gfc_expr ¶Expressions and “values”, including constants, variable-, array- and
component-references as well as complex expressions consisting of operators and
function calls are internally represented as one or a whole tree of
gfc_expr objects. The member expr_type specifies the overall
type of an expression (for instance, EXPR_CONSTANT for constants or
EXPR_VARIABLE for variable references). The members ts and
rank as well as shape, which can be NULL, specify
the type, rank and, if applicable, shape of the whole expression or expression
tree of which the current structure is the root. where is the locus of
this expression in the source code.
Depending on the flavor of the expression being described by the object
(that is, the value of its expr_type member), the corresponding structure
in the value union will usually contain additional data describing the
expression’s value in a type-specific manner. The ref member is used to
build chains of (array-, component- and substring-) references if the expression
in question contains such references, see below for details.
Scalar constants are represented by gfc_expr nodes with their
expr_type set to EXPR_CONSTANT. The constant’s value shall
already be known at compile-time and is stored in the logical,
integer, real, complex or character struct inside
value, depending on the constant’s type specification.
Operator-expressions are expressions that are the result of the execution of
some operator on one or two operands. The expressions have an expr_type
of EXPR_OP. Their value.op structure contains additional data.
op1 and optionally op2 if the operator is binary point to the
two operands, and operator or uop describe the operator that
should be evaluated on these operands, where uop describes a user-defined
operator.
If the expression is the return value of a function-call, its expr_type
is set to EXPR_FUNCTION, and symtree must point to the symtree
identifying the function to be called. value.function.actual holds the
actual arguments given to the function as a linked list of
gfc_actual_arglist nodes.
The other members of value.function describe the function being called
in more detail, containing a link to the intrinsic symbol or user-defined
function symbol if the call is to an intrinsic or external function,
respectively. These values are determined during resolution-phase from the
structure’s symtree member.
A special case of function calls are “component calls” to type-bound
procedures; those have the expr_type EXPR_COMPCALL with
value.compcall containing the argument list and the procedure called,
while symtree and ref describe the object on which the procedure
was called in the same way as a EXPR_VARIABLE expression would.
See Type-bound Procedures.
Array- and structure-constructors (one could probably call them “array-” and
“derived-type constants”) are gfc_expr structures with their
expr_type member set to EXPR_ARRAY or EXPR_STRUCTURE,
respectively. For structure constructors, symtree points to the
derived-type symbol for the type being constructed.
The values for initializing each array element or structure component are
stored as linked-list of gfc_constructor nodes in the
value.constructor member.
NULL is a special value for pointers; it can be of different base types.
Such a NULL value is represented in the internal tree by a
gfc_expr node with expr_type EXPR_NULL. If the base type
of the NULL expression is known, it is stored in ts (that’s for
instance the case for default-initializers of ALLOCATABLE components),
but this member can also be set to BT_UNKNOWN if the information is not
available (for instance, when the expression is a pointer-initializer
NULL()).
Variable references are gfc_expr structures with their expr_type
set to EXPR_VARIABLE; their symtree should point to the variable
that is referenced.
For this type of expression, it’s also possible to chain array-, component-
or substring-references to the original expression to get something like
‘struct%component(2:5)’, where component is either an array or
a CHARACTER member of struct that is of some derived-type. Such a
chain of references is achieved by a linked list headed by ref of the
gfc_expr node. For the example above it would be (‘==|’ is the
last NULL pointer):
EXPR_VARIABLE(struct) ==> REF_COMPONENT(component) ==> REF_ARRAY(2:5) ==|
If component is a string rather than an array, the last element would be
a REF_SUBSTRING reference, of course. If the variable itself or some
component referenced is an array and the expression should reference the whole
array rather than being followed by an array-element or -section reference, a
REF_ARRAY reference must be built as the last element in the chain with
an array-reference type of AR_FULL. Consider this example code:
TYPE :: mytype INTEGER :: array(42) END TYPE mytype TYPE(mytype) :: variable INTEGER :: local_array(5) CALL do_something (variable%array, local_array)
The gfc_expr nodes representing the arguments to the ‘do_something’
call will have a reference-chain like this:
EXPR_VARIABLE(variable) ==> REF_COMPONENT(array) ==> REF_ARRAY(FULL) ==| EXPR_VARIABLE(local_array) ==> REF_ARRAY(FULL) ==|
EXPR_SUBSTRING is a special type of expression that encodes a substring
reference of a constant string, as in the following code snippet:
x = "abcde"(1:2)
In this case, value.character contains the full string’s data as if it
was a string constant, but the ref member is also set and points to a
substring reference as described in the subsection above.