[flang][OpenMP] reduction with minus operator not supported

[mwetzstein]

Currently, reduction operations with minus are not supported and trigger

error: The minus reduction operator is deprecated since OpenMP 5.2 and is not supported in the REDUCTION clause.


Such minus reductions are currently used in various production codes, so sticking to OpenMP 5.2 and not supporting them will break such applications and require source code changes.

I request supporting the minus operator in reductions, so that flang can compile such applications out of the box without source code changes in the applications.

Here is a small reproducer which just does a simple reduction loop over one array: omp_minus_reproducer.F90.zip

[mjklemm]

The OpenMP specification has removed support for the - reduction operator in version 5.2 of the OpenMP API specification.

As per the semantics of OpenMP, a - reduction is the same as a + reduction and this is the reason it was removed from the OpenMP language. This is also the reason, why the production code should move to + instead.

[jprotze]

The OpenMP specification has removed support for the - reduction operator in version 5.2 of the OpenMP API specification.

The operator was deprecated in 5.2 and removed in 6.0.

An option would be to trigger a warning instead of an error and use the implementation of the + reduction operator.