Unifying i387 support

[Andrew Cagney]

Jim Blandy wrote:
> 
> We would like make it possible to write code that accesses the i387
> registers which can be shared amongst all the i386-based targets.
> However, at present, each target lays the registers out differently in
> GDB's map, and #defines different symbols to say where things are.
> 
> There are two approaches to fix this.
> 
> 1) We could choose a common set of symbols, assign them useful
>    semantics, and make each target #define them appropriately.  Then,
>    generic i387 code can just use them.  Each target will continue to
>    lay out registers in GDB's map as it pleases.
> 
> 2) We could choose a single register layout for all i387 targets to
>    use, and require targets to reform the data they receive from the
>    OS via ptrace or whatever to fit that layout.  Generic i387 code
>    can be written to assume this layout.

I also have a strong preference for two.

Another issue to keep in mind is remote.c.  It assumes that the order
registers appear in a G packet corresponds to their internal-to-gdb
register numbering.  It relies heavily on REGISTER_RAW_SIZE().

I'm not adverse to ideas such as there being a macro that maps a
target-G-packet register onto an internal register.  Kind of like the
RAW/VIRTUAL stuff but in the correct place :-).  The alternative would
be to parameterize the target with specific call backs.

	Andrew