Handle dynamic DW_AT_data_bit_offset

In Ada, a field can have a dynamic bit offset in its enclosing record.

In DWARF 3, this was handled using a dynamic
DW_AT_data_member_location, combined with a DW_AT_bit_offset -- this
combination worked out ok because in practice GNAT only needs a
dynamic byte offset with a fixed offset within the byte.

However, this approach was deprecated in DWARF 4 and then removed in
DWARF 5.  No replacement approach was given, meaning that in strict
mode there is no way to express this.

This is a DWARF bug, see

    https://dwarfstd.org/issues/250501.1.html

In a discussion on the DWARF mailing list, a couple people mentioned
that compilers could use the obvious extension of a dynamic
DW_AT_data_bit_offset.  I've implemented this for LLVM:

    llvm/llvm-project#141106

In preparation for that landing, this patch implements support for
this construct in gdb.

New in v2: renamed some constants and added a helper method, per
Simon's review.

New in v3: more renamings.

Approved-By: Simon Marchi <[email protected]>

Author: tromey

gdb/dwarf2/call-site.h

@@ -198,7 +198,7 @@ struct call_site
   struct call_site *tail_call_next = nullptr;
 
   /* * Describe DW_AT_call_target.  Missing attribute uses
-     FIELD_LOC_KIND_DWARF_BLOCK with FIELD_DWARF_BLOCK == NULL.  */
+     FIELD_LOC_KIND_DWARF_BLOCK_ADDR with FIELD_DWARF_BLOCK == NULL.  */
 
   struct call_site_target target {};
 

gdb/dwarf2/read.c

@@ -9999,7 +9999,7 @@ handle_member_location (struct die_info *die, struct dwarf2_cu *cu,
 	      dlbaton->per_objfile = per_objfile;
 	      dlbaton->per_cu = cu->per_cu;
 
-	      field->set_loc_dwarf_block (dlbaton);
+	      field->set_loc_dwarf_block_addr (dlbaton);
 	    }
 	}
       else
@@ -10010,7 +10010,28 @@ handle_member_location (struct die_info *die, struct dwarf2_cu *cu,
     {
       attr = dwarf2_attr (die, DW_AT_data_bit_offset, cu);
       if (attr != nullptr)
-	field->set_loc_bitpos (attr->unsigned_constant ().value_or (0));
+	{
+	  if (attr->form_is_constant ())
+	    field->set_loc_bitpos (attr->unsigned_constant ().value_or (0));
+	  else if (attr->form_is_block ())
+	    {
+	      /* This is a DWARF extension.  See
+		 https://dwarfstd.org/issues/250501.1.html.  */
+	      dwarf2_per_objfile *per_objfile = cu->per_objfile;
+	      dwarf2_locexpr_baton *dlbaton
+		= OBSTACK_ZALLOC (&per_objfile->objfile->objfile_obstack,
+				  dwarf2_locexpr_baton);
+	      dlbaton->data = attr->as_block ()->data;
+	      dlbaton->size = attr->as_block ()->size;
+	      dlbaton->per_objfile = per_objfile;
+	      dlbaton->per_cu = cu->per_cu;
+
+	      field->set_loc_dwarf_block_bitpos (dlbaton);
+	    }
+	  else
+	    complaint (_("Unsupported form %s for DW_AT_data_bit_offset"),
+		       dwarf_form_name (attr->form));
+	}
     }
 }
 

gdb/gdb-gdb.py.in

@@ -164,8 +164,10 @@ class StructMainTypePrettyPrinter:
             return "physaddr = 0x%x" % loc_val["physaddr"]
         elif loc_kind == "FIELD_LOC_KIND_PHYSNAME":
             return "physname = %s" % loc_val["physname"]
-        elif loc_kind == "FIELD_LOC_KIND_DWARF_BLOCK":
-            return "dwarf_block = %s" % loc_val["dwarf_block"]
+        elif loc_kind == "FIELD_LOC_KIND_DWARF_BLOCK_ADDR":
+            return "dwarf_block_addr = %s" % loc_val["dwarf_block"]
+        elif loc_kind == "FIELD_LOC_KIND_DWARF_BLOCK_BITPOS":
+            return "dwarf_block_bitpos = %s" % loc_val["dwarf_block"]
         else:
             return "m_loc = ??? (unsupported m_loc_kind value)"
 

gdb/gdbtypes.c

@@ -2121,7 +2121,7 @@ is_dynamic_type_internal (struct type *type, bool top_level)
 	      return true;
 	    /* If the field is at a fixed offset, then it is not
 	       dynamic.  */
-	    if (type->field (i).loc_kind () != FIELD_LOC_KIND_DWARF_BLOCK)
+	    if (!type->field (i).loc_is_dwarf_block ())
 	      continue;
 	    /* Do not consider C++ virtual base types to be dynamic
 	       due to the field's offset being dynamic; these are
@@ -2723,7 +2723,7 @@ resolve_dynamic_field (struct field &field,
 {
   gdb_assert (!field.is_static ());
 
-  if (field.loc_kind () == FIELD_LOC_KIND_DWARF_BLOCK)
+  if (field.loc_is_dwarf_block ())
     {
       dwarf2_locexpr_baton *field_loc
 	= field.loc_dwarf_block ();
@@ -2736,10 +2736,15 @@ resolve_dynamic_field (struct field &field,
       prop.set_locexpr (&baton);
 
       CORE_ADDR vals[1] = {addr_stack->addr};
-      CORE_ADDR addr;
-      if (dwarf2_evaluate_property (&prop, frame, addr_stack, &addr, vals))
+      CORE_ADDR addr_or_bitpos;
+      if (dwarf2_evaluate_property (&prop, frame, addr_stack,
+				    &addr_or_bitpos, vals))
 	{
-	  field.set_loc_bitpos (TARGET_CHAR_BIT * (addr - addr_stack->addr));
+	  if (field.loc_kind () == FIELD_LOC_KIND_DWARF_BLOCK_ADDR)
+	    field.set_loc_bitpos (TARGET_CHAR_BIT
+				  * (addr_or_bitpos - addr_stack->addr));
+	  else
+	    field.set_loc_bitpos (addr_or_bitpos);
 
 	  if (field_loc->is_field_location)
 	    {
@@ -4415,7 +4420,8 @@ check_types_equal (struct type *type1, struct type *type2,
 					       field2->loc_physname ()))
 		return false;
 	      break;
-	    case FIELD_LOC_KIND_DWARF_BLOCK:
+	    case FIELD_LOC_KIND_DWARF_BLOCK_ADDR:
+	    case FIELD_LOC_KIND_DWARF_BLOCK_BITPOS:
 	      {
 		struct dwarf2_locexpr_baton *block1, *block2;
 
@@ -5560,8 +5566,12 @@ copy_type_recursive (struct type *type, copied_types_hash_t &copied_types)
 	      new_type->field (i).set_loc_physname
 		(xstrdup (type->field (i).loc_physname ()));
 	      break;
-	    case FIELD_LOC_KIND_DWARF_BLOCK:
-	      new_type->field (i).set_loc_dwarf_block
+	    case FIELD_LOC_KIND_DWARF_BLOCK_ADDR:
+	      new_type->field (i).set_loc_dwarf_block_addr
+		(type->field (i).loc_dwarf_block ());
+	      break;
+	    case FIELD_LOC_KIND_DWARF_BLOCK_BITPOS:
+	      new_type->field (i).set_loc_dwarf_block_bitpos
 		(type->field (i).loc_dwarf_block ());
 	      break;
 	    default:

gdb/gdbtypes.h

@@ -480,7 +480,10 @@ enum field_loc_kind
     FIELD_LOC_KIND_ENUMVAL,	/**< enumval */
     FIELD_LOC_KIND_PHYSADDR,	/**< physaddr */
     FIELD_LOC_KIND_PHYSNAME,	/**< physname */
-    FIELD_LOC_KIND_DWARF_BLOCK	/**< dwarf_block */
+    /* A DWARF block that computes the address of the field.  */
+    FIELD_LOC_KIND_DWARF_BLOCK_ADDR,	/**< dwarf_block */
+    /* A DWARF block that computes the bit offset of the field.  */
+    FIELD_LOC_KIND_DWARF_BLOCK_BITPOS,
   };
 
 /* * A discriminant to determine which field in the
@@ -616,6 +619,13 @@ struct field
     return m_loc_kind;
   }
 
+  /* Return true if this location has either "DWARF block" kind.  */
+  bool loc_is_dwarf_block () const
+  {
+    return (m_loc_kind == FIELD_LOC_KIND_DWARF_BLOCK_ADDR
+	    || m_loc_kind == FIELD_LOC_KIND_DWARF_BLOCK_BITPOS);
+  }
+
   LONGEST loc_bitpos () const
   {
     gdb_assert (m_loc_kind == FIELD_LOC_KIND_BITPOS);
@@ -666,13 +676,19 @@ struct field
 
   dwarf2_locexpr_baton *loc_dwarf_block () const
   {
-    gdb_assert (m_loc_kind == FIELD_LOC_KIND_DWARF_BLOCK);
+    gdb_assert (loc_is_dwarf_block ());
     return m_loc.dwarf_block;
   }
 
-  void set_loc_dwarf_block (dwarf2_locexpr_baton *dwarf_block)
+  void set_loc_dwarf_block_addr (dwarf2_locexpr_baton *dwarf_block)
+  {
+    m_loc_kind = FIELD_LOC_KIND_DWARF_BLOCK_ADDR;
+    m_loc.dwarf_block = dwarf_block;
+  }
+
+  void set_loc_dwarf_block_bitpos (dwarf2_locexpr_baton *dwarf_block)
   {
-    m_loc_kind = FIELD_LOC_KIND_DWARF_BLOCK;
+    m_loc_kind = FIELD_LOC_KIND_DWARF_BLOCK_BITPOS;
     m_loc.dwarf_block = dwarf_block;
   }
 

gdb/gnu-v3-abi.c

@@ -476,7 +476,7 @@ gnuv3_baseclass_offset (struct type *type, int index,
     return TYPE_BASECLASS_BITPOS (type, index) / 8;
 
   /* If we have a DWARF expression for the offset, evaluate it.  */
-  if (type->field (index).loc_kind () == FIELD_LOC_KIND_DWARF_BLOCK)
+  if (type->field (index).loc_kind () == FIELD_LOC_KIND_DWARF_BLOCK_ADDR)
     {
       struct dwarf2_property_baton baton;
       baton.property_type

gdb/python/py-type.c

@@ -158,7 +158,7 @@ convert_field (struct type *type, int field)
 	}
       else
 	{
-	  if (type->field (field).loc_kind () == FIELD_LOC_KIND_DWARF_BLOCK)
+	  if (type->field (field).loc_is_dwarf_block ())
 	    arg = gdbpy_ref<>::new_reference (Py_None);
 	  else
 	    arg = gdb_py_object_from_longest (type->field (field).loc_bitpos ());

gdb/testsuite/gdb.dwarf2/dynamic-bit-offset.exp

@@ -0,0 +1,95 @@
+# 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/>.
+
+# Test DW_AT_data_bit_offset with an expression.  This is a DWARF
+# extension, but expected to be in DWARF 6.  See
+# https://dwarfstd.org/issues/250501.1.html
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+require dwarf2_support
+
+standard_testfile ada-array-bound.c -debug.S
+
+# Set up the DWARF for the test.
+
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    global srcdir subdir srcfile
+
+    cu {} {
+	DW_TAG_compile_unit {
+		{DW_AT_language @DW_LANG_Ada95}
+		{DW_AT_name	$srcfile}
+	} {
+	    declare_labels byte array struct
+
+	    byte: DW_TAG_base_type {
+		{DW_AT_byte_size 1 DW_FORM_sdata}
+		{DW_AT_encoding	 @DW_ATE_unsigned}
+		{DW_AT_name	 byte}
+	    }
+
+	    array: DW_TAG_array_type {
+		{DW_AT_name array_type}
+		{DW_AT_type :$byte}
+	    } {
+		DW_TAG_subrange_type {
+		    {DW_AT_type	       :$byte}
+		    {DW_AT_upper_bound 3 DW_FORM_sdata}
+		}
+	    }
+
+	    struct: DW_TAG_structure_type {
+		{DW_AT_name discriminated}
+		{DW_AT_byte_size 4 DW_FORM_sdata}
+	    } {
+		DW_TAG_member {
+		    {DW_AT_name disc}
+		    {DW_AT_type :$byte}
+		    {DW_AT_data_member_location 0 DW_FORM_sdata}
+		}
+
+		# We know this is always at offset 1 but use an
+		# expression just to test this code path.  This is a
+		# DWARF extension.  See
+		# https://dwarfstd.org/issues/250501.1.html.
+		DW_TAG_member {
+		    {DW_AT_name nums}
+		    {DW_AT_type :$array}
+		    {DW_AT_data_bit_offset {DW_OP_lit8} SPECIAL_expr}
+		}
+	    }
+
+	    DW_TAG_variable {
+		{DW_AT_name "value"}
+		{DW_AT_type :$struct}
+		{DW_AT_external 1 DW_FORM_flag}
+		{DW_AT_location {DW_OP_addr [gdb_target_symbol "our_data"]}
+		    SPECIAL_expr}
+	    }
+	}
+    }
+}
+
+if {[prepare_for_testing "failed to prepare" ${testfile} \
+	 [list $srcfile $asm_file] {nodebug}]} {
+    return -1
+}
+
+gdb_test_no_output "set language ada"
+gdb_test "print value" \
+    [string_to_regexp " = (disc => 3, nums => (7, 11, 13))"]