http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47409
--- Comment #14 from Jason Merrill <jason at gcc dot gnu.org> 2013-02-06 14:59:57 UTC --- (In reply to comment #13) > But, what to do about unions? The standard says that only one union member is > active, but which one it is? I think the compiler generally can't know. So, > do we just ignore unions and expand them always as we used to? Pick up the > first union member (or randomly or preferably one with volatile)? C++ defines copy of a (trivially copyable) union to copy the object representation, which is not volatile unless the whole union is volatile. I can't find anything relevant in C11. There is also C++ DR 496: http://www.open-std.org/JTC1/SC22/WG21/docs/cwg_toc.html#496 It seems the effect of this change will be to make assignment of a union with a volatile field ill-formed in C++ unless the union has a user-provided assignment operator. I think we just ignore unions, at least in the middle end. > What about bitfields? Does it have to be per bitfield assignment, or can we > e.g. assign the whole representative field at a time? I think we should follow the memory model data race rules here; volatile accesses are done per memory location, rather than per field.
