Parallel Programming in Fortran 95 using OpenMP - People

46 3. PRIVATE, SHARED & Co.
Operator/Intrinsic Initialization
+ 0
∗ 1
- 0
.AND.
.TRUE.
.OR.
.FALSE.
.EQV.
.TRUE.
.NEQV.
.FALSE.
MAX
smallest representable number
MIN
largest representable number
IAND
all bits on
IOR 0
IEOR 0
Table 3.1: Initialization rules for variables included in REDUCTION clauses.
operator
= +, *, -, .AND., .OR., .EQV. or .NEQV.
intrincic_procedure = MAX, MIN, IAND, IOR or IEOR
The variable x, affected by the REDUCTION clause, must be of scalar nature and of
intrinsic type. Even though, x may be the scalar entry in an array which is not of
deferred shape or assumed size. If this is the case, the computational overhead due to the
creation and initialization of the private copies of the variables can be very large if x is a
large array.
Obviously, the expression expr must also be a scalar expression that does not make use
of the value of x, since this magnitude is only known at the end of the REDUCTION process.
Also, although it seems that the intrinsic procedures do not fulfill this condition, this is
not true, since only procedures of associative nature are allowed in the REDUCTION clause.
In an opening-directive it is possible to specify any number of REDUCTION clauses, but
a variable can appear only once in these REDUCTION clauses.
3.2 Other clauses
Additionally to the previously presented data scope attribute clauses, a few more clauses
are available, which are presented in the following subsections.
3.2.1 IF(scalar logical expression)
Maybe it is not always interesting to run certain source code lines in parallel, for example
if the computational cost of creating and closing the parallel region exceeds the possible
gain achieved by running the source code lines in parallel. In the following example
!$OMP PARALLEL IF(N > 1000)
!$OMP DO