B.1.1.1 Issues Between Sun-Style Fortran Directives and OpenMP

• Scoping of private variables must be declared explicitly with OpenMP. With Sun directives, the compiler uses its own default scoping rules for variables not explicitly scoped in a PRIVATE or SHARED clause: all scalars are treated as PRIVATE, and all array references are SHARED. With OpenMP, the default data scope is SHARED unless a DEFAULT(PRIVATE) clause appears on the PARALLEL DO directive. A DEFAULT(NONE) clause causes the compiler to flag variables not scoped explicitly. However, see 4.4 Some Tips on Using Nested Parallelism for information on autoscoping in Fortran.

• Since there is no DOSERIAL directive, mixing automatic and explicit OpenMP parallelization may have different effects: some loops may be automatically parallelized that would not have been with Sun directives.

• OpenMP provides a richer parallelism model by providing parallel regions and parallel sections. It could be possible to get better performance by redesigning the parallelism strategies of a program that uses Sun directives to take advantage of these features of OpenMP.