insv vs one-bit fields

[Paul Schlie]

Richard Henderson writes:
> On Fri, Sep 02, 2005 at 09:40:20PM -0400, DJ Delorie wrote:
> http://gcc.gnu.org/ml/gcc/2005-09/msg00064.html
> > So... why is it illegal to put a constant into a single bit field?
> 
> Probably because it was more efficient to use some other pattern
> for some other target.
>
> But there's absolutely zero chance you can reliably use a volatile
> bit field to emit a given insn sequence necessary for accessing a
> hardware register.

- Out of curiosity, how's that?

  As it would seem that as HW control/I/O registers are often typically
  mapped into a processor's data memory address space, they may be
  correspondingly addressable via a read/mask/write as any N bit field
  may be?

  Candidly the only potential problem I see may occur if multiple
  independent volatile bit fields of any size are defined within the same
  uniquely addressable UNIT, as then an access to any would necessitate an
  access to all (although even this is likely fine, as if in fact the bit
  fields were defined in this way, it would be unavoidably physically
  required).