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Interpretation of Element Array Assignments

Execution of an element array assignment consists of the following steps:

1. Evaluation in any order of the subscript and stride expressions in the forall-triplet-spec-list. The set of valid combinations of index-name values is then the Cartesian product of the sets defined by these triplets.
2. Evaluation of the scalar-mask-expr for all valid combinations of index-name values. The mask elements may be evaluated in any order. The set of active combinations of index-name values is the subset of the valid combinations for which the mask evaluates to .TRUE.
3. Evaluation in any order of the expr and all expressions within variable (in the case of assignment-stmt) or target and all expressions within pointer-object (in the case of pointer-assignment-stmt.) of the forall-assignment for all active combinations of index-name values. In the case of pointer assignment where the target is not a pointer, the evaluation consists of identifying the object referenced rather than computing its value.
4. Assignment of the computed expr values to the corresponding variable locations (in the case of assignment-stmt) or the association of the target values with the corresponding pointer-object locations (in the case of pointer-assignment-stmt) for all active combinations of index-name values. The assignments or associations may be made in any order. In the case of a pointer assignment where the target is not a pointer, this assignment consists of associating the pointer-object with the object referenced.

If the scalar mask expression is omitted, it is as if it were present with the value .TRUE.

The scope of an index-name is the FORALL statement itself.

A forall-stmt is not HPF-conforming if the result of evaluating any expression in the forall-header affects or is affected by the evaluation of any other expression in the forall-header.

• []Rationale. This is consistent with the handling of DO loop bounds and strides. Disallowing references to impure functions in a forall-triplet-spec-list was suggested, but the analogy to DO bounds was considered too strong to overlook. Note that the scalar-mask-expr can only invoke pure functions, which are side-effect free. Therefore, the scalar-mask-expr cannot affect the values of the bounds. (End of rationale.)

A forall-stmt is not HPF-conforming if it causes any atomic data object to be assigned more than one value. A data object is atomic if it contains no subobjects. For the purposes of this restriction, any assignment (including array assignment or assignment to a variable of derived type) to a non-atomic object is considered to assign to all subobjects contained by that object.

• []Rationale. For example, an integer variable is an atomic object, but an array of integers is an object that is not atomic. Similarly, assignment to an array section is equivalent to assignments to each individual element (which may require further reductions when the array contains objects of derived type). This restriction allows cases such as

  FORALL ( i = 1:10 ) a(indx(i)) = b(i) if and only if indx contains no repeated values. Note that it restricts FORALL behavior, but not syntax. Syntactic restrictions to enforce this behavior would be either incomplete (ie. allow undefined behavior) or exclude conceptually legal programs.

  Since a function called from a forall-asssignment must be pure, it is impossible for that function's evaluation to affect other expressions' evaluations, either for the same combination of index-name values or for a different combination. In addition, it is possible that the compiler can perform more extensive optimizations because all functions are pure. (End of rationale.)

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