### 20.1 The Inside of a `GTY(())`

Sometimes the C code is not enough to fully describe the type structure. Extra information can be provided with `GTY` options and additional markers. Some options take a parameter, which may be either a string or a type name, depending on the parameter. If an option takes no parameter, it is acceptable either to omit the parameter entirely, or to provide an empty string as a parameter. For example, `GTY ((skip))` and `GTY ((skip ("")))` are equivalent.

When the parameter is a string, often it is a fragment of C code. Four special escapes may be used in these strings, to refer to pieces of the data structure being marked:

`%h`
  The current structure.
`%1`
  The structure that immediately contains the current structure.
`%0`
  The outermost structure that contains the current structure.
`%a`
  A partial expression of the form `[i1][i2]...` that indexes the array item currently being marked.

For instance, suppose that you have a structure of the form

         struct A {
           ...
         };
         struct B {
           struct A foo[12];
         };

and `b` is a variable of type `struct B`. When marking ‘`b.foo[11]`’, `%h` would expand to ‘`b.foo[11]`’, `%0` and `%1` would both expand to ‘`b`’, and `%a` would expand to ‘`[11]`’.

As in ordinary C, adjacent strings will be concatenated; this is helpful when you have a complicated expression.

         GTY ((chain_next ("TREE_CODE (&%h.generic) == INTEGER_TYPE"
                           " ? TYPE_NEXT_VARIANT (&%h.generic)"
                           " : TREE_CHAIN (&%h.generic)")))

The available options are:

`length ("`expression`")`
  There are two places the type machinery will need to be explicitly told the length of an array. The first case is when a structure ends in a variable-length array, like this:

                struct rtvec_def GTY(()) {
                  int num_elem;		/* number of elements */
                  rtx GTY ((length ("%h.num_elem"))) elem[1];
                };

  In this case, the `length` option is used to override the specified array length (which should usually be `1`). The parameter of the option is a fragment of C code that calculates the length.

  The second case is when a structure or a global variable contains a pointer to an array, like this:

                tree *
                  GTY ((length ("%h.regno_pointer_align_length"))) regno_decl;

  In this case, `regno_decl` has been allocated by writing something like

                  x->regno_decl =
                    ggc_alloc (x->regno_pointer_align_length * sizeof (tree));

  and the `length` provides the length of the field.

  This second use of `length` also works on global variables, like:

             static GTY((length ("reg_base_value_size")))
               rtx *reg_base_value;

`skip`
  If `skip` is applied to a field, the type machinery will ignore it. This is somewhat dangerous; the only safe use is in a union when one field really isn't ever used.

`desc ("`expression`")`
`tag ("`constant`")`
`default`
  The type machinery needs to be told which field of a `union` is currently active. This is done by giving each field a constant `tag` value, and then specifying a discriminator using `desc`. The value of the expression given by `desc` is compared against each `tag` value, each of which should be different. If no `tag` is matched, the field marked with `default` is used if there is one, otherwise no field in the union will be marked.

  In the `desc` option, the “current structure” is the union that it discriminates. Use `%1` to mean the structure containing it. There are no escapes available to the `tag` option, since it is a constant.

  For example,

                struct tree_binding GTY(())
                {
                  struct tree_common common;
                  union tree_binding_u {
                    tree GTY ((tag ("0"))) scope;
                    struct cp_binding_level * GTY ((tag ("1"))) level;
                  } GTY ((desc ("BINDING_HAS_LEVEL_P ((tree)&%0)"))) xscope;
                  tree value;
                };

  In this example, the value of BINDING_HAS_LEVEL_P when applied to a `struct tree_binding *` is presumed to be 0 or 1. If 1, the type mechanism will treat the field `level` as being present and if 0, will treat the field `scope` as being present.

`param_is (`type`)`
`use_param`
  Sometimes it's convenient to define some data structure to work on generic pointers (that is, `PTR`) and then use it with a specific type. `param_is` specifies the real type pointed to, and `use_param` says where in the generic data structure that type should be put.

  For instance, to have a `htab_t` that points to trees, one would write the definition of `htab_t` like this:

                typedef struct GTY(()) {
                  ...
                  void ** GTY ((use_param, ...)) entries;
                  ...
                } htab_t;

  and then declare variables like this:

                  static htab_t GTY ((param_is (union tree_node))) ict;

`param` n`_is (`type`)`
`use_param` n
  In more complicated cases, the data structure might need to work on several different types, which might not necessarily all be pointers. For this, `param1_is` through `param9_is` may be used to specify the real type of a field identified by `use_param1` through `use_param9`.

`use_params`
  When a structure contains another structure that is parameterized, there's no need to do anything special, the inner structure inherits the parameters of the outer one. When a structure contains a pointer to a parameterized structure, the type machinery won't automatically detect this (it could, it just doesn't yet), so it's necessary to tell it that the pointed-to structure should use the same parameters as the outer structure. This is done by marking the pointer with the `use_params` option.

`deletable`
  `deletable`, when applied to a global variable, indicates that when garbage collection runs, there's no need to mark anything pointed to by this variable, it can just be set to `NULL` instead. This is used to keep a list of free structures around for re-use.

`if_marked ("`expression`")`
  Suppose you want some kinds of object to be unique, and so you put them in a hash table. If garbage collection marks the hash table, these objects will never be freed, even if the last other reference to them goes away. GGC has special handling to deal with this: if you use the `if_marked` option on a global hash table, GGC will call the routine whose name is the parameter to the option on each hash table entry. If the routine returns nonzero, the hash table entry will be marked as usual. If the routine returns zero, the hash table entry will be deleted.

  The routine `ggc_marked_p` can be used to determine if an element has been marked already; in fact, the usual case is to use `if_marked ("ggc_marked_p")`.

`maybe_undef`
  When applied to a field, `maybe_undef` indicates that it's OK if the structure that this fields points to is never defined, so long as this field is always `NULL`. This is used to avoid requiring backends to define certain optional structures. It doesn't work with language frontends.

`nested_ptr (`type`, "`to expression`", "`from expression`")`
  The type machinery expects all pointers to point to the start of an object. Sometimes for abstraction purposes it's convenient to have a pointer which points inside an object. So long as it's possible to convert the original object to and from the pointer, such pointers can still be used. type is the type of the original object, the to expression returns the pointer given the original object, and the from expression returns the original object given the pointer. The pointer will be available using the `%h` escape.

`chain_next ("`expression`")`
`chain_prev ("`expression`")`
  It's helpful for the type machinery to know if objects are often chained together in long lists; this lets it generate code that uses less stack space by iterating along the list instead of recursing down it. `chain_next` is an expression for the next item in the list, `chain_prev` is an expression for the previous item. For singly linked lists, use only `chain_next`; for doubly linked lists, use both. The machinery requires that taking the next item of the previous item gives the original item.

`reorder ("`function name`")`
  Some data structures depend on the relative ordering of pointers. If the precompiled header machinery needs to change that ordering, it will call the function referenced by the `reorder` option, before changing the pointers in the object that's pointed to by the field the option applies to. The function must take four arguments, with the signature ‘`void *, void *, gt_pointer_operator, void *`’. The first parameter is a pointer to the structure that contains the object being updated, or the object itself if there is no containing structure. The second parameter is a cookie that should be ignored. The third parameter is a routine that, given a pointer, will update it to its correct new value. The fourth parameter is a cookie that must be passed to the second parameter.

  PCH cannot handle data structures that depend on the absolute values of pointers. `reorder` functions can be expensive. When possible, it is better to depend on properties of the data, like an ID number or the hash of a string instead.

`special ("`name`")`
  The `special` option is used to mark types that have to be dealt with by special case machinery. The parameter is the name of the special case. See `gengtype.c` for further details. Avoid adding new special cases unless there is no other alternative.