B.2 Converting Legacy C Pragmas

The C compiler accepts legacy pragmas for explicit parallelization. These are described in the C User’s Guide. As with the Fortran directives, these are only suggestions.

The legacy parallelization pragmas are:

Table B–5 Converting Legacy C Parallelization Pragmas to OpenMP

Legacy C Pragma	Equivalent OpenMP Pragma
#pragma MP taskloop [clauses]	#pragma omp parallel for [clauses]
#pragma MP serial_loop	No exact equivalent. You can use  #pragma omp master loop
#pragma MP serial_loop_nested	No exact equivalent. You can use  #pragma omp master loopnest

The taskloop pragma can take on one or more of the following optional clauses.

Table B–6 taskloop Optional Clauses and OpenMP Equivalents

taskloop Clause	OpenMP parallel for Equivalent Clause
maxcpus(n)	No exact equivalent. Use num_threads(n)
private(v1,v2,...)	private(v1,v2,...)
shared(v1,v2,...)	shared(v1,v2,...)
readonly(v1,v2,...)	No exact equivalent. You can achieve the same effect by using firstprivate(v1,v2,...).
storeback(v1,v2,...)	You can achieve the same effect by using lastprivate(v1,v2,...).
savelast	No exact equivalent. You can achieve the same effect by using lastprivate(v1,v2,...).
reduction(v1,v2,...)	reduction(operator:v1,v2,...). Must supply the reduction operator as well as the list of variables.
schedtype(spec)	schedule(spec) (See Table B–7)

The schedtype(spec) clause accepts the following scheduling specifications.

Table B–7 SCHEDTYPE Scheduling and OpenMP schedule Equivalents

schedtype(spec)	OpenMP schedule( spec) Clause Equivalent
SCHEDTYPE(STATIC)	schedule(static)
SCHEDTYPE(SELF(chunksize))	schedule(dynamic,chunksize) Note: Default chunksize is 1.
SCHEDTYPE(FACTORING(m))	No exact equivalent.
SCHEDTYPE(GSS(m))	schedule(guided, m) Default m is 1.

B.2.1 Issues Between Legacy C Pragmas and OpenMP

• OpenMP scopes variables declared within a parallel construct as private. A default(none) clause on a #pragma omp parallel for directive causes the compiler to flag variables not scoped explicitly.

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

• Because OpenMP provides a richer parallelism model, it is often possible to get better performance by redesigning the parallelism strategies of a program that uses legacy C directives to take advantage of these features.