Canonical Loop Form

The loops associated with a loop-associated directive have canonical loop form if they conform to the following:





for (init-expr; test-expr; incr-expr) structured-block




init-expr	One of the following:
	var = lb
	integer-type var = lb
	random-access-iterator-type var = lb
	pointer-type var = lb

test-expr	One of the following:
	var relational-op b
	b relational-op var

incr-expr	One of the following:
	++var
	var++
	- - var
	var - -
	var += incr
	var - = incr
	var = var + incr
	var = incr + var
	var = var - incr

var	One of the following:
	A variable of a signed or unsigned integer type.
	For C++, a variable of a random access iterator type.
	For C, a variable of a pointer type.
	This variable must not be modified during the execution of the for-loop other than in incr-expr.
relational-op	One of the following:
	<
	<=
	>
	>=
	!=

lb and b	Expressions of a type compatible with the type of var that are loop invariant with respect to the outermost associated loop or are one of the following (where var-outer, a1, and a2 have a type compatible with the type of var, var-outer is var from an outer associated loop, and a1 and a2 are loop invariant integer expressions with respect to the outermost loop):
	var-outer
	var-outer + a2
	a2 + var-outer
	var-outer - a2
	a2 - var-outer
	a1 * var-outer
	a1 * var-outer + a2
	a2 + a1 * var-outer
	a1 * var-outer - a2
	a2 - a1 * var-outer
	var-outer * a1
	var-outer * a1 + a2
	a2 + var-outer * a1
	var-outer * a1 - a2
	a2 - var-outer * a1

incr	An integer expression that is loop invariant with respect to the outermost associated loop.

The loops associated with a loop-associated directive have canonical loop form if each of them is a do-loop that is a do-construct or an inner-shared-do-construct as defined by the Fortran standard. If an end do directive follows a do-construct in which several loop statements share a DO termination statement, then the directive can only be specified for the outermost of these DO statements.

The do-stmt for any do-loop must conform to the following:





DO [ label ] var =lb ,b [ ,incr ]




var	A variable of integer type.

lb and b	Expressions of a type compatible with the type of var that are loop invariant with respect to the outermost associated loop or are one of the following (where var-outer, a1, and a2 have a type compatible with the type of var, var-outer is var from an outer associated loop, and a1 and a2 are loop invariant integer expressions with respect to the outermost loop):
	var-outer
	var-outer + a2
	a2 + var-outer
	var-outer - a2
	a2 - var-outer
	a1 * var-outer
	a1 * var-outer + a2
	a2 + a1 * var-outer
	a1 * var-outer - a2
	a2 - a1 * var-outer
	var-outer * a1
	var-outer * a1 + a2
	a2 + var-outer * a1
	var-outer * a1 - a2
	a2 - var-outer * a1

incr	An integer expression that is loop invariant with respect to the outermost associated loop. If it is not explicitly specified, its value is assumed to be 1.

If var is of a random access iterator type, then the type is the type that would be used by std::distance applied to variables of the type of var.

If var is of a pointer type, then the type is ptrdiff_t.

A range-based for loop that is valid in the base language and has a begin value that satisfies the random access iterator requirement has canonical loop form. Range-based for loops are of the following form:

for (range-decl: range-expr) structured-block

The begin-expr and end-expr expressions are derived from range-expr by the base language and assigned to variables to which this specification refers as __begin and __end respectively. Both __begin and __end are privatized. For the purpose of the rest of the standard __begin is the iteration variable of the range-for loop.

If test-expr is of the form var relational-op b and relational-op is < or <= then incr-expr must cause var to increase on each iteration of the loop. If test-expr is of the form var relational-op b and relational-op is > or >= then incr-expr must cause var to decrease on each iteration of the loop.
If test-expr is of the form b relational-op var and relational-op is < or <= then incr-expr must cause var to decrease on each iteration of the loop. If test-expr is of the form b relational-op var and relational-op is > or >= then incr-expr must cause var to increase on each iteration of the loop.
If test-expr is of the form b != var or var != b then incr-expr must cause var either to increase on each iteration of the loop or to decrease on each iteration of the loop.
If relational-op is != and incr-expr is of the form that has incr then incr must be a constant expression and evaluate to -1 or 1.

In the simd construct the only random access iterator types that are allowed for var are pointer types.
The range-expr of a range-for loop must be loop invariant with respect to the outermost associated loop, and must not reference iteration variables of any associated loops.
The loops associated with an ordered clause with a parameter may not include range-for loops.

2.9.1 Canonical Loop Form