Technical data

Writing Parallel Fortran

For multiprocessing to work correctly, the iterations of the loop must not

depend on each other; each iteration must stand alone and produce the same

answer regardless of whether any other iteration of the loop is executed. Not

all DO loops have this property, and loops without it cannot be correctly

executed in parallel. However, any of the loops encountered in practice ﬁt

this model. Further, many loops that cannot be run in parallel in their

original form can be rewritten to run wholly or partially in parallel.

To provide compatibility for existing parallel programs, Silicon Graphics has

chosen to adopt the syntax for parallelism used by Sequent Computer

Corporation. This syntax takes the form of compiler directives embedded in

comments. These fairly high level directives provide a convenient method

for you to describe a parallel loop, while leaving the details to the Fortran

compiler. For advanced users, there are a number of special routines that

permit more direct control over the parallel execution. (Refer to “Advanced

Features” on page 97 for more information.)

Writing Parallel Fortran

The Fortran compiler accepts directives that cause it to generate code that

can be run in parallel. The compiler directives look like Fortran comments:

they begin with a C in column one. If multiprocessing is not turned on, these

statements are treated as comments. This allows the identical source to be

compiled with a single-processing compiler or by Fortran without the

multiprocessing option. The directives are distinguished by having a $ as the

second character. There are six directives that are supported:

C$DOACROSS, C$&, C$, C$MP_SCHEDTYPE, C$CHUNK, and

C$COPYIN. The C$COPYIN directive is described in “Local COMMON

Blocks” on page 102. This section describes the others.

C$DOACROSS

The essential compiler directive is C$DOACROSS. This directs the compiler

to generate special code to run iterations of the DO loop in parallel. The

C$DOACROSS statement applies only to the next statement (which must

be a DO loop).