--conda-forge-previews.netlify.app/docs/maintainer/knowledge_base/#openmp

[matthewfeickert]

👋 Ping to @conda-forge/core for review.

[isuruf]

I wouldn't recommend vcomp14 as it's not updated anymore by microsoft and they are switching to llvm openmp soon.

[matthewfeickert]

I wouldn't recommend vcomp14 as it's not updated anymore by microsoft and they are switching to llvm openmp soon.

Oh that's good news (I think). For the immediate time period, there are llvm-openmp build for win-64, so should existing recipes that currently are using vcomp14 start switching to llvm-openmp?

[isuruf]

Yes, but it needs a separate option. See https://learn.microsoft.com/en-us/cpp/build/reference/openmp-enable-openmp-2-0-support?view=msvc-170

[matthewfeickert]

Yes, but it needs a separate option. See https://learn.microsoft.com/en-us/cpp/build/reference/openmp-enable-openmp-2-0-support?view=msvc-170

Okay, so seems like we need to use the /openmp:llvm option for Windows. So need to make it clear that people either need to set that in their code or set it through something like

build:
   script:
    - if: win
      then: set "CXXFLAGS=%CXXFLAGS% /openmp:llvm"
...

[matthewfeickert]

@conda-forge/core I think this is ready for review again.

The linter error I don't think is associated with my PR:

### Errors in build/docs/maintainer/infrastructure/index.html

* [404] [https://github.com/conda-forge/staged-recipes/blob/main/.github/workflows/automate-review-labels.yml](https://github.com/conda-forge/staged-recipes/blob/main/.github/workflows/automate-review-labels.yml) | Failed: Network error: Not Found

[matthewfeickert]

👋 ping to @conda-forge/core for review.

[matthewfeickert]

👋 ping to @conda-forge/core.

[chrisburr]

I think this looks like it's ready to merged. @h-vetinari/@isuruf as you know better than I do for Windows, do you agree?

[matthewfeickert]

Thanks for the review @isuruf. Though the "checks links" linter that is failing given the state of the HEAD of main is keeping all checks from completing and so auto-merge won't pass. Can you please manually merge this?

[matthewfeickert]

cc @conda-forge/core RE: manual merge request in #2804 (comment).