Hi folks,

I'm continuing the work to enhance DFP type support on GDB. DFP types
are nativelly supported on the Power6 processor, thus they are stored in
registers as so. This patch handles the printing and setting of
Decimal128 types directly out of/in the registers.

A Decimal128 type is stored using pairs of Floating Point registers, and
it always starts in an even-numbered register. So, we could, for
example, have a Decimal128 var taking up FPR0-FPR1, FPR10-FPR11, but
never FPR1-FPR2 or FPR23-24.

Right now GDB isn't able to print those vars as it doesn't know they
take up more than one register.

The idea to overcome this was to create a new set of 16 Decimal128-typed
pseudo-registers called DL (Standing for "D"ecimal "L"ong Double) that
would represent all 16 pairs of Floating Point Registers.. By using a
pseudo-register mechanism, we're able to force the DL/FPR registers to
map directly into the FPR/DL registers.

Benefits of this change include:

* Better printing support for Decimal128 types. We just print the
register as is. No need to include casting tricks and arrays to print
that value.

* Straightforward way of setting Decimal128 values directly in the
registers. We just "set $dl0=1.23dl", for example, and it's
automatically set.

* Overview of the state of 16 Decimal128 registers using the "info reg
all" command.


Supporting the patch there is also a number of other modifications, as
listed:

* DFP types are now global since they were included as predefined types
in the "target-descriptions.c" file.

* A simple testcase making sure the registers are working as expected.

* Piece of documentation for the GDB manual.

* Fix to display a 128-bit hex literal representation of Decimal128
types (using "info reg").

Also note that this support will be available for both ppc64 and ppc32.

Looking forward to have comments and suggestions for improvements.

Regards,
Luis