[PATCH][4.8] C++ memory model bitfield handling rewrite

[Joseph S. Myers]

On Tue, 7 Feb 2012, Richard Guenther wrote:

> If the C frontend would stop using DECL_INITIAL temporarily for
> bitfield FIELD_DECLs we could avoid adding a new member to struct
> tree_field_decl - Joseph, is it possible to avoid using DECL_INITIAL?

C++ does the same thing with DECL_INITIAL so I'd expect that to need to 
change as well - Jason?  C only needs an integer width within a limited 
range; C++ may actually need a general expression here.

C currently uses a TREE_LIST of field declarations when parsing a 
structure.  It should be reasonably straightforward to change that to a 
VEC of structures storing both the declaration and the width, so the width 
no longer need go in DECL_INITIAL of the declaration, though it will be 
necessary to check for other code using DECL_INITIAL to check for 
bit-fields.  For example, c-common.c:handle_packed_attribute uses 
DECL_INITIAL like that; it should be possible to make both C and C++ set 
DECL_C_BIT_FIELD early enough that such DECL_INITIAL checks can be 
replaced by a more meaningful DECL_C_BIT_FIELD check.

-- 
Joseph S. Myers
joseph@codesourcery.com