General Form of Element Array Assignment

XBNF forall-stmt -to -rule to to is FORALL forall-header forall-assignment

forall-header -to -rule to to is ( forall-triplet-spec-list [ , scalar-mask-expr ] ) XBNF

• Any procedure referenced in the scalar-mask-expr of a forall-header must be pure, as defined in Section .

• []Rationale. Pure functions are guaranteed to be free of side effects. Therefore, they are safe to invoke in the scalar-mask-expr.

  Note that functions referenced in the forall-triplet-spec-list are not syntactically constrained as the scalar-mask-expr is. This is consistent with the handling of bounds expressions in DO loops. (End of rationale.)

XBNF forall-triplet-spec -to -rule to to is index-name =index.html subscript : subscript [ : stride ] XBNF

• index-name must be a scalar integer variable.

• A subscript or stride in a forall-triplet-spec-list must not contain a reference to any index-name in the forall-triplet-spec-list in which it appears.

XBNF forall-assignment -to -rule to to is assignment-stmt -to or pointer-assignment-stmt XBNF

• Any procedure referenced in a forall-assignment, including one referenced by a defined operation or assignment, must be pure as defined in Section .

• []Rationale. Pure functions are guaranteed to have no side effects, and thus have an unambiguous meaning when used in a FORALL statement. Experience also suggests that they form a useful class of functions for use in scientific computation, and are particularly useful when applied as data-parallel operations. For these reasons, there was a strong consensus to allow their use in FORALL. More general functions called from FORALL were also considered, but eventually rejected for lack of agreement on their desirability, ease of implementation, or the semantics of complex cases they allowed. (End of rationale.)

To determine the set of permitted values for each index-name in the forall-header, we introduce some simplifying notation. In the forall-triplet-spec, let

• be first subscript (``lower bound'');

• be second subscript (``upper bound'');

• be the stride; and

• be .

If stride is missing, it is as if it were present with the value 1. Stride must not have the value 0. The set of permitted values is determined on entry to the statement and is , . If for some index-name, the forall-assignment is not executed.

A FORALL statement assigns to memory locations specified by the forall-assignment for permitted values of the index-name variables. A program that causes multiple values to be assigned to the same location is not HPF-conforming and therefore has no defined meaning. This is a semantic constraint rather than a syntactic constraint, however; in general, it cannot be checked during compilation.