Make gengtype more robust against user error

	* doc/gty.texi ("Inheritance and GTY"): Make it clear that
	to use autogenerated markers for a class-hierarchy, every class
	must have a GTY marker.
	* gengtype.h (struct type): Add linked list of subclasses to
	the "s" member of the union.
	(add_subclass): New decl.
	* gengtype-state.c (read_state_struct_type): Set up subclass
	linked list.
	* gengtype.c (get_ultimate_base_class): New.
	(add_subclass): New.
	(new_structure): Set up subclass linked list.
	(set_gc_used_type): Propagate usage information to subclasses.
	(output_mangled_typename): Use get_ultimate_base_class.
	(walk_subclasses): Use the subclass linked list, avoiding an
	O(N^2) when writing out all types.
	(walk_type): Issue an error if the base class is missing a tag,
	rather than generating bogus C code.  Add a gcc_unreachable
	default case, in case people omit tags from concrete subclasses,
	or get the values wrong.
	(write_func_for_structure): Issue an error for subclasses for
	which the base doesn't have a "desc", since otherwise the
	autogenerated routines for the base would silently fail to visit
	any subclass fields.
	(write_root): Use get_ultimate_base_class, tweaking constness of
	tp to match that function's signature.

From-SVN: r204148

1 files changed, 7 insertions, 0 deletions

diff --git a/gcc/doc/gty.texi b/gcc/doc/gty.texi
index 090f6a6..a64d110 100644
--- a/gcc/doc/gty.texi
+++ b/gcc/doc/gty.texi
@@ -497,6 +497,13 @@ 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.
 
+Every class in the hierarchy must have a @code{GTY(())} marker, as
+gengtype will only attempt to parse classes that have such a marker
+@footnote{Classes lacking such a marker will not be identified as being
+part of the hierarchy, and so the marking routines will not handle them,
+leading to a assertion failure within the marking routines due to an
+unknown tag value (assuming that assertions are enabled).}.
+
 @smallexample
 class GTY((desc("%h.kind"), tag("0"))) example_base
 @{