[tests][cmake] an update of ONEDPL_BACKEND variable description

[MikeDvorskiy]

[tests][cmake] an update of ONEDPL_BACKEND variable description.

[dmitriy-sobolev]

Suggested change

	| ONEDPL_BACKEND | STRING | Threading backend; supported values: tbb, dpcpp, dpcpp_only, tbb, omp, serial, ...; the default value is defined by compiler: dpcpp for DPC++ and tbb for others. These values may conflict with explicit set variables like `-DCMAKE_CXX_FLAGS="-DONEDPL_USE_DPCPP_BACKEND=0"` etc. | tbb/dpcpp |
	| ONEDPL_BACKEND | STRING | Threading backend; supported values: `dpcpp`, `dpcpp_only`, `tbb`, `omp`, `serial`. The default value is `dpcpp` if SYCL support can be detected and `tbb` otherwise. Avoid defining backend control macros (`ONEDPL_USE_TBB_BACKEND`, `ONEDPL_USE_OPENMP_BACKEND`, `ONEDPL_USE_DPCPP_BACKEND`) directly, as they may conflict with the `ONEDPL_BACKEND` setting. | tbb/dpcpp |

[dmitriy-sobolev]

The suggestion above also replaces

the default value is defined by compiler: dpcpp for DPC++ and tbb for others.

with

The default value is dpcpp if SYCL support can be detected and tbb otherwise.

It aligns the note with the relevant behaviour.

[akukanov]

Avoid defining backend control macros (ONEDPL_USE_TBB_BACKEND, ONEDPL_USE_OPENMP_BACKEND, ONEDPL_USE_DPCPP_BACKEND) directly

Would not it be better to describe what takes precedence in this case? Or is it not deterministic?

[dmitriy-sobolev]

It is deterministic. See the mapping table below.

ONEDPL_BACKEND cmake option	oneDPL backend macro
dpcpp (default if SYCL is available)	-
dpcpp_only	ONEDPL_USE_TBB_BACKEND=0
tbb (default if SYCL is not available)	ONEDPL_USE_DPCPP_BACKEND=0
omp	ONEDPL_USE_DPCPP_BACKEND=0; ONEDPL_USE_OPENMP_BACKEND=1; ONEDPL_USE_TBB_BACKEND=0
serial	ONEDPL_USE_DPCPP_BACKEND=0; ONEDPL_USE_OPENMP_BACKEND=0; ONEDPL_USE_TBB_BACKEND=0

I struggle to find a common logic between the options for a crisp description...

---

This is orthogonal to the question above, but I would prefer this set of options:

cmake flag and its options	oneDPL backend macro
ONEDPL_HOST_PARALLEL_BACKEND
auto (default)	(select the backend basing on the internal header heuristics)
tbb	ONEDPL_USE_TBB_BACKEND=1
openmp	ONEDPL_USE_OPENMP_BACKEND=1
serial	ONEDPL_USE_OPENMP_BACKEND=0; ONEDPL_USE_TBB_BACKEND=0
ONEDPL_DEVICE_BACKEND
auto (default)	(select the backend basing on the internal header heuristics)
dpcpp	ONEDPL_USE_DPCPP_BACKEND=1
off	ONEDPL_USE_DPCPP_BACKEND=0

I am going to create an issue with a feature request.