openmp/system: new recipe

[valgur]

Adds a new openmp/system package for a much more convenient and less bug-prone OpenMP support in recipes.

This avoids the need for listing the appropriate flags in every recipe using OpenMP, e.g. https://github.com/conan-io/conan-center-index/blob/master/recipes/lightgbm/all/conanfile.py#L123-L133

It relies on the default OpenMP provided by the compiler for compilers that ship with one (GCC and MSVC) and falls back to llvm-openmp from CCI for others (Clang and AppleClang).

transitive_headers=True, transitive_libs=True can be used to set the correct visibility for libraries that use #pragma omp in their public headers.

See #24577 for more context.

Requires

• llvm-openmp: match FindOpenMP.cmake output, add MSVC support #22353
• llvm-openmp: fix incompatibilities with official FindOpenMP, add v18.1.8, simplify patches, add MSVC support #24584

[conan-center-bot]

Conan v1 pipeline ✔️

All green in build 11 (fda846f782bf90dfa592b294bee0f26f1acc6fd7):

• openmp/system:
  All packages built successfully! (All logs)

---

Conan v2 pipeline ✔️

Note: Conan v2 builds are now mandatory. Please read our discussion about it.

All green in build 11 (fda846f782bf90dfa592b294bee0f26f1acc6fd7):

• openmp/system:
  All packages built successfully! (All logs)

[jacobfriedman]

Why not just use the llvm-openmp recipe, ignore the wrapping, and append the openmp argument in downstream recipes?

Edit: Adding yet another openmp to conan is not ideal. I recommend recipes add the argument, rather than bloating the ecosystem @jcar87 @planetmarshall @memsharded

[memsharded]

Edit: Adding yet another openmp to conan is not ideal. I recommend recipes add the argument,

Recall that it is also possible to inject compiler flags with tools.build:cxxflags, so adding arbitrary flags can be done from the user side without really modifying libraries.

[valgur]

Edit: Adding yet another openmp to conan is not ideal. I recommend recipes add the argument,

Recall that it is also possible to inject compiler flags with tools.build:cxxflags, so adding arbitrary flags can be done from the user side without really modifying libraries.

@memsharded I do recall that, but the proposed openmp/system is mostly concerned with correct handling OpenMP as a part of the dependency graph (that is, correct visibility of the runtime library dependency via the transitive_libs=True attribute and propagating the preprocessor flag via transitive_headers=True for public headers that rely on OpenMP features) rather than a mere optimization flag. Also, I hope you're not seriously suggesting force-enabling OpenMP for all recipes that might support it (#24577 (comment))?

[jacobfriedman]

I would explicitly define requirements in packages, rather than wrapping them, which is a workaround. I wouldn't expect force-enabling openmp, rather, it would be an option- for all recipes that might support it.