[tree-ssa, RFC] SSA_NAMES and overload of chain field

[law@redhat.com]

In message <20031128224306.GO1906@kam.mff.cuni.cz>, Jan Hubicka writes:
 >Hi,
 >the chain filed is special because GGC is aware of fact that the chain
 >is used as linked list:
 >union lang_tree_node
 >  GTY((desc ("TREE_CODE (&%h.generic) == IDENTIFIER_NODE"),
 >       chain_next ("TREE_CODE (&%h.generic) == INTEGER_TYPE ? (union lang_tre
 >e_node *)TYPE_NEXT_VARIANT (&%h.generic) : (union lang_tree_node *)TREE_CHAIN
 > (&%h.generic)")))
 >
 >The use of the field in SSA_NAME_DEF_STMT breaks this expectation and
 >confuses GGC when multiple SSA_NAMES point to same DEF_STMT.
 >
 >The attached patch fixes it by avoiding the overload of chain and ading
 >separate field for this, but I am open for further suggestions.
 >
 >Bootstrapped/regtested i686-pc-gnu-linux in isolation
 >and additionally x86_64-linux in combination with two patches sent
 >earlier.
 >Honza
 >
 >2003-11-28  Jan Hubicka  <jh@suse.cz>
 >	* tree.h (SSA_NAME_DEF_STMT): Use def_stmt field.
 >	(tree_ssa_name): def_stmt.
Use the annotation field or something like that.   Otherwise you end up
throwing SSA_NAMEs into a different GC pagesize.

However, before doing that, I'd like a clearer explanatation of why using
the TREE_CHAIN field is causing problems.

Jeff