DECLARE REDUCTION

!$OMP DECLARE REDUCTION (reduction-identifier : type-list : combiner) [initializer-clause]

The DECLARE REDUCTION directive is a specification directive. It can appear in a specification part of a subroutine, function, main program, module, or block construct.

User-defined (custom) reductions can be defined using the DECLARE REDUCTION directive. The reduction is identified by the reduction-identifier and the associated type from type-list. The reduction-identifier can be used in a REDUCTION clause in another OpenMP* directive anywhere it is accessible by use or host association.

A DECLARE REDUCTION directive cannot redefine a predefined reduction-identifier (see the table of implicitly defined reduction identifiers in the REDUCTION clause section).

If a type in type-list has deferred or assumed-length type parameters, the reduction-identifier can be used in a REDUCTION clause with a variable of the same type and kind type parameter as type, regardless of the length parameter with which the variable is declared. The length parameter of a character type must be a constant, colon, or asterisk. An accessible reduction-identifier defined with a deferred or assumed-length character type cannot appear in another DECLARE REDUCTION directive with a type-list item of type character with the same kind type parameter.

The accessibility of a reduction-identifier is determined by the same rules as for other Fortran entities; it can be declared PUBLIC or PRIVATE, be made accessible or blocked by a USE or IMPORT statement, and it can be renamed. If the reduction-identifier is the same as a generic name that is also the name of a derived type, the accessibility of the reduction-identifier is the same as that of the generic name.

If a subroutine or function used in initializer-expression or combiner is not an intrinsic procedure, it must have an accessible interface. Defined operators and defined assignments used in initializer or combiner must have accessible interfaces. All subroutines, functions, defined operators and defined assignments used in initializer or combiner must have accessible interfaces in the subprogram in which the corresponding REDUCTION clause appears. Procedures referenced in combiner and initializer cannot be alternate return subprograms.

The initial value of a user-defined reduction is not known before it is specified. The initializer-clause can be used to specify an initial value for the reduction variable. The initializer-clause will be executed to establish initial values for the private copies of reduction list items indicated in a REDUCTION clause that specifies the reduction-identifier.

If initializer is not specified, private reduction variables are initialized as follows:

• If the reduction variable is type COMPLEX, REAL, or INTEGER, the default initializer is the value zero.

• If the reduction variable specified in list of the REDUCTION clause is LOGICAL, the default initializer is the value .FALSE..

• If the reduction variable is of a default initialized derived type, the default initializer value is used.

• Otherwise, the initial value is unspecified.

If initializer is used in a target region, then a DECLARE TARGET construct must be specified for any procedures that are executed during the execution of combiner or initializer.

If the execution of combiner or initializer results in the execution of an OpenMP* construct or an OpenMP* API call, the behavior is undefined. If the variable omp_orig is defined during execution of initializer, the behavior is unspecified.

Example

Consider that a DECLARE REDUCTION directive is used to declare a sum reduction for an integer component of a type my_type that is identified by the reduction-identifier '+'. It is then used in a REDUCTION clause of a parallel region to produce the sum of the thread numbers (numbered 1 thru 4) of the region:

module types
  type my_type
    integer :: component
  end type 
  interface operator(+)
    module procedure :: my_add
  end interface
!$omp declare reduction (+ : my_type : omp_out = omp_out + omp_in) initializer (omp_priv = my_type (/0/))
... 
  contains
    function my_add (a1, a2)
      type(my_type),intent(in) :: a1, a2
      type(my_type)            :: my_add
      my_add%component = a1%component + a2%component
      return
    end function my_add
end module types

program main
  use types
  use omp_lib
  type(my_type) :: my_var
!$omp parallel reduction (+ : my_var) num_threads(4)
  my_var%component = omp_get_thread_num() + 1
!$omp end parallel
  print *, "sum of thread numbers is ", my_var%component
end program

The output of the program follows:

sum of thread numbers is 10