[PATCH 1/3] Fix PR12616 - gdb does not implement DW_AT_data_bit_offset

Andreas Arnez
2016-11-14T15:04:04Z

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.
---
gdb/dwarf2read.c | 4 ++
gdb/testsuite/gdb.dwarf2/nonvar-access.exp | 103 +++++++++++++++++++++++++++++
2 files changed, 107 insertions(+)
create mode 100644 gdb/testsuite/gdb.dwarf2/nonvar-access.exp

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/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>"
--
2.3.0