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?
