Parallel Programming in Fortran 95 using OpenMP - People

20 2. OpenMP constructs
• If the WORKSHARE directive is applied to a forall statement, the evaluation of the
mask expression, expressions occurring in the specification of the iteration space,
and the masked assignments are workshared.
• For ATOMIC directives and their corresponding assignments, the update of each scalar
variable is a single unit of work.
• For CRITICAL constructs, each construct is a single unit of work.
• If none of the rules above applies to a portion of the statement in the block of code,
then this portion is treated as a single unit of work.
Therefore, the application of the previous rules delivers a large amount of units of work
which are distributed over the different threads; how they are distributed is OpenMPimplementation
dependent.
In order to correctly use the !$OMP WORKSHARE directive, it is necessary to take into
account the following restrictions:
• Thecodeenclosedinsidethe!$OMP WORKSHARE/!$OMP END WORKSHARE directive-pair
must be a structured block; no branching into or out of the block of code is allowed.
• The enclosed code must only contain array assignment statements, scalar assignment
statements, forall and where statements and !$OMP ATOMIC and !$OMP CRITICAL
directives.
• The enclosed code must not contain any user defined function calls unless the function
is pure, which means it is free of side effects and has been declared with the
keyword elemental.
• Variables, which are referenced or modified inside the scope of the !$OMP WORKSHARE/!$OMP
END WORKSHARE directive-pair, must have the SHARED attribute; otherwise, the results
are unspecified.
The scope of the !$OMP WORKSHARE directive is limited to the lexical extent of the
directive-pair.
2.2 Combined parallel work-sharing constructs
The combined parallel work-sharing constructs are shortcuts for specifying a parallel
region that contains only one work-sharing construct. The behavior of these directives is
identical to that of explicitly specifying an !$OMP PARALLEL/!$OMP END PARALLEL directivepair
enclosing a single work-sharing construct.
The reason of existence of these shortcuts is to give the OpenMP-implementation a
way of reducing the overhead cost associated to both OpenMP directive-pairs, when they
appear together.