[offload] [cmake] Standalone build test logic is broken

[mgorny]

I'm trying to figure out how to build it post-openmp/offload split, and the test logic just doesn't seem to make any sense to me.

So far I've noticed that:

• offload reuses the OpenMPTesting logic that tries to use OPENMP_TEST_{C,CXX}_COMPILER
• however, the cmake/DetectTestCompiler directories has not been copied from openmp, so the whole logic fails
• test directory instead uses CMAKE_{C,CXX}_COMPILER

Even if I copy the directory, ensure that C/C++ compiler is clang and make the whole CMake pass, the test target seems to be broken:

$ ninja check-libomptarget
ninja: error: 'test/omp', needed by 'test/CMakeFiles/check-libomptarget', missing and no known rule to make it

And if I remove the wrong dependency from test/CMakeLists.txt, the tests fail because of:

# .---command stderr------------
# | x86_64-pc-linux-gnu-clang: error: '-fopenmp-targets' must be used in conjunction with a '-fopenmp' option compatible with offloading; e.g., '-fopenmp=libomp' or '-fopenmp=libiomp5'
# `-----------------------------
# error: command failed with exit status: 1

[mgorny]

CC @jdoerfert

[llvmbot]

@llvm/issue-subscribers-offload

Author: Michał Górny (mgorny)

I'm trying to figure out how to build it post-openmp/offload split, and the test logic just doesn't seem to make any sense to me.

So far I've noticed that:

• offload reuses the OpenMPTesting logic that tries to use OPENMP_TEST_{C,CXX}_COMPILER
• however, the cmake/DetectTestCompiler directories has not been copied from openmp, so the whole logic fails
• test directory instead uses CMAKE_{C,CXX}_COMPILER

Even if I copy the directory, ensure that C/C++ compiler is clang and make the whole CMake pass, the test target seems to be broken:

$ ninja check-libomptarget
ninja: error: 'test/omp', needed by 'test/CMakeFiles/check-libomptarget', missing and no known rule to make it

And if I remove the wrong dependency from test/CMakeLists.txt, the tests fail because of:

# .---command stderr------------
# | x86_64-pc-linux-gnu-clang: error: '-fopenmp-targets' must be used in conjunction with a '-fopenmp' option compatible with offloading; e.g., '-fopenmp=libomp' or '-fopenmp=libiomp5'
# `-----------------------------
# error: command failed with exit status: 1

[jhuber6]

I think we should be able to set up the build system to work fine standalone. We should rewrite all of the offloading CMake honestly. I'll try to get on that once my other overhaul patches land.