16.1 The Inside of a GTY(())

Sometimes the C code is not enough to fully describe the type structure. Extra information can be provided by using more GTY markers. These markers can be placed:

• In a structure definition, before the open brace;
• In a global variable declaration, after the keyword static or extern; and
• In a structure field definition, before the name of the field.

The format of a marker is

GTY (([name] ([param]), [name] ([param]) ...))

When the parameter is a string, often it is a fragment of C code. Three special escapes may be available:

%h

This expands to an expression that evaluates to the current structure.

%1

This expands to an expression that evaluates to the structure that immediately contains the current structure.

%0

This expands to an expression that evaluates to the outermost structure that contains the current structure.

%a

This expands to the string of the form [i1][i2]... that indexes the array item currently being marked. For instance, if the field being marked is foo, then %1.foo%a is the same as %h.

The available options are:

length

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

tag

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's supposed to be 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

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 should write

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

paramn_is

use_paramn

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

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.

chain_next

chain_prev

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. The machinery requires that taking the next item of the previous item gives the original item.

reorder

Some data structures depend on the relative ordering of pointers. If the type 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 be of the type void ()(void *, void *, gt_pointer_operator, void *). The second parameter is the pointed-to object; the third parameter is a routine that, given a pointer, can update it to its new value. The fourth parameter is a cookie to be passed to the third parameter. The first parameter is the structure that contains the object, or the object itself if it is a structure.

No data structure may depend on the absolute value of pointers. Even relying on relative orderings and using reorder functions can be expensive. It is better to depend on properties of the data, like an ID number or the hash of a string instead.

special

The special option is used for those bizarre cases that are just too hard to deal with otherwise. Don't use it for new code.