S390: Fix internal error with stackless inferior

This fixes a GDB internal error that may occur when the inferior has no
valid stack pointer in r15.

gdb/testsuite/ChangeLog:

	* gdb.arch/s390-stackless.S: New.
	* gdb.arch/s390-stackless.exp: New.

gdb/ChangeLog:

	* s390-linux-tdep.c (s390_backchain_frame_unwind_cache): Avoid
	exception when attempting to access the inferior's backchain.

1 files changed, 39 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.arch/s390-stackless.exp b/gdb/testsuite/gdb.arch/s390-stackless.exp
new file mode 100644
index 0000000..b4aedb5
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/s390-stackless.exp
@@ -0,0 +1,39 @@
+# 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 2 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/>.
+
+if {![istarget "s390*-*-*"]} then {
+    verbose "Skipping s390 stackless test."
+    return
+}
+
+standard_testfile .S
+
+if { [prepare_for_testing $testfile.exp $testfile $srcfile] } {
+    return -1
+}
+
+# Run until SIGSEGV.
+gdb_run_cmd
+
+set test "run until SIGSEGV"
+gdb_test_multiple "" $test {
+    -re "Program received signal SIGSEGV.*$gdb_prompt $" {
+	pass $test
+    }
+}
+
+# Ensure that 'info registers' works properly and does not generate
+# an internal-error.
+gdb_test "info registers" "r0.*" "info registers"