If we are in C++ mode and include <math.h> (not <cmath>) first, we still
need to make sure <cmath> is read first. The problem otherwise is that
we haven't seen the declarations of the math.h functions when the system
math.h includes our cmath overlay. However, our cmath overlay, or better
the underlying overlay, e.g. CUDA, uses the math.h functions. Since we
haven't declared them yet we get errors. CUDA avoids this by eagerly
declaring all math functions (in the __device__ space) but we cannot do
this. Instead we break the dependence by forcing cmath to go first.
Reviewed By: JonChesterfield
Differential Revision: https://reviews.llvm.org/D77774
For OpenMP target regions to piggy back on the CUDA/AMDGPU/... implementation of math functions,
we include the appropriate definitions inside of an `omp begin/end declare variant match(device={arch(nvptx)})` scope.
This way, the vendor specific math functions will become specialized versions of the system math functions.
When a system math function is called and specialized version is available the selection logic introduced in D75779
instead call the specialized version. In contrast to the code path we used so far, the system header is actually included.
This means functions without specialized versions are available and so are macro definitions.
This should address PR42061, PR42798, and PR42799.
Reviewed By: ye-luo
Differential Revision: https://reviews.llvm.org/D75788
Johannes Doerfert