Collaboration diagram for nesting_info:

Data Fields
struct nesting_info *	outer
struct nesting_info *	inner
struct nesting_info *	next
struct pointer_map_t *	field_map
struct pointer_map_t *	var_map
struct pointer_set_t *	mem_refs
bitmap	suppress_expansion
tree	context
tree	new_local_var_chain
tree	debug_var_chain
tree	frame_type
tree	frame_decl
tree	chain_field
tree	chain_decl
tree	nl_goto_field
bool	any_parm_remapped
bool	any_tramp_created
char	static_chain_added

Detailed Description

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version.

GCC is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with GCC; see the file COPYING3. If not see http://www.gnu.org/licenses/. The object of this pass is to lower the representation of a set of nested functions in order to expose all of the gory details of the various nonlocal references. We want to do this sooner rather than later, in order to give us more freedom in emitting all of the functions in question.

Back in olden times, when gcc was young, we developed an insanely complicated scheme whereby variables which were referenced nonlocally were forced to live in the stack of the declaring function, and then the nested functions magically discovered where these variables were placed. In order for this scheme to function properly, it required that the outer function be partially expanded, then we switch to compiling the inner function, and once done with those we switch back to compiling the outer function. Such delicate ordering requirements makes it difficult to do whole translation unit optimizations involving such functions.

The implementation here is much more direct. Everything that can be referenced by an inner function is a member of an explicitly created structure herein called the "nonlocal frame struct". The incoming static chain for a nested function is a pointer to this struct in the parent. In this way, we settle on known offsets from a known base, and so are decoupled from the logic that places objects in the function's stack frame. More importantly, we don't have to wait for that to happen – since the compilation of the inner function is no longer tied to a real stack frame, the nonlocal frame struct can be allocated anywhere. Which means that the outer function is now inlinable.

Theory of operation here is very simple. Iterate over all the statements in all the functions (depth first) several times, allocating structures and fields on demand. In general we want to examine inner functions first, so that we can avoid making changes to outer functions which are unnecessary.

The order of the passes matters a bit, in that later passes will be skipped if it is discovered that the functions don't actually interact at all. That is, they're nested in the lexical sense but could have been written as independent functions without change.