mips-tdep: Make FCRs always 32-bit

Fix a regression from commit f8b73d13b7ca ("Target-described register
support for MIPS"),
<https://sourceware.org/ml/gdb-patches/2007-05/msg00340.html>,
<https://sourceware.org/ml/gdb-patches/2007-06/msg00256.html>, which
caused Floating Point Control Registers (FCRs) to be shown as 64-bit
with 64-bit targets.

This came from the legacy register format where all raw registers
matched the width of the architecture regardless of their actual size.
The correct size was then set in `mips_register_type' for cooked
registers presented to the user, which in the case of FCRs meant the
cooked size was always forced to 32 bits, reflecting their actual
hardware size, even though the raw format carried them in 64-bit
quantities on 64-bit targets.  The upper 32 bits carried in the raw FCR
format have always been don't-cares, not actually retrieved from
hardware and never written back.

With the introduction of XML register descriptions the layout of
previously defined raw registers has been preserved, so as to keep
existing register handling code unchanged and make it easier for GDB and
`gdbserver' to interact with each other whether neither, either or both
parties talking over RSP support XML register descriptions.  For the
XML-described case however `mips_register_type' is not used in raw to
cooked register conversion, so any special cases coded there are not
taken into account.

Instead a new function, `mips_pseudo_register_type', has been introduced
to handle size conversion, however lacking the special case for FCRs for
the Linux and the now defunct IRIX target.  The correct size has been
maintained for embedded targets however, due to the bundling of FCRs
with the embedded registers under the `rawnum >= MIPS_EMBED_FP0_REGNUM +
32' condition.

Add the missing case to `mips_pseudo_register_type' then, referring to
the FCR indices explicitly, and observing that between
`MIPS_EMBED_FP0_REGNUM + 32' and `MIPS_FIRST_EMBED_REGNUM' there is an
unused register slot whose contents are ignored so with the removal of
embedded FCRs from under that condition we don't have to care about it
and we can refer to the embedded registers starting from
MIPS_FIRST_EMBED_REGNUM instead.

Add a test case too so that we have means to check automatically that
the correct user-visible size of FCRs is maintained.

	gdb/
	* mips-tdep.c (mips_pseudo_register_type): Make FCRs always
	32-bit.

	gdb/testsuite/
	* gdb.arch/mips-fcr.exp: New test.
	* gdb.arch/mips-fcr.c: Source for the new test.

1 files changed, 22 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.arch/mips-fcr.c b/gdb/testsuite/gdb.arch/mips-fcr.c
new file mode 100644
index 00000000000..1545b21709f
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/mips-fcr.c
@@ -0,0 +1,22 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2016 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+int
+main (void)
+{
+  return 0;
+}