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Add documentation about gengtype and inheritance

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* doc/gty.texi (GTY Options): Add note about inheritance to
	description of desc and tag.
	(Inheritance and GTY): New.

From-SVN: r204147
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David Malcolm 2013-10-29 01:10:20 +00:00 committed by David Malcolm
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@ -1,3 +1,9 @@
2013-10-29 David Malcolm <dmalcolm@redhat.com>
* doc/gty.texi (GTY Options): Add note about inheritance to
description of desc and tag.
(Inheritance and GTY): New.
2013-10-29 David Malcolm <dmalcolm@redhat.com>
* gengtype-parse.c (opts_have): Drop "static" so that

52
gcc/doc/gty.texi
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@ -87,6 +87,7 @@ data members.
@menu
* GTY Options:: What goes inside a @code{GTY(())}.
* Inheritance and GTY:: Adding GTY to a class hierarchy.
* User GC:: Adding user-provided GC marking routines.
* GGC Roots:: Making global variables GGC roots.
* Files:: How the generated files work.
@ -234,6 +235,10 @@ In this example, the value of BINDING_HAS_LEVEL_P when applied to a
mechanism will treat the field @code{level} as being present and if 0,
will treat the field @code{scope} as being present.
The @code{desc} and @code{tag} options can also be used for inheritance
to denote which subclass an instance is. See @ref{Inheritance and GTY}
for more information.
@findex param_is
@findex use_param
@item param_is (@var{type})
@ -470,6 +475,53 @@ fields is completely handled by user-provided routines. See section
@ref{User GC} for details on what functions need to be provided.
@end table
@node Inheritance and GTY
@section Support for inheritance
gengtype has some support for simple class hierarchies. You can use
this to have gengtype autogenerate marking routines, provided:
@itemize @bullet
@item
There must be a concrete base class, with a discriminator expression
that can be used to identify which subclass an instance is.
@item
Only single inheritance is used.
@item
None of the classes within the hierarchy are templates.
@end itemize
If your class hierarchy does not fit in this pattern, you must use
@ref{User GC} instead.
The base class and its discriminator must be identified using the ``desc''
option. Each concrete subclass must use the ``tag'' option to identify
which value of the discriminator it corresponds to.
@smallexample
class GTY((desc("%h.kind"), tag("0"))) example_base
@{
public:
int kind;
tree a;
@};
class GTY((tag("1")) some_subclass : public example_base
@{
public:
tree b;
@};
class GTY((tag("2")) some_other_subclass : public example_base
@{
public:
tree c;
@};
@end smallexample
The generated marking routines for the above will contain a ``switch''
on ``kind'', visiting all appropriate fields. For example, if kind is
2, it will cast to ``some_other_subclass'' and visit fields a, b, and c.
@node User GC
@section Support for user-provided GC marking routines
@cindex user gc