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Next: DYNAMIC Directive Up: Data Alignment and Previous: DISTRIBUTE and REDISTRIBUTE

ALIGN and REALIGN Directives

The ALIGN directive is used to specify that certain data objects are to be mapped in the same way as certain other data objects. Operations between aligned data objects are likely to be more efficient than operations between data objects that are not known to be aligned (because two objects that are aligned are intended to be mapped to the same abstract processor). The ALIGN directive is designed to make it particularly easy to specify explicit mappings for all the elements of an array at once. While objects can be aligned in some cases through careful use of matching DISTRIBUTE directives, ALIGN is more general and frequently more convenient.

The REALIGN directive is similar to the ALIGN directive but is considered executable. An array (or template) may be realigned at any time, provided it has been declared DYNAMIC (see Section ) Unlike redistribution (see Section ), realigning a data object does not cause any other object to be remapped. (However, realignment of even a single object, if it is large, could require a lot of computational and communication effort at run time; the programmer must take care when using this feature.)

The ALIGN directive may appear only in the specification-part of a scoping unit. The REALIGN directive is similar but may appear only in the execution-part of a scoping unit. The principal difference between ALIGN and REALIGN is that ALIGN must contain only a specification-expr as a subscript or in a subscript-triplet, whereas in REALIGN such subscripts may be any integer expressions. Another difference is that ALIGN is an attribute, and so can be combined with other attributes as part of a combined-directive, whereas REALIGN is not an attribute (although a REALIGN statement may be written in the style of attributed syntax, using ``::'' punctuation).

Formally, the syntax of ALIGN and REALIGN is as follows:

XBNF align-directive -to -rule to to is ALIGN alignee align-directive-stuff

realign-directive -to -rule to to is REALIGN alignee align-directive-stuff -to or REALIGN align-attribute-stuff :: alignee-list

align-directive-stuff -to -rule to to is ( align-source-list ) align-with-clause

align-attribute-stuff -to -rule to to is [ ( align-source-list ) ] align-with-clause

alignee -to -rule to to is object-name

align-source -to -rule to to is : -to or * -to or align-dummy

align-dummy -to -rule to to is scalar-int-variable XBNF

• An object-name mentioned as an alignee may not appear as a distributee in a DISTRIBUTE or REDISTRIBUTE directive.
• Any alignee that appears in a REALIGN directive must have the DYNAMIC attribute (see Section ).

• The align-source-list (and its surrounding parentheses) must be omitted if the alignee is scalar. (In some cases this will preclude the use of the statement form of the directive.)
• If the align-source-list is present, its length must equal the rank of the alignee.
• An align-dummy must be a named variable.
• An object may not have both the INHERIT attribute and the ALIGN attribute. (However, an object with the INHERIT attribute may appear as an alignee in a REALIGN directive, provided that it does not appear as a distributee in a DISTRIBUTE or REDISTRIBUTE directive.)

Note that the possibility of an ALIGN directive of the form _=13_}

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