### 23.1 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 of non-atomic objects. The first case is when a structure ends in a variable-length array, like this:

      struct GTY(()) rtvec_def {
        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:

      struct gimple_omp_for_iter * GTY((length ("%h.collapse"))) iter;

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

        x->iter = ggc_alloc_cleared_vec_gimple_omp_for_iter (collapse);

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

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

      static GTY((length("reg_known_value_size"))) rtx *reg_known_value;

  Note that the `length` option is only meant for use with arrays of non-atomic objects, that is, objects that contain pointers pointing to other GTY-managed objects. For other GC-allocated arrays and strings you should use `atomic`.

`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.

`for_user`

  Use this to mark types that need to be marked by user gc routines, but are not refered to in a template argument. So if you have some user gc type T1 and a non user gc type T2 you can give T2 the for_user option so that the marking functions for T1 can call non mangled functions to mark T2.

`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 GTY(()) tree_binding
      {
        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.

  The `desc` and `tag` options can also be used for inheritance to denote which subclass an instance is. See Inheritance and GTY for more information.

`cache`

  When the `cache` option is applied to a global variable gt_clear_cache is called on that variable between the mark and sweep phases of garbage collection. The gt_clear_cache function is free to mark blocks as used, or to clear pointers in the variable.

`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.

`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")`
`chain_circular ("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. `chain_circular` is similar to `chain_next`, but can be used for circular single linked lists.

`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.

`atomic`

  The `atomic` option can only be used with pointers. It informs the GC machinery that the memory that the pointer points to does not contain any pointers, and hence it should be treated by the GC and PCH machinery as an “atomic” block of memory that does not need to be examined when scanning memory for pointers. In particular, the machinery will not scan that memory for pointers to mark them as reachable (when marking pointers for GC) or to relocate them (when writing a PCH file).

  The `atomic` option differs from the `skip` option. `atomic` keeps the memory under Garbage Collection, but makes the GC ignore the contents of the memory. `skip` is more drastic in that it causes the pointer and the memory to be completely ignored by the Garbage Collector. So, memory marked as `atomic` is automatically freed when no longer reachable, while memory marked as `skip` is not.

  The `atomic` option must be used with great care, because all sorts of problem can occur if used incorrectly, that is, if the memory the pointer points to does actually contain a pointer.

  Here is an example of how to use it:

      struct GTY(()) my_struct {
        int number_of_elements;
        unsigned int * GTY ((atomic)) elements;
      };

  In this case, `elements` is a pointer under GC, and the memory it points to needs to be allocated using the Garbage Collector, and will be freed automatically by the Garbage Collector when it is no longer referenced. But the memory that the pointer points to is an array of `unsigned int` elements, and the GC must not try to scan it to find pointers to mark or relocate, which is why it is marked with the `atomic` option.

  Note that, currently, global variables cannot be marked with `atomic`; only fields of a struct can. This is a known limitation. It would be useful to be able to mark global pointers with `atomic` to make the PCH machinery aware of them so that they are saved and restored correctly to PCH files.

`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.

`user`

  The `user` option indicates that the code to mark structure fields is completely handled by user-provided routines. See section User GC for details on what functions need to be provided.