[PATCH] Fix handling of DW_AT_data_bit_offset

[Tom Tromey <tromey@adacore.com>]

A newer version of GCC will now emit member locations using just
DW_AT_data_bit_offset, like:

 <3><14fe>: Abbrev Number: 1 (DW_TAG_member)
    <14ff>   DW_AT_name        : (indirect string, offset: 0x215e): nb_bytes
    <1503>   DW_AT_decl_file   : 1
    <1503>   DW_AT_decl_line   : 10
    <1504>   DW_AT_decl_column : 7
    <1505>   DW_AT_type        : <0x150b>
    <1509>   DW_AT_bit_size    : 31
    <150a>   DW_AT_data_bit_offset: 64

whereas earlier versions would emit something like:

 <3><164f>: Abbrev Number: 7 (DW_TAG_member)
    <1650>   DW_AT_name        : (indirect string, offset: 0x218d): nb_bytes
    <1654>   DW_AT_decl_file   : 1
    <1655>   DW_AT_decl_line   : 10
    <1656>   DW_AT_decl_column : 7
    <1657>   DW_AT_type        : <0x165f>
    <165b>   DW_AT_byte_size   : 4
    <165c>   DW_AT_bit_size    : 31
    <165d>   DW_AT_bit_offset  : 1
    <165e>   DW_AT_data_member_location: 8

That is, DW_AT_data_member_location is not emitted any more.  This is
a change due to the switch to DWARF 5 by default.

This change pointed out an existing bug in gdb, namely that the
attr_to_dynamic_prop depends on the presence of
DW_AT_data_member_location.  This patch moves the handling of
DW_AT_data_bit_offset into handle_data_member_location, and updates
attr_to_dynamic_prop to handle this new case.

A new test case is included.  This test fails with GCC 11, but passes
with an earlier version of GCC.
---
 gdb/dwarf2/read.c                          | 49 ++++++++++++++--------
 gdb/testsuite/gdb.ada/packed_record.exp    | 36 ++++++++++++++++
 gdb/testsuite/gdb.ada/packed_record/pr.adb | 35 ++++++++++++++++
 3 files changed, 103 insertions(+), 17 deletions(-)
 create mode 100644 gdb/testsuite/gdb.ada/packed_record.exp
 create mode 100644 gdb/testsuite/gdb.ada/packed_record/pr.adb

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 2e85deeaa39..c7a5362342d 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -14337,14 +14337,14 @@ dwarf2_access_attribute (struct die_info *die, struct dwarf2_cu *cu)
     }
 }
 
-/* Look for DW_AT_data_member_location.  Set *OFFSET to the byte
-   offset.  If the attribute was not found return 0, otherwise return
-   1.  If it was found but could not properly be handled, set *OFFSET
-   to 0.  */
+/* Look for DW_AT_data_member_location or DW_AT_data_bit_offset.  Set
+   *OFFSET to the byte offset.  If the attribute was not found return
+   0, otherwise return 1.  If it was found but could not properly be
+   handled, set *OFFSET to 0.  */
 
 static int
-handle_data_member_location (struct die_info *die, struct dwarf2_cu *cu,
-			     LONGEST *offset)
+handle_member_location (struct die_info *die, struct dwarf2_cu *cu,
+			LONGEST *offset)
 {
   struct attribute *attr;
 
@@ -14368,15 +14368,25 @@ handle_data_member_location (struct die_info *die, struct dwarf2_cu *cu,
 
       return 1;
     }
+  else
+    {
+      attr = dwarf2_attr (die, DW_AT_data_bit_offset, cu);
+      if (attr != nullptr)
+	{
+	  *offset = attr->constant_value (0);
+	  return 1;
+	}
+    }
 
   return 0;
 }
 
-/* Look for DW_AT_data_member_location and store the results in FIELD.  */
+/* Look for DW_AT_data_member_location or DW_AT_data_bit_offset and
+   store the results in FIELD.  */
 
 static void
-handle_data_member_location (struct die_info *die, struct dwarf2_cu *cu,
-			     struct field *field)
+handle_member_location (struct die_info *die, struct dwarf2_cu *cu,
+			struct field *field)
 {
   struct attribute *attr;
 
@@ -14418,6 +14428,12 @@ handle_data_member_location (struct die_info *die, struct dwarf2_cu *cu,
       else
 	dwarf2_complex_location_expr_complaint ();
     }
+  else
+    {
+      attr = dwarf2_attr (die, DW_AT_data_bit_offset, cu);
+      if (attr != nullptr)
+	SET_FIELD_BITPOS (*field, attr->constant_value (0));
+    }
 }
 
 /* Add an aggregate field to the field list.  */
@@ -14479,7 +14495,7 @@ dwarf2_add_field (struct field_info *fip, struct die_info *die,
 	}
 
       /* Get bit offset of field.  */
-      handle_data_member_location (die, cu, fp);
+      handle_member_location (die, cu, fp);
       attr = dwarf2_attr (die, DW_AT_bit_offset, cu);
       if (attr != nullptr && attr->form_is_constant ())
 	{
@@ -14526,10 +14542,6 @@ 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)
-				+ attr->constant_value (0)));
 
       /* Get name of field.  */
       fieldname = dwarf2_name (die, cu);
@@ -14590,7 +14602,7 @@ dwarf2_add_field (struct field_info *fip, struct die_info *die,
   else if (die->tag == DW_TAG_inheritance)
     {
       /* C++ base class field.  */
-      handle_data_member_location (die, cu, fp);
+      handle_member_location (die, cu, fp);
       FIELD_BITSIZE (*fp) = 0;
       fp->set_type (die_type (die, cu));
       FIELD_NAME (*fp) = fp->type ()->name ();
@@ -18305,6 +18317,9 @@ attr_to_dynamic_prop (const struct attribute *attr, struct die_info *die,
       if (target_attr == NULL)
 	target_attr = dwarf2_attr (target_die, DW_AT_data_member_location,
 				   target_cu);
+      if (target_attr == nullptr)
+	target_attr = dwarf2_attr (target_die, DW_AT_data_bit_offset,
+				   target_cu);
       if (target_attr == NULL)
 	{
 	  const char *name = var_decl_name (target_die, target_cu);
@@ -18348,11 +18363,11 @@ attr_to_dynamic_prop (const struct attribute *attr, struct die_info *die,
 	      }
 	    break;
 	  case DW_AT_data_member_location:
+	  case DW_AT_data_bit_offset:
 	    {
 	      LONGEST offset;
 
-	      if (!handle_data_member_location (target_die, target_cu,
-						&offset))
+	      if (!handle_member_location (target_die, target_cu, &offset))
 		return 0;
 
 	      baton = XOBNEW (obstack, struct dwarf2_property_baton);
diff --git a/gdb/testsuite/gdb.ada/packed_record.exp b/gdb/testsuite/gdb.ada/packed_record.exp
new file mode 100644
index 00000000000..20793067d06
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/packed_record.exp
@@ -0,0 +1,36 @@
+# Copyright 2021 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 "ada.exp"
+
+if { [skip_ada_tests] } { return -1 }
+
+standard_ada_testfile pr
+
+foreach_with_prefix scenario {all minimal} {
+    set flags [list debug additional_flags=-fgnat-encodings=$scenario]
+
+    if {[gdb_compile_ada "${srcfile}" "${binfile}" executable $flags] != ""} {
+	return -1
+    }
+
+    clean_restart ${testfile}
+
+    set bp_location [gdb_get_line_number "STOP" ${testdir}/pr.adb]
+    runto "pr.adb:$bp_location"
+
+    gdb_test "print var" \
+	"= \\(length => 11, length_t => 23, bytes => 13, msg => hello, val => \"abcdefghijk\"\\)"
+}
diff --git a/gdb/testsuite/gdb.ada/packed_record/pr.adb b/gdb/testsuite/gdb.ada/packed_record/pr.adb
new file mode 100644
index 00000000000..7abfda9085b
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/packed_record/pr.adb
@@ -0,0 +1,35 @@
+--  Copyright 2021 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/>.
+
+procedure pr is
+
+   type Report is (Hello, Goodbye);
+
+   type Response (Length : integer ) is record
+      Length_t : Integer;
+      Bytes : Natural;
+      Msg : Report;
+      Val : String(1..Length);
+   end record;
+
+   pragma pack (Response);
+
+   Var : Response(11) := (11, 23, 13, Hello, "abcdefghijk");
+
+begin
+
+   null; -- STOP
+
+end pr;
-- 
2.26.3