[openmp] Allow testing OpenMP without a full clang build tree

[mstorsjo]

Having a build tree with "not" and "FileCheck" is still required, but if Clang isn't configured in that build, run the tests with the same compiler CMake uses. This is how testing worked in the standalone build configurations that now have been removed.

[Meinersbur]

Could you also add logic that disables testing again when OPENMP_TEST_C_COMPILER is not set despite no clang target available?

[Meinersbur]

Suggested change

	set(CLANG_DEPEND)
	set(EXTRA_CHECK_DEPENDS "")

set(CLANG_DEPEND) is equivalent to unset(CLANG_DEPEND)

[Meinersbur]

Consider adding that you need OPENMP_TEST_C_COMPILER/OPENMP_TEST_CXX_COMPILER if you want testing without clang in LLVM_BINARY_DIR

[Meinersbur]

Suggested change

	It will use Clang from `LLVM_BINARY_DIR` for running the regression tests, if Clang is included in that build.
	Clang/Flang from `LLVM_BINARY_DIR` will be used for testing if available, otherwise `CMAKE_C_COMPILER`/`CMAKE_CXX_COMPILER`/`CMAKE_Fortran_COMPILER`. Tests are also only expected to work with Clang/Flang built from the same Git commit, so `OPENMP_TEST_C_COMPILER`/`OPENMP_TEST_CXX_COMPILER`/`OPENMP_TEST_Fortran_COMPILER` can be used to explicitly set the test compilers.

[Meinersbur]

If not too much work, could you make OPENMP_TEST_Fortran_COMPILER be handled the same way?

[mstorsjo]

Could you also add logic that disables testing again when OPENMP_TEST_C_COMPILER is not set despite no clang target available?

As we implicitly set OPENMP_TEST_C_COMPILER from CMAKE_C_COMPILER, so the only case that would hit is if the user manually sets OPENMP_TEST_C_COMPILER to an empty string. So we shouldn't need that, as configuring the project does require a working CMAKE_C_COMPILER to begin with.

If not too much work, could you make OPENMP_TEST_Fortran_COMPILER be handled the same way?

Sure, I tried to make that mostly consistent. There's some extra logic for printing/disabling that if that's unavailable, that I left untouched for now.

Now whether a build with LLVM_ENABLE_PROJECTS="clang;flang" LLVM_ENABLE_RUNTIMES=openmp will build flang before attempting to configure openmp or not, I'm not sure; currently it builds clang before configuring openmp at least. (That might require changes to llvm-project/llvm/runtimes if we'd want to add such an implicit dependency there. But I feel that's out of scope for this PR in any case.)

[Meinersbur]

Suggested change

	unset(EXTRA_CHECK_DEPENDS)
	set(EXTRA_CHECK_DEPENDS "")

By set(EXTRA_CHECK_DEPENDS) and unset(EXTRA_CHECK_DEPENDS) being equivalent I meant to not use it. unsettin the value will mean that the value of outer namespace becomes visible:

function(foo)
  set(EXTRA_CHECK_DEPENDS) # or unset(EXTRA_CHECK_DEPENDS) 
  message("EXTRA_CHECK_DEPENDS='${EXTRA_CHECK_DEPENDS}'")
endfunction()


set(EXTRA_CHECK_DEPENDS "Outer namespace value" CACHE STRING "Example variable")
foo()

$ cmake -P unset.cmake
EXTRA_CHECK_DEPENDS='Outer namespace value'

The CMake manual puts it like this:

[mstorsjo]

Oh, right, sorry, I missed the point there! Will fix.

[Meinersbur]

Suggested change

	It will use Clang from `LLVM_BINARY_DIR` for running the regression tests, if Clang is included in that build.
	Clang/Flang from `LLVM_BINARY_DIR` will be used for testing if available, otherwise `CMAKE_C_COMPILER`/`CMAKE_CXX_COMPILER`/`CMAKE_Fortran_COMPILER`. Tests are also only expected to work with Clang/Flang built from the same Git commit, so `OPENMP_TEST_C_COMPILER`/`OPENMP_TEST_CXX_COMPILER`/`OPENMP_TEST_Fortran_COMPILER` can be used to explicitly set the test compilers.

[Meinersbur]

LGTM, thank you

[mstorsjo]

Thanks for the reviews!

Would you consider this patch for a backport to 22.x? It wouldn't be a direct match though, but would need to be adjusted to how the code was before removing the standalone build mode. My desire would be to have a single way to run the tests (in my setup) for both the 22.x and main branches.

[Meinersbur]

22.1 has just been released: https://discourse.llvm.org/t/llvm-22-1-0-released/89950

If you want to prepare a PR for 22.1.1, I would review it. It will depend on the release manager whether they will accept non-bugfix build system changes.