DECLARE VARIANT

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ID 767251

Date 10/31/2024

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DECLARE VARIANT

OpenMP* Fortran Compiler Directive: Identifies a variant of a base procedure and specifies the context in which this variant is used.

Syntax

!$OMP DECLARE VARIANT ([base-proc-name:]variant-proc-name) clause[[[,] clause]... ]

base-proc-name

Is the name of a base procedure. It is the name that appears in a procedure reference and is replaced by the variant name if the procedure reference appears in the OpenMP* context specified by the MATCH clause. base-proc-name must have an accessible explicit interface.

variant-proc-name

Is the name of the variant procedure that is to be called instead of the base procedure if the base procedure is called from an OpenMP* context that matches the context specified by the MATCH clause.

clause

Is one or more of the following:

ADJUST_ARGS (adjust-op : argument-list)

Causes the adjust-op operation to be performed to each argument specified in argument-list before calling the variant procedure.

adjust-op is either need_device_ptr or nothing.

If need_device_ptr is specified, it causes the listed arguments to be converted to corresponding device pointers of the default device. An argument in the argument-list following need_device_ptr must be of type C_PTR.

If nothing is specified, the arguments listed are passed without modification.

Multiple ADJUST_ARG clauses can appear for a DECLARE VARIANT directive.
APPEND_ARGS (append-op [[, append-op] … ]

Causes additional arguments to be passed to the call at the end of the argument list of the base procedure.

append-op is:

INTEROP (modifier-list)

where modifier-list is a valid modifier-list accepted in an INIT clause of the INTEROP directive.

Only one APPEND_ARGS clause can appear in a DECLARE VARIANT directive.
MATCH (context-selector-specification-list)

Specifies OpenMP* context selectors that determine if a variant function is to be used as a base function replacement in the specified context. Any variable referenced in an expression of a context selector must be accessible at the call site to the base function.

An APPEND_ARGS clause or ADJUST_ARGS clause is not permitted unless a MATCH clause also appears that specifies CONSTRUCT= {DISPATCH} as a selector.

The DECLARE VARIANT directive is a declarative directive and must appear in the specification part of a subroutine or function, or in an interface in an interface block. It is a pure directive, so it can appear in a Fortran PURE procedure. It identifies the name of a variant procedure that is to be called instead of the base procedure when the call appears in a context that matches the context-selector-specification in the MATCH clause.

If base-proc-name is not specified, the name of the procedure containing the directive is the base-proc-name. base-proc-name must not be a dummy procedure name, a statement function name, a generic name, a procedure pointer, or an alternate entry name.

If a DECLARE VARIANT directive appears in an interface body for a procedure, it must match a DECLARE VARIANT directive in the definition of that procedure. If a DECLARE VARIANT directive appears for a procedure with an explicit interface, and the definition of that procedure also contains a DECLARE VARIANT directive for that procedure, the two directives must match.

Multiple DECLARE VARIANT directives can associate different variant-proc-names with the same base-proc-name. If more than one DECLARE VARIANT associates the same variant-proc-name with the same base-proc-name, then the context-selector-specification must be the same for all such directives.

A variant procedure must have the same interface characteristics as the base procedure.

When the ADJUST_ARGS clause is specified, an argument with the is-device-ptr property in its interoperability requirement set will be passed as is. Otherwise, the argument will be converted in the same way that a USE_DEVICE_PTR clause in a TARGET DATA construct converts a pointer list item into a device pointer.

When the APPEND_ARGS clause appears, the following occurs:

For each modifier specified, an additional argument of type omp_interop_kind from the interoperability requirement set of the encountering task is added to the end of the argument list of the base procedure.

The ordering of the appended arguments is the same as the order of the modifiers that are specified in modifier-list in parentheses following the INTEROP keyword in the APPEND_ARGS clause.
Each argument is constructed as if an INTEROP construct specifying an INIT clause with the corresponding modifier was present.
If the interoperability requirement set contains properties that could be used as INTEROP construct clauses, it is as if the INTEROP construct also contained those clauses, and the properties will be removed from the interoperability requirement set.
Each appended argument is destroyed after the selected variant completes executions as if the INTEROP construct contained a DESTROY clause.

If the variant is invoked by a DISPATCH construct that contains an INTEROP clause with n variables specified, the first n modifiers specified in the APPEND_ARGS clause are ignored and replaced by the n variables specified in the INTEROP clause of the DISPATCH directive. The order of these n variables appearing in the argument list is the same order that they are specified in the INTEROP clause of the DISPATCH directive.

If there are m modifiers specified in the APPEND_ARG clause, and m > n, an argument for each of the remaining m - n modifiers in the APPEND_ARGS clause is constructed and appended to the end of the argument list in the same order in which they appear in the APPEND_ARGS clause.

Calling a procedure variant directly by variant-proc-name within an OpenMP* context that is different than the context specified in the MATCH clause is non-conforming.

Example

The DECLARE VARIANT directive in the module procedure vecadd_base identifies the procedure vecadd_gpu_offload as a variant that is to replace references to vecadd_base when called from a DISPATCH construct and a GEN device is available for offloading. Notice that vecadd_base does not have any dummy arguments, while vecadd_gpu_offload has a single C_PTR dummy argument.

MODULE vecadd
  INTEGER,PARAMETER  :: n = 1024
CONTAINS
  FUNCTION vecadd_gpu_offload (ptr) RESULT (res)
   USE,INTRINSIC :: ISO_C_BINDING, ONLY : c_ptr
   !$DEC ATTRIBUTES NOINLINE :: vecadd_gpu_offload
    TYPE (c_ptr)      :: ptr
    REAL              :: res
    REAL,DIMENSION(n) :: a, b
    INTEGER           :: k

!$omp TARGET PARALLEL DO REDUCTION (+: res) MAP(TO: a, b)
   DO k= 0, n - 1
     a(k) = k
     b(k) = k + 1
     res   = a(k) + b(k)
    END DO
!$omp END TARGET PARALLEL DO 
    PRINT *, "GPU version of vecadd called"
  END FUNCTION vecadd_gpu_offload

  FUNCTION vecadd_base ()RESULT (res)
    !$DEC ATTRIBUTES NOINLINE :: vecadd_base
    !$OMP DECLARE VARIANT (vecadd_gpu_offload) &
    !$OMP&                ,MATCH (DEVICE = {ARCH (gen)} ))
    REAL              :: res
    REAL,DIMENSION(n) :: a, b
    INTEGER           :: k

!$omp PARALLEL DO REDUCTION (+: res)
   DO k = 1, n
      a(k) = k
      b(k) = k + 1
      res   = a(k) + b(k)
    END DO
!$omp END PARALLEL DO
    PRINT *, "CPU version of vecadd called"
  END FUNCTION vecadd_base
END MODULE vecadd

PROGRAM main
  USE vecadd
  REAL    :: result = 0.0

  !$OMP DISPATCH 
  result = vecadd_base ()

  IF (result == 1048576.0) then
    PRINT *, "PASSED: correct results"
  ELSE 
    PRINT *, "FAILED: incorrect results"
  ENDIF
END PROGRAM

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DECLARE VARIANT

Syntax

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