Fix internal error on DW_OP_bregx(-1)

https://bugzilla.redhat.com/show_bug.cgi?id=1270564#c15
https://bugzilla.redhat.com/attachment.cgi?id=1081772

clang-3.5.0-9.fc22.x86_64
 <3><22b2>: Abbrev Number: 69 (DW_TAG_variable)
    <22b3>   DW_AT_location    : 7 byte block: 92 ff ff ff ff f 0	(DW_OP_bregx: 4294967295 (r-1) 0)
    <22bb>   DW_AT_name        : (indirect string, offset: 0x2a36): texture_data
    <22c1>   DW_AT_type        : <0x1d3>

(gdb) p variable
warning: Unmapped DWARF Register #-1 encountered.
regcache.c:177: internal-error: register_size: Assertion `regnum >= 0 && regnum < (gdbarch_num_regs (gdbarch) + gdbarch_num_pseudo_regs
(gdbarch))' failed.
[...]
Quit this debugging session? (y or n) FAIL: gdb.dwarf2/dw2-regno-invalid.exp: p variable (GDB internal error)

-> (x86_64)
(gdb) p variable
warning: Unmapped DWARF Register #-1 encountered.
Invalid register #-1, expecting 0 <= # < 220
(gdb) PASS: gdb.dwarf2/dw2-regno-invalid.exp: p variable
-> (i386)
(gdb) p variable
Invalid register #104, expecting 0 <= # < 104
(gdb) PASS: gdb.dwarf2/dw2-regno-invalid.exp: p variable

GDB calls gdbarch_dwarf2_reg_to_regnum() first which returns -1 in the x86_64
case
  if (regnum == -1)
    warning (_("Unmapped DWARF Register #%d encountered."), reg);
but in i386 case it does:
  /* This will hopefully provoke a warning.  */
  return gdbarch_num_regs (gdbarch) + gdbarch_num_pseudo_regs (gdbarch);
and the default implementation is a nop, leaving whatever register number
the DWARF specified.

gdb/ChangeLog
2015-10-20  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* findvar.c (address_from_register): Check REGNUM validity.

gdb/testsuite/ChangeLog
2015-10-20  Jan Kratochvil  <jan.kratochvil@redhat.com>
	    Pedro Alves  <palves@redhat.com>

	* gdb.dwarf2/dw2-regno-invalid.exp: New file.
	* lib/dwarf.exp (Dwarf): Add DW_OP_bregx.

1 files changed, 71 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.dwarf2/dw2-regno-invalid.exp b/gdb/testsuite/gdb.dwarf2/dw2-regno-invalid.exp
new file mode 100644
index 0000000..a7d77c5
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-regno-invalid.exp
@@ -0,0 +1,71 @@
+# Copyright 2015 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/>.
+load_lib dwarf.exp
+
+# Test GDB does not internal error on DWARF register number -1 which is invalid
+# in DWARF.  clang-3.5.0-9.fc22.x86_64 produced it inside DW_AT_location.
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+standard_testfile .S main.c
+
+# Make some DWARF for the test.
+set asm_file [standard_output_file $srcfile]
+Dwarf::assemble $asm_file {
+    cu {} {
+	compile_unit {
+	    {low_pc main DW_FORM_addr}
+	    {high_pc main+0x10000 DW_FORM_addr}
+	} {
+	    declare_labels integer_label
+
+	    integer_label: DW_TAG_base_type {
+		{DW_AT_byte_size 4 DW_FORM_sdata}
+		{DW_AT_encoding  @DW_ATE_signed}
+		{DW_AT_name      integer}
+	    }
+
+	    DW_TAG_subprogram {
+		{name main}
+		{DW_AT_external 1 flag}
+		{low_pc main DW_FORM_addr}
+		{high_pc main+0x10000 DW_FORM_addr}
+	    } {
+		DW_TAG_variable {
+		    {DW_AT_name bregx}
+			{DW_AT_type :$integer_label}
+			{DW_AT_external 1 flag}
+		    {DW_AT_location {
+			DW_OP_bregx 0xffffffff 0
+		    } SPECIAL_expr}
+		}
+	    }
+	}
+    }
+}
+
+if { [prepare_for_testing ${testfile}.exp ${testfile} \
+			  [list $srcfile2 $asm_file] {nodebug}] } {
+    return -1
+}
+
+runto_main
+
+# Variable 'bregx' needs to be inside main as for global symbols GDB does not
+# support non-trivial DW_AT_location.  Former GDB printed internal error here.
+gdb_test "p bregx"