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.

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	Oracle Solaris Studio 12.2: OpenMP API User's Guide