- the position and/or sizes of fields vary from object to object of the
- same type, i.e., where some other field in the object contains a value
- that is used in the computation of another field's offset or size.
-
- For example, a record type with a discriminant in Ada is such a type.
- This mechanism is also used to create "fat pointers" for unconstrained
- array types in Ada; the fat pointer is a structure one of whose fields is
- a pointer to the actual array type and the other field is a pointer to a
- template, which is a structure containing the bounds of the array. The
- bounds in the type pointed to by the first field in the fat pointer refer
- to the values in the template.
-
- These "self-references" are doing using a PLACEHOLDER_EXPR. This is a
- node that will later be replaced with the object being referenced. Its type
- is that of the object and selects which object to use from a chain of
- references (see below).
-
- When we wish to evaluate a size or offset, we check it is contains a
- placeholder. If it does, we construct a WITH_RECORD_EXPR that contains
- both the expression we wish to evaluate and an expression within which the
- object may be found. The latter expression is the object itself in
- the simple case of an Ada record with discriminant, but it can be the
- array in the case of an unconstrained array.
-
- In the latter case, we need the fat pointer, because the bounds of the
- array can only be accessed from it. However, we rely here on the fact that
- the expression for the array contains the dereference of the fat pointer
- that obtained the array pointer.
+ some field in an object of the type contains a value that is used in
+ the computation of another field's offset or size and/or the size of
+ the type. The positions and/or sizes of fields can vary from object
+ to object of the same type or even for one and the same object within
+ its scope.
+
+ Record types with discriminants in Ada or schema types in Pascal are
+ examples of such types. This mechanism is also used to create "fat
+ pointers" for unconstrained array types in Ada; the fat pointer is a
+ structure one of whose fields is a pointer to the actual array type
+ and the other field is a pointer to a template, which is a structure
+ containing the bounds of the array. The bounds in the type pointed
+ to by the first field in the fat pointer refer to the values in the
+ template.
+
+ When you wish to construct such a type you need "self-references"
+ that allow you to reference the object having this type from the
+ TYPE node, i.e. without having a variable instantiating this type.
+
+ Such a "self-references" is done using a PLACEHOLDER_EXPR. This is
+ a node that will later be replaced with the object being referenced.
+ Its type is that of the object and selects which object to use from
+ a chain of references (see below). No other slots are used in the
+ PLACEHOLDER_EXPR.
+
+ For example, if your type FOO is a RECORD_TYPE with a field BAR,
+ and you need the value of <variable>.BAR to calculate TYPE_SIZE
+ (FOO), just substitute <variable> above with a PLACEHOLDER_EXPR
+ whose TREE_TYPE is FOO. Then construct your COMPONENT_REF with
+ the PLACEHOLDER_EXPR as the first operand (which has the correct
+ type). Later, when the size is needed in the program, the back-end
+ will find this PLACEHOLDER_EXPR and generate code to calculate the
+ actual size at run-time. In the following, we describe how this
+ calculation is done.
+
+ When we wish to evaluate a size or offset, we check whether it
+ contains a PLACEHOLDER_EXPR. If it does, we construct a
+ WITH_RECORD_EXPR that contains both the expression we wish to
+ evaluate and an expression within which the object may be found.
+ The latter expression is the object itself in the simple case of an
+ Ada record with discriminant, but it can be the array in the case of
+ an unconstrained array.
+
+ In the latter case, we need the fat pointer, because the bounds of
+ the array can only be accessed from it. However, we rely here on the
+ fact that the expression for the array contains the dereference of
+ the fat pointer that obtained the array pointer.