Fix crash when reading dummy CUs.

Dummy CUs are used by the incremental linker to pre-allocate space
in the output file. They have a DWARF header but no contents.

gdb/ChangeLog:

	* dwarf2read.c (dwarf2_per_cu_data): Add comment.
	(load_cu): Handle dummy CUs.
	(dw2_do_instantiate_symtab, process_queuef): Ditto.
	(dwarf2_fetch_die_loc_sect_off, dwarf2_fetch_constant_bytes): Ditto.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/dw2-dummy-cu.S: New file.
	* gdb.dwarf2/dw2-dummy-cu.exp: New file.

1 files changed, 39 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.dwarf2/dw2-dummy-cu.exp b/gdb/testsuite/gdb.dwarf2/dw2-dummy-cu.exp
new file mode 100644
index 0000000..a5f6ca5
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-dummy-cu.exp
@@ -0,0 +1,39 @@
+# 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
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+standard_testfile .S
+set executable ${testfile}
+
+if  { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" object {}] != "" } {
+    return -1
+}
+
+set saved_gdbflags $GDBFLAGS
+set GDBFLAGS "$GDBFLAGS -readnow"
+
+clean_restart $executable
+
+# Something simple to verify gdb didn't crash, and has read in whatever symbol
+# info is there.
+gdb_test "info fun main" "main_end"
+
+set GDBFLAGS $saved_gdbflags