GCC uses some fairly sophisticated memory management techniques, which involve determining information about GCC’s data structures from GCC’s source code and using this information to perform garbage collection and implement precompiled headers.
A full C++ parser would be too complicated for this task, so a limited
subset of C++ is interpreted and special markers are used to determine
what parts of the source to look at. All struct, union
and template structure declarations that define data structures
that are allocated under control of the garbage collector must be
marked. All global variables that hold pointers to garbage-collected
memory must also be marked. Finally, all global variables that need
to be saved and restored by a precompiled header must be marked. (The
precompiled header mechanism can only save static variables if they’re
scalar. Complex data structures must be allocated in garbage-collected
memory to be saved in a precompiled header.)
The full format of a marker is
GTY (([option] [(param)], [option] [(param)] …))
but in most cases no options are needed. The outer double parentheses
are still necessary, though: GTY(()). Markers can appear:
static or
extern; and
Here are some examples of marking simple data structures and globals.
struct GTY(()) tag
{
fields…
};
typedef struct GTY(()) tag
{
fields…
} *typename;
static GTY(()) struct tag *list; /* points to GC memory */
static GTY(()) int counter; /* save counter in a PCH */
The parser understands simple typedefs such as
typedef struct tag *name; and
typedef int name;.
These don’t need to be marked.
However, in combination with GTY, avoid using typedefs such as
typedef int_hash<…> name;
for these generate infinite-recursion code.
See PR103157.
Instead, you may use
struct name : int_hash<…> {};,
for example.
Since gengtype’s understanding of C++ is limited, there are
several constructs and declarations that are not supported inside
classes/structures marked for automatic GC code generation. The
following C++ constructs produce a gengtype error on
structures/classes marked for automatic GC code generation:
If you have a class or structure using any of the above constructs,
you need to mark that class as GTY ((user)) and provide your
own marking routines (see section Support for user-provided GC marking routines for details).
It is always valid to include function definitions inside classes.
Those are always ignored by gengtype, as it only cares about
data members.