Memory Management and Type Information
**************************************

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 and union 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:

  * 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.

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.

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 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;

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 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.

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;

paramN_is (TYPE)
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 ("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").

mark_hook ("HOOK-ROUTINE-NAME")

     If provided for a structure or union type, the given HOOK-ROUTINE-NAME (between double-quotes) is the name of a routine called when the garbage collector has just marked the data as reachable. This routine should not change the data, or call any ggc routine. Its only argument is a pointer to the just marked (const) structure or union.

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.

variable_size

     The type machinery expects the types to be of constant size.  When this is not true, for example, with structs that have array fields or unions, the type machinery cannot tell how many bytes need to be allocated at each allocation.  The variable_size is used to mark such types. The type machinery then provides allocators that take a parameter indicating an exact size of object being allocated.

     For example,

          struct GTY((variable_size)) sorted_fields_type {
            int len;
            tree GTY((length ("%h.len"))) elts[1];
          };

     Then the objects of struct sorted_fields_type are allocated in GC memory as follows:

            field_vec = ggc_alloc_sorted_fields_type (size);

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.

Marking Roots for the Garbage Collector
=======================================

In addition to keeping track of types, the type machinery also locates the global variables (roots) that the garbage collector starts at.  Roots must be declared using one of the following syntaxes:

  * extern GTY(([OPTIONS])) TYPE NAME;

  * static GTY(([OPTIONS])) TYPE NAME;

The syntax

  * GTY(([OPTIONS])) TYPE NAME;

is not accepted.  There should be an extern declaration of such a variable in a header somewhere---mark that, not the definition.  Or, if the variable is only used in one file, make it static.

Source Files Containing Type Information
========================================

Whenever you add GTY markers to a source file that previously had none, or create a new source file containing GTY markers, there are three things you need to do:

 1. You need to add the file to the list of source files the type machinery scans.  There are four cases:

      a. For a back-end file, this is usually done automatically; if not, you should add it to target_gtfiles in the appropriate port's entries in config.gcc.

      b. For files shared by all front ends, add the filename to the GTFILES variable in Makefile.in.

      c. For files that are part of one front end, add the filename to the gtfiles variable defined in the appropriate config-lang.in.  For C, the file is c-config-lang.in. Headers should appear before non-headers in this list.

      d. For files that are part of some but not all front ends, add the filename to the gtfiles variable of all the front ends that use it.

 2. If the file was a header file, you'll need to check that it's included in the right place to be visible to the generated files.  For a back-end header file, this should be done automatically.  For a front-end header file, it needs to be included by the same file that includes gtype-LANG.h.  For other header files, it needs to be included in gtype-desc.c, which is a generated file, so add it to ifiles in open_base_file in gengtype.c.

     For source files that aren't header files, the machinery will generate a header file that should be included in the source file you just changed. The file will be called gt-PATH.h where PATH is the pathname relative to the gcc directory with slashes replaced by -, so for example the header file to be included in cp/parser.c is called gt-cp-parser.c.  The generated header file should be included after everything else in the source file.  Don't forget to mention this file as a dependency in the Makefile!

For language frontends, there is another file that needs to be included somewhere.  It will be called gtype-LANG.h, where LANG is the name of the subdirectory the language is contained in.

Plugins can add additional root tables.  Run the gengtype utility in plugin mode as gengtype -P pluginout.h SOURCE-DIR FILE-LIST PLUGIN*.C with your plugin files PLUGIN*.C using GTY to generate the PLUGINOUT.H file. The GCC build tree is needed to be present in that mode.

How to invoke the garbage collector
===================================

The GCC garbage collector GGC is only invoked explicitly. In contrast with many other garbage collectors, it is not implicitly invoked by allocation routines when a lot of memory has been consumed. So the only way to have GGC reclaim storage it to call the ggc_collect function explicitly. This call is an expensive operation, as it may have to scan the entire heap. Beware that local variables (on the GCC call stack) are not followed by such an invocation (as many other garbage collectors do): you should reference all your data from static or external GTY-ed variables, and it is advised to call ggc_collect with a shallow call stack. The GGC is an exact mark and sweep garbage collector (so it does not scan the call stack for pointers). In practice GCC passes don't often call ggc_collect themselves, because it is called by the pass manager between passes.