Handle field with dynamic bit offset

I discovered that GCC emitted incorrect DWARF for the test case
included in this patch.  Eric wrote a fix for GCC, but then he found
that gdb crashed on the resulting file.

This test has a field that is at a non-constant bit offset from the
start of the type.  DWARF 5 does not allow for this situation (I've
sent a report to the DWARF list), but DWARF 3 did allow for this via a
combination of an expression for the byte offset and then the use of
DW_AT_bit_offset.  This looks like:

 <5><117a>: Abbrev Number: 17 (DW_TAG_member)
    <117b>   DW_AT_name        : (indirect string, offset: 0x1959): another_field
...
    <1188>   DW_AT_bit_offset  : 6
    <1189>   DW_AT_data_member_location: 6 byte block: 99 3d 1 0 0 22 	(DW_OP_call4: <0x1193>; DW_OP_plus)
...
 <3><1193>: Abbrev Number: 2 (DW_TAG_dwarf_procedure)
    <1194>   DW_AT_location    : 15 byte block: 97 94 1 37 1a 32 1e 23 7 38 1b 31 1c 23 3 	(DW_OP_push_object_address; DW_OP_deref_size: 1; DW_OP_lit7; DW_OP_and; DW_OP_lit2; DW_OP_mul; DW_OP_plus_uconst: 7; DW_OP_lit8; DW_OP_div; DW_OP_lit1; DW_OP_minus; DW_OP_plus_uconst: 3)

Now, that combination is not fully general, in that the bit offset
must be a constant -- only the byte offset may really vary.  However,
I couldn't come up with a situation where full generality is needed,
mainly because GNAT won't seem to pack fields into the padding of a
variable-length array.

Meanwhile, the reason for the gdb crash is that the code handling
DW_AT_bit_offset assumes that the byte offset is a constant.  This
causes an assertion failure.

This patch arranges for DW_AT_bit_offset to be applied during field
resolution, when needed.

2 files changed, 91 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.ada/dyn-bit-offset.exp b/gdb/testsuite/gdb.ada/dyn-bit-offset.exp
new file mode 100644
index 0000000..19d16b1
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/dyn-bit-offset.exp
@@ -0,0 +1,46 @@
+# Copyright 2025 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"
+
+require allow_ada_tests
+
+standard_ada_testfile exam
+
+set flags {debug}
+if {[ada_minimal_encodings]} {
+    lappend flags additional_flags=-fgnat-encodings=minimal
+}
+
+if {[gdb_compile_ada "${srcfile}" "${binfile}" executable $flags] != ""} {
+    return -1
+}
+
+clean_restart ${testfile}
+
+set bp_location [gdb_get_line_number "STOP" ${testdir}/exam.adb]
+runto "exam.adb:$bp_location"
+
+gdb_test_multiple "print spr" "" {
+    -re -wrap " = \\(discr => 3, array_field => \\(-5, -6, -7\\), field => -4, another_field => -6\\)" {
+	pass $gdb_test_name
+    }
+    -re -wrap " = \\(discr => 3, array_field => \\(-5, -6, -7\\), field => -4, another_field => -4\\)" {
+	# A known GCC bug.
+	xfail $gdb_test_name
+    }
+}
+
+gdb_test "print spr.field" " = -4"
diff --git a/gdb/testsuite/gdb.ada/dyn-bit-offset/exam.adb b/gdb/testsuite/gdb.ada/dyn-bit-offset/exam.adb
new file mode 100644
index 0000000..a882afd
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/dyn-bit-offset/exam.adb
@@ -0,0 +1,45 @@
+--  Copyright 2025 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 Exam is
+   type Small is range -7 .. -4;
+   for Small'Size use 2;
+
+   type Packed_Array is array (Integer range <>) of Small;
+   pragma pack (Packed_Array);
+
+   subtype Range_Int is Natural range 0 .. 7;
+
+   type Some_Packed_Record (Discr : Range_Int := 3) is record
+      Array_Field : Packed_Array (1 .. Discr);
+      Field: Small;
+      case Discr is
+         when 3 =>
+            Another_Field : Small;
+         when others =>
+            null;
+      end case;
+   end record;
+   pragma Pack (Some_Packed_Record);
+   pragma No_Component_Reordering (Some_Packed_Record);
+
+   SPR : Some_Packed_Record := (Discr => 3,
+                                Field => -4,
+                                Another_Field => -6,
+                                Array_Field => (-5, -6, -7));
+
+begin
+   null;                        --  STOP
+end Exam;