Doc for -fcall-used-REG and -fcall-saved-REG

[Gavin Romig-Koch <gavin at cygnus dot com>]

>From invoke.texi, for -fcall-used-REG and -fcall-saved-REG:

   Use of this flag for a register that has a fixed pervasive role in the
   machine's execution model, such as the stack pointer or frame pointer,
   will produce disastrous results.

I'm writing a patch to have the compiler emit an error message 
if these options are used on the frame pointer or the stack pointer,
and was going to update the doc to reflect this, so some thing 
like:

   Use of this flag for the stack pointer or the frame pointer is
   an error.

My question is whether we need to keep the above comment for 
other fixed pervasive role registers (other than the sp and fp);
something like:

   Use of this flag for the stack pointer or the frame pointer is
   an error.  Use of this flag for a register that has a fixed 
   pervasive role in the machine's execution model, other than 
   the stack pointer and frame pointer, will produce disastrous
   results.

Or wether we know that there are no other such fixed registers,
and can drop that second sentance?  Does anyone know of other
fixed pervasive role (perhaps machine specific) registers  
exist?

I think the "will produce disastrous results" thing is a cop out,
and I'd like to replace it with an actual real life error message,
if we can hunt down all the bad cases.

                                         -gavin...