HP Fortran Programmer Guide (766160-001, March 2014)

unit or until superseded by a later directive. For more information about this option, see the Parallel
Programming Guide for HP-UX Systems.
NOTE: The + Ovectorizeoption is valid only on the PA-RISC 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: *$* CONCURRENTIZE and DIR$
CONCURenable parallelization; *$* NOCONCURRENTIZEand DIR$ NOCONCURdisable
parallelization.
The parallelization directives are effective only if you have compiled and linked the program with
the +Oparallel and the +O3option. 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remains in effect for the rest of the program unit or until superseded by a later directive.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 *$* CONCURRENTIZEand DIR$ 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$ IVDEPdirective 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.
NOTE: Using these directives to incorrectly assert that a loop has no data dependences can result
in the loop producing wrong answers.
Other conditions may limit the compiler's efforts to parallelize, such as the presence of the VD$
NOCONCURdirective. Such conditions may prevent parallelization even if you use a directive to
disable dependence checking.
Controlling checks for side effects
The compiler will not parallelize a loop with an embedded call to a routine if the compiler finds
that the routine has side effects. However, if you know that a routine that is called inside of a loop
does not have side effects, you can insert the DIR$ NO SIDE EFFECTSdirective in front of the
loop to force the compiler to ignore any side effects in the referenced routine when it determines
whether to parallelize the loop.
This directive affects only the immediately following loop.
Compatibility directives 129