Fix PR12616 - gdb does not implement DW_AT_data_bit_offset

The DW_AT_data_bit_offset attribute was introduced by DWARF V4 and
allows specifying the offset of a data member within its containing
entity.  But although the new attribute was intended to replace
DW_AT_bit_offset for this purpose, GDB ignores it, and thus GCC still
emits DW_AT_bit_offset instead.  See also
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71669.

This change fixes GDB's lack of support for DW_AT_data_bit_offset and
adds an appropriate test case.

gdb/ChangeLog:

	PR gdb/12616
	* dwarf2read.c (dwarf2_add_field): Handle the DWARF V4 attribute
	DW_AT_data_bit_offset.

gdb/testsuite/ChangeLog:

	PR gdb/12616
	* gdb.dwarf2/nonvar-access.exp: New testcase.  Check that GDB
	respects the DW_AT_data_bit_offset attribute.

4 files changed, 119 insertions, 0 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index a2a11e2..22f02cc 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,9 @@
+2016-11-24  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
+	PR gdb/12616
+	* dwarf2read.c (dwarf2_add_field): Handle the DWARF V4 attribute
+	DW_AT_data_bit_offset.
+
 2016-11-23  Pedro Alves  <palves@redhat.com>
 
 	* Makefile.in (SFILES): Add common/run-time-clock.c.
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 1ad6b00..558159a 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -12584,6 +12584,10 @@ dwarf2_add_field (struct field_info *fip, struct die_info *die,
 				 - bit_offset - FIELD_BITSIZE (*fp)));
 	    }
 	}
+      attr = dwarf2_attr (die, DW_AT_data_bit_offset, cu);
+      if (attr != NULL)
+	SET_FIELD_BITPOS (*fp, (FIELD_BITPOS (*fp)
+				+ dwarf2_get_attr_constant_value (attr, 0)));
 
       /* Get name of field.  */
       fieldname = dwarf2_name (die, cu);
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index c34b37d..e9dc36f 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2016-11-24  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
+	PR gdb/12616
+	* gdb.dwarf2/nonvar-access.exp: New testcase.  Check that GDB
+	respects the DW_AT_data_bit_offset attribute.
+
 2016-11-22  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	* gdb.base/default.exp: Fix output of "set language".
diff --git a/gdb/testsuite/gdb.dwarf2/nonvar-access.exp b/gdb/testsuite/gdb.dwarf2/nonvar-access.exp
new file mode 100644
index 0000000..21532a6
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/nonvar-access.exp
@@ -0,0 +1,103 @@
+# 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/>.
+
+# Test accessing "non-variable" variables, i.e., variables which are
+# optimized to a constant DWARF location expression and/or
+# partially/fully optimized out.
+
+load_lib dwarf.exp
+
+if {![dwarf2_support]} { return 0 }
+
+standard_testfile main.c nonvar-access-dw.S
+
+# Make some DWARF for the test.
+set asm_file [standard_output_file $srcfile2]
+
+Dwarf::assemble $asm_file {
+    cu {} {
+	compile_unit {
+	    {DW_AT_name main.c}
+	} {
+	    declare_labels int_type_label short_type_label
+	    declare_labels struct_s_label
+
+	    int_type_label: base_type {
+		{name "int"}
+		{encoding @DW_ATE_signed}
+		{byte_size 4 DW_FORM_sdata}
+	    }
+
+	    struct_s_label: structure_type {
+		{name s}
+		{byte_size 4 DW_FORM_sdata}
+	    } {
+		member {
+		    {name a}
+		    {type :$int_type_label}
+		    {data_member_location 0 DW_FORM_udata}
+		    {bit_size 8 DW_FORM_udata}
+		}
+		member {
+		    {name b}
+		    {type :$int_type_label}
+		    {data_bit_offset 8 DW_FORM_udata}
+		    {bit_size 24 DW_FORM_udata}
+		}
+	    }
+
+	    DW_TAG_subprogram {
+		{name main}
+		{DW_AT_external 1 flag}
+		{low_pc [gdb_target_symbol main] DW_FORM_addr}
+		{high_pc [gdb_target_symbol main]+0x10000 DW_FORM_addr}
+	    } {
+		DW_TAG_variable {
+		    {name undef_int}
+		    {type :$int_type_label}
+		}
+		DW_TAG_variable {
+		    {name undef_s}
+		    {type :$struct_s_label}
+		}
+		DW_TAG_variable {
+		    {name def_s}
+		    {type :$struct_s_label}
+		    {location {
+			const1u 0
+			stack_value
+			bit_piece 8 0
+			const1s -1
+			stack_value
+			bit_piece 24 0
+		    } SPECIAL_expr}
+		}
+	    }
+	}
+    }
+}
+
+if { [prepare_for_testing ${testfile}.exp ${testfile} \
+	  [list $srcfile $asm_file] {nodebug}] } {
+    return -1
+}
+
+if ![runto_main] {
+    return -1
+}
+
+gdb_test "print def_s" " = \\{a = 0, b = -1\\}"
+gdb_test "print undef_int" " = <optimized out>"
+gdb_test "print undef_s.a" " = <optimized out>"