bitfields: types vs modes?

[DJ Delorie <dj at redhat dot com>]

One of our customers has a chip with memory-mapped peripheral
registers that need to be accessed in a specific mode.  The registers
represent bitfields within the hardware, so a volatile struct is an
obvious choice to represent them in C.

However, gcc has a very simplistic heuristic for deciding what mode to
use to access bitfields in structures - it uses either the biggest or
smallest mode practical.  This offers the programmer no way to tell
gcc what mode the accesses need to be in, aside from manually
reading/writing memory with integer types and casting.

Options?  My thought, after some internal discussion, is that (if the
target chooses) we allow gcc to honor the type of a volatile bitfield
as the mode as long as it can do so without otherwise violating the
structure's layout.  Some new hook will be needed for the backend, and
perhaps a -W option for when the type cannot be honored.

I.e. if the programmer is careful enough to properly lay out the
struct, the programmer should get what the programmer asks for.

Comments?  Alternatives?