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Syntax of Data Alignment and Distribution Directives

Specification directives in HPF have two forms: specification statements, analogous to the DIMENSION and ALLOCATABLE statements of Fortran 90; and an attribute form analogous to type declaration statements in Fortran 90 using the ``::'' punctuation.

The attribute form allows more than one attribute to be described in a single directive. HPF goes beyond Fortran 90 in not requiring that the first attribute, or indeed any of them, be a type specifier.

For syntactic convenience, the executable directives REALIGN and REDISTRIBUTE also come in two forms (statement form and attribute form) but may not be combined with other attributes in a single directive.

H301	combined-directive	is 	combined-attribute-list :: entity-decl-list

H302	combined-attribute	is   ALIGN align-attribute-stuff
				or   DISTRIBUTE dist-attribute-stuff
				or   DYNAMIC
				or   INHERIT
				or   TEMPLATE
				or   PROCESSORS
				or   DIMENSION ( explicit-shape-spec-list )

Constraint:	The same combined-attribute must not appear more
		than once in a given combined-directive.

Constraint:	If the DIMENSION attribute appears in a
		combined-directive, any entity to which it
		applies must be declared with the HPF TEMPLATE
		or PROCESSORS type specifier.

If the DISTRIBUTE attribute is present, then every name declared in the entity-decl-list is considered to be a distributee and is subject to the constraints listed in section 3.3. If the ALIGN attribute is present, then every name declared in the entity-decl-list is considered to be an alignee and is subject to the constraints listed in section 3.4.

The HPF keywords PROCESSORS and TEMPLATE play the role of type specifiers in declaring processor arrangements and templates. The HPF keywords ALIGN, DISTRIBUTE, DYNAMIC, and INHERIT play the role of attributes. Attributes referring to processor arrangements, to templates, or to entities with other types (such as REAL) may be combined in an HPF directive without having the type specifier appear.

No entity may be given a particular attribute more than once.

Dimension information may be specified after an object-name or in a DIMENSION attribute. If both are present, the one after the object-name overrides the DIMENSION attribute (this is consistent with the Fortran 90 standard). For example, in:

!HPF$ TEMPLATE,DIMENSION(64,64) :: A,B,C(32,32),D

If a specification expression includes a reference to the value of an element of an array specified in the same specification-part, any explicit mapping or INHERIT attribute for the array must be completely specified in prior specification-directives. (This restriction is inspired by and extends section 7.1.6.2 of the Fortran 90 standard, which states in part: If a specification expression includes a reference to the value of an element of an array specified in the same specification-part, the array bounds must be specified in a prior declaration.

A comment on asterisks: The asterisk character ``*'' appears in the syntax rules for HPF alignment and distribution directives in three distinct roles:

• When a lone asterisk appears as a member of a parenthesized list, it indicates either a collapsed mapping, wherein many elements of an array may be mapped to the same abstract processor, or a replicated mapping, wherein each element of an array may be mapped to many abstract processors. See the syntax rules for align-source and align-subscript (see Section ) and for dist-format (see Section ).

• When an asterisk appears before a left parenthesis ``('' or after the keyword WITH or ONTO, it indicates that the directive constitutes an assertion about the current mapping of a dummy argument on entry to a subprogram, rather than a request for a desired mapping of that dummy argument. This use of the asterisk may appear only in directives that apply to dummy arguments (see Section ).

• When an asterisk appears in an align-subscript-use expression, it represents the usual integer multiplication operator.

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