1 @c Copyright (C) 2002, 2003, 2004
2 @c Free Software Foundation, Inc.
3 @c This is part of the GCC manual.
4 @c For copying conditions, see the file gcc.texi.
7 @chapter Memory Management and Type Information
11 GCC uses some fairly sophisticated memory management techniques, which
12 involve determining information about GCC's data structures from GCC's
13 source code and using this information to perform garbage collection and
14 implement precompiled headers.
16 A full C parser would be too complicated for this task, so a limited
17 subset of C is interpreted and special markers are used to determine
18 what parts of the source to look at. All @code{struct} and
19 @code{union} declarations that define data structures that are
20 allocated under control of the garbage collector must be marked. All
21 global variables that hold pointers to garbage-collected memory must
22 also be marked. Finally, all global variables that need to be saved
23 and restored by a precompiled header must be marked. (The precompiled
24 header mechanism can only save static variables if they're scalar.
25 Complex data structures must be allocated in garbage-collected memory
26 to be saved in a precompiled header.)
28 The full format of a marker is
30 GTY (([@var{option}] [(@var{param})], [@var{option}] [(@var{param})] @dots{}))
33 but in most cases no options are needed. The outer double parentheses
34 are still necessary, though: @code{GTY(())}. Markers can appear:
38 In a structure definition, before the open brace;
40 In a global variable declaration, after the keyword @code{static} or
43 In a structure field definition, before the name of the field.
46 Here are some examples of marking simple data structures and globals.
49 struct @var{tag} GTY(())
54 typedef struct @var{tag} GTY(())
59 static GTY(()) struct @var{tag} *@var{list}; /* @r{points to GC memory} */
60 static GTY(()) int @var{counter}; /* @r{save counter in a PCH} */
63 The parser understands simple typedefs such as
64 @code{typedef struct @var{tag} *@var{name};} and
65 @code{typedef int @var{name};}.
66 These don't need to be marked.
69 * GTY Options:: What goes inside a @code{GTY(())}.
70 * GGC Roots:: Making global variables GGC roots.
71 * Files:: How the generated files work.
75 @section The Inside of a @code{GTY(())}
77 Sometimes the C code is not enough to fully describe the type
78 structure. Extra information can be provided with @code{GTY} options
79 and additional markers. Some options take a parameter, which may be
80 either a string or a type name, depending on the parameter. If an
81 option takes no parameter, it is acceptable either to omit the
82 parameter entirely, or to provide an empty string as a parameter. For
83 example, @code{@w{GTY ((skip))}} and @code{@w{GTY ((skip ("")))}} are
86 When the parameter is a string, often it is a fragment of C code. Four
87 special escapes may be used in these strings, to refer to pieces of
88 the data structure being marked:
90 @cindex % in GTY option
93 The current structure.
95 The structure that immediately contains the current structure.
97 The outermost structure that contains the current structure.
99 A partial expression of the form @code{[i1][i2]...} that indexes
100 the array item currently being marked.
103 For instance, suppose that you have a structure of the form
113 and @code{b} is a variable of type @code{struct B}. When marking
114 @samp{b.foo[11]}, @code{%h} would expand to @samp{b.foo[11]},
115 @code{%0} and @code{%1} would both expand to @samp{b}, and @code{%a}
116 would expand to @samp{[11]}.
118 As in ordinary C, adjacent strings will be concatenated; this is
119 helpful when you have a complicated expression.
122 GTY ((chain_next ("TREE_CODE (&%h.generic) == INTEGER_TYPE"
123 " ? TYPE_NEXT_VARIANT (&%h.generic)"
124 " : TREE_CHAIN (&%h.generic)")))
128 The available options are:
132 @item length ("@var{expression}")
134 There are two places the type machinery will need to be explicitly told
135 the length of an array. The first case is when a structure ends in a
136 variable-length array, like this:
138 struct rtvec_def GTY(()) @{
139 int num_elem; /* @r{number of elements} */
140 rtx GTY ((length ("%h.num_elem"))) elem[1];
144 In this case, the @code{length} option is used to override the specified
145 array length (which should usually be @code{1}). The parameter of the
146 option is a fragment of C code that calculates the length.
148 The second case is when a structure or a global variable contains a
149 pointer to an array, like this:
152 GTY ((length ("%h.regno_pointer_align_length"))) regno_decl;
154 In this case, @code{regno_decl} has been allocated by writing something like
157 ggc_alloc (x->regno_pointer_align_length * sizeof (tree));
159 and the @code{length} provides the length of the field.
161 This second use of @code{length} also works on global variables, like:
163 static GTY((length ("reg_base_value_size")))
170 If @code{skip} is applied to a field, the type machinery will ignore it.
171 This is somewhat dangerous; the only safe use is in a union when one
172 field really isn't ever used.
177 @item desc ("@var{expression}")
178 @itemx tag ("@var{constant}")
181 The type machinery needs to be told which field of a @code{union} is
182 currently active. This is done by giving each field a constant
183 @code{tag} value, and then specifying a discriminator using @code{desc}.
184 The value of the expression given by @code{desc} is compared against
185 each @code{tag} value, each of which should be different. If no
186 @code{tag} is matched, the field marked with @code{default} is used if
187 there is one, otherwise no field in the union will be marked.
189 In the @code{desc} option, the ``current structure'' is the union that
190 it discriminates. Use @code{%1} to mean the structure containing it.
191 There are no escapes available to the @code{tag} option, since it is a
196 struct tree_binding GTY(())
198 struct tree_common common;
199 union tree_binding_u @{
200 tree GTY ((tag ("0"))) scope;
201 struct cp_binding_level * GTY ((tag ("1"))) level;
202 @} GTY ((desc ("BINDING_HAS_LEVEL_P ((tree)&%0)"))) xscope;
207 In this example, the value of BINDING_HAS_LEVEL_P when applied to a
208 @code{struct tree_binding *} is presumed to be 0 or 1. If 1, the type
209 mechanism will treat the field @code{level} as being present and if 0,
210 will treat the field @code{scope} as being present.
214 @item param_is (@var{type})
217 Sometimes it's convenient to define some data structure to work on
218 generic pointers (that is, @code{PTR}) and then use it with a specific
219 type. @code{param_is} specifies the real type pointed to, and
220 @code{use_param} says where in the generic data structure that type
223 For instance, to have a @code{htab_t} that points to trees, one would
224 write the definition of @code{htab_t} like this:
226 typedef struct GTY(()) @{
228 void ** GTY ((use_param, @dots{})) entries;
232 and then declare variables like this:
234 static htab_t GTY ((param_is (union tree_node))) ict;
237 @findex param@var{n}_is
238 @findex use_param@var{n}
239 @item param@var{n}_is (@var{type})
240 @itemx use_param@var{n}
242 In more complicated cases, the data structure might need to work on
243 several different types, which might not necessarily all be pointers.
244 For this, @code{param1_is} through @code{param9_is} may be used to
245 specify the real type of a field identified by @code{use_param1} through
251 When a structure contains another structure that is parameterized,
252 there's no need to do anything special, the inner structure inherits the
253 parameters of the outer one. When a structure contains a pointer to a
254 parameterized structure, the type machinery won't automatically detect
255 this (it could, it just doesn't yet), so it's necessary to tell it that
256 the pointed-to structure should use the same parameters as the outer
257 structure. This is done by marking the pointer with the
258 @code{use_params} option.
263 @code{deletable}, when applied to a global variable, indicates that when
264 garbage collection runs, there's no need to mark anything pointed to
265 by this variable, it can just be set to @code{NULL} instead. This is used
266 to keep a list of free structures around for re-use.
269 @item if_marked ("@var{expression}")
271 Suppose you want some kinds of object to be unique, and so you put them
272 in a hash table. If garbage collection marks the hash table, these
273 objects will never be freed, even if the last other reference to them
274 goes away. GGC has special handling to deal with this: if you use the
275 @code{if_marked} option on a global hash table, GGC will call the
276 routine whose name is the parameter to the option on each hash table
277 entry. If the routine returns nonzero, the hash table entry will
278 be marked as usual. If the routine returns zero, the hash table entry
281 The routine @code{ggc_marked_p} can be used to determine if an element
282 has been marked already; in fact, the usual case is to use
283 @code{if_marked ("ggc_marked_p")}.
288 When applied to a field, @code{maybe_undef} indicates that it's OK if
289 the structure that this fields points to is never defined, so long as
290 this field is always @code{NULL}. This is used to avoid requiring
291 backends to define certain optional structures. It doesn't work with
296 @item chain_next ("@var{expression}")
297 @itemx chain_prev ("@var{expression}")
299 It's helpful for the type machinery to know if objects are often
300 chained together in long lists; this lets it generate code that uses
301 less stack space by iterating along the list instead of recursing down
302 it. @code{chain_next} is an expression for the next item in the list,
303 @code{chain_prev} is an expression for the previous item. For singly
304 linked lists, use only @code{chain_next}; for doubly linked lists, use
305 both. The machinery requires that taking the next item of the
306 previous item gives the original item.
309 @item reorder ("@var{function name}")
311 Some data structures depend on the relative ordering of pointers. If
312 the precompiled header machinery needs to change that ordering, it
313 will call the function referenced by the @code{reorder} option, before
314 changing the pointers in the object that's pointed to by the field the
315 option applies to. The function must take four arguments, with the
316 signature @samp{@w{void *, void *, gt_pointer_operator, void *}}.
317 The first parameter is a pointer to the structure that contains the
318 object being updated, or the object itself if there is no containing
319 structure. The second parameter is a cookie that should be ignored.
320 The third parameter is a routine that, given a pointer, will update it
321 to its correct new value. The fourth parameter is a cookie that must
322 be passed to the second parameter.
324 PCH cannot handle data structures that depend on the absolute values
325 of pointers. @code{reorder} functions can be expensive. When
326 possible, it is better to depend on properties of the data, like an ID
327 number or the hash of a string instead.
330 @item special ("@var{name}")
332 The @code{special} option is used to mark types that have to be dealt
333 with by special case machinery. The parameter is the name of the
334 special case. See @file{gengtype.c} for further details. Avoid
335 adding new special cases unless there is no other alternative.
339 @section Marking Roots for the Garbage Collector
340 @cindex roots, marking
341 @cindex marking roots
343 In addition to keeping track of types, the type machinery also locates
344 the global variables (@dfn{roots}) that the garbage collector starts
345 at. Roots must be declared using one of the following syntaxes:
349 @code{extern GTY(([@var{options}])) @var{type} @var{name};}
351 @code{static GTY(([@var{options}])) @var{type} @var{name};}
357 @code{GTY(([@var{options}])) @var{type} @var{name};}
360 is @emph{not} accepted. There should be an @code{extern} declaration
361 of such a variable in a header somewhere---mark that, not the
362 definition. Or, if the variable is only used in one file, make it
366 @section Source Files Containing Type Information
367 @cindex generated files
368 @cindex files, generated
370 Whenever you add @code{GTY} markers to a source file that previously
371 had none, or create a new source file containing @code{GTY} markers,
372 there are three things you need to do:
376 You need to add the file to the list of source files the type
377 machinery scans. There are four cases:
381 For a back-end file, this is usually done
382 automatically; if not, you should add it to @code{target_gtfiles} in
383 the appropriate port's entries in @file{config.gcc}.
386 For files shared by all front ends, add the filename to the
387 @code{GTFILES} variable in @file{Makefile.in}.
390 For files that are part of one front end, add the filename to the
391 @code{gtfiles} variable defined in the appropriate
392 @file{config-lang.in}. For C, the file is @file{c-config-lang.in}.
395 For files that are part of some but not all front ends, add the
396 filename to the @code{gtfiles} variable of @emph{all} the front ends
401 If the file was a header file, you'll need to check that it's included
402 in the right place to be visible to the generated files. For a back-end
403 header file, this should be done automatically. For a front-end header
404 file, it needs to be included by the same file that includes
405 @file{gtype-@var{lang}.h}. For other header files, it needs to be
406 included in @file{gtype-desc.c}, which is a generated file, so add it to
407 @code{ifiles} in @code{open_base_file} in @file{gengtype.c}.
409 For source files that aren't header files, the machinery will generate a
410 header file that should be included in the source file you just changed.
411 The file will be called @file{gt-@var{path}.h} where @var{path} is the
412 pathname relative to the @file{gcc} directory with slashes replaced by
413 @verb{|-|}, so for example the header file to be included in
414 @file{objc/objc-parse.c} is called @file{gt-objc-objc-parse.c}. The
415 generated header file should be included after everything else in the
416 source file. Don't forget to mention this file as a dependency in the
420 If a new @file{gt-@var{path}.h} file is needed, you need to arrange to
421 add a @file{Makefile} rule that will ensure this file can be built.
422 This is done by making it a dependency of @code{s-gtype}, like this:
424 gt-path.h : s-gtype ; @true
428 For language frontends, there is another file that needs to be included
429 somewhere. It will be called @file{gtype-@var{lang}.h}, where
430 @var{lang} is the name of the subdirectory the language is contained in.
431 It will need @file{Makefile} rules just like the other generated files.