HP Fortran Programmer's Guide (March 2010)

Using Fortran directives
Compatibility directives
Chapter 9 225
To use the vectorization directive, you must compile and link with the +Ovectorize option.
The directive applies to the beginning of the next loop and remains in effect for the rest of the
program unit or until superseded by a later directive. For more information about this option,
see the Parallel Programming Guide for HP-UX Systems.
Controlling parallelization
HP Fortran can parallelize eligible program loops by distributing different iterations of the
loop to different processors for parallel execution on a multiprocessor machine. The following
directives provide local control over parallelization:
• *$* [NO]CONCURRENTIZE
• DIR$ [NO]CONCUR
These directives have both enable and disable versions: *$* CONCURRENTIZE and
DIR$ CONCUR enable parallelization; *$* NOCONCURRENTIZE and DIR$ NOCONCUR disable
parallelization.
The parallelization directives are effective only if you have compiled and linked the program
with the +Oparallel and the +O3 option. Each directive applies to the beginning of the next
loop and remains in effect for the rest of the program unit or until superseded by a later
directive.
The compiler considers the *$* CONCURRENTIZE and DIR$ CONCUR directives as requests to
parallelize a loop. If the compiler cannot profitably or safely parallelize the loop, it ignores the
directive. For information about conditions that can inhibit parallelization, see the Parallel
Programming Guide for HP-UX Systems.
Controlling dependence checks
The compiler will not parallelize a loop where it detects a possible data dependence, even if
you use an option or directive that specifically requests parallelization. However, if you know
that there is no actual data dependence in the loop in question, you can insert one of the
following directives just before the loop:
• DIR$ IVDEP
• FPP$ NODEPCHK
• VD$ NODEPCHK
The effect of these directives is to cause the compiler to ignore data dependences within the
next loop when determining whether to parallelize. The DIR$ IVDEP directive differs from the
other two in that it causes the compiler to ignore only array-based dependences, but not
scalar-based. All three directives apply to the next loop only.