Support DWARF tags for C++11 variadic templates

This patch adds support for DW_TAG_GNU_formal_parameter_pack and
DW_TAG_GNU_template_parameter_pack, added to DWARF in March 2009 for
C++11 variadic templates[1].

They are not currently emitted thanks to a typo[2] but the fix is
trivial[3] and has been repeatedly submitted[4] to gcc; I'm not sure
what else I can do to get it accepted; regardless, anyone building their
own compiler can still make use of this.

This implementation synthesizes type and parameter names as T#n, p#n
e.g. Args#1, args#1. This is a pretty simple approach but it seems to
work OK and is compatible with the old style type and parameter names
emitted by old versions of gcc and when it's writing stabs+ format,
meaning that any debugger scripts will continue to work.

1. http://wiki.dwarfstd.org/index.php?title=C%2B%2B0x:_Variadic_templates
2. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70536
3. ecatmur/gcc#5
4. https://gcc.gnu.org/pipermail/gcc-patches/2023-January/609659.html
PR. https://sourceware.org/bugzilla/show_bug.cgi?id=17272

Author: ecatmur

gdb/dwarf2/read.c

@@ -12103,7 +12103,8 @@ read_func_scope (struct die_info *die, struct dwarf2_cu *cu)
   for (child_die = die->child; child_die; child_die = child_die->sibling)
     {
       if (child_die->tag == DW_TAG_template_type_param
-	  || child_die->tag == DW_TAG_template_value_param)
+	  || child_die->tag == DW_TAG_template_value_param
+	  || child_die->tag == DW_TAG_GNU_template_parameter_pack)
 	{
 	  templ_func = new (&objfile->objfile_obstack) template_symbol;
 	  templ_func->subclass = SYMBOL_TEMPLATE;
@@ -12152,6 +12153,23 @@ read_func_scope (struct die_info *die, struct dwarf2_cu *cu)
 	      if (arg != NULL)
 		template_args.push_back (arg);
 	    }
+	  else if (child_die->tag == DW_TAG_GNU_template_parameter_pack)
+	    {
+	      struct die_info *pack_die;
+	      for (pack_die = child_die->child; pack_die; pack_die = pack_die->sibling)
+		{
+		  struct symbol *arg = new_symbol (pack_die, NULL, cu);
+
+		  if (arg != NULL)
+		    template_args.push_back (arg);
+		}
+	    }
+	  else if (child_die->tag == DW_TAG_GNU_formal_parameter_pack)
+	    {
+	      struct die_info *pack_die;
+	      for (pack_die = child_die->child; pack_die; pack_die = pack_die->sibling)
+		process_die (pack_die, cu);
+	    }
 	  else
 	    process_die (child_die, cu);
 	  child_die = child_die->sibling;
@@ -16639,6 +16657,11 @@ read_subroutine_type (struct die_info *die, struct dwarf2_cu *cu)
 	{
 	  if (child_die->tag == DW_TAG_formal_parameter)
 	    nparams++;
+	  else if (child_die->tag == DW_TAG_GNU_formal_parameter_pack)
+	    {
+	      child_die = child_die->child;
+	      continue;
+	    }
 	  else if (child_die->tag == DW_TAG_unspecified_parameters)
 	    ftype->set_has_varargs (true);
 
@@ -16714,6 +16737,11 @@ read_subroutine_type (struct die_info *die, struct dwarf2_cu *cu)
 	      ftype->field (iparams).set_type (arg_type);
 	      iparams++;
 	    }
+	  else if (child_die->tag == DW_TAG_GNU_formal_parameter_pack)
+	    {
+	      child_die = child_die->child;
+	      continue;
+	    }
 	  child_die = child_die->sibling;
 	}
     }
@@ -20847,13 +20875,16 @@ new_symbol (struct die_info *die, struct type *type, struct dwarf2_cu *cu,
 	sym->set_type (type);
       else
 	sym->set_type (die_type (die, cu));
-      attr = dwarf2_attr (die,
+      struct die_info *line_file_die = die;
+      if (die->tag == DW_TAG_formal_parameter && die->parent && die->parent->tag == DW_TAG_GNU_formal_parameter_pack)
+	line_file_die = die->parent;
+      attr = dwarf2_attr (line_file_die,
 			  inlined_func ? DW_AT_call_line : DW_AT_decl_line,
 			  cu);
       if (attr != nullptr)
 	sym->set_line (attr->constant_value (0));
 
-      attr = dwarf2_attr (die,
+      attr = dwarf2_attr (line_file_die,
 			  inlined_func ? DW_AT_call_file : DW_AT_decl_file,
 			  cu);
       if (attr != nullptr && attr->is_nonnegative ())
@@ -21073,6 +21104,8 @@ new_symbol (struct die_info *die, struct type *type, struct dwarf2_cu *cu,
 	    list_to_add = cu->list_in_scope;
 	  }
 	  break;
+	case DW_TAG_GNU_formal_parameter_pack:
+	  break;
 	case DW_TAG_unspecified_parameters:
 	  /* From varargs functions; gdb doesn't seem to have any
 	     interest in this information, so just ignore it for now.
@@ -22081,8 +22114,11 @@ dwarf2_name (struct die_info *die, struct dwarf2_cu *cu)
   if (attr_name == nullptr
       && die->tag != DW_TAG_namespace
       && die->tag != DW_TAG_class_type
+      && die->tag != DW_TAG_formal_parameter
       && die->tag != DW_TAG_interface_type
       && die->tag != DW_TAG_structure_type
+      && die->tag != DW_TAG_template_type_param
+      && die->tag != DW_TAG_template_value_param
       && die->tag != DW_TAG_namelist
       && die->tag != DW_TAG_union_type
       && die->tag != DW_TAG_template_type_param
@@ -22111,9 +22147,36 @@ dwarf2_name (struct die_info *die, struct dwarf2_cu *cu)
 	return attr_name;
       return CP_ANONYMOUS_NAMESPACE_STR;
 
-    /* DWARF does not actually require template tags to have a name.  */
+    case DW_TAG_formal_parameter:
     case DW_TAG_template_type_param:
     case DW_TAG_template_value_param:
+      if (!attr
+	  && die->parent
+	  && (die->parent->tag == DW_TAG_GNU_formal_parameter_pack
+	      || die->parent->tag == DW_TAG_GNU_template_parameter_pack))
+	{
+	  const char *parent_name;
+	  int ordinal = 0;
+	  struct die_info *child_die;
+	  size_t size;
+	  char *name;
+	  parent_name = dwarf2_name(die->parent, cu);
+	  if (!parent_name)
+	    return NULL;
+	  for (child_die = die->parent->child; child_die != die; child_die = child_die->sibling)
+	    ++ordinal;
+	  size = snprintf(NULL, 0, "%s#%d", parent_name, ordinal) + 1;
+	  name = ((char *) obstack_alloc (&cu->per_objfile->per_bfd->obstack, size));
+	  snprintf(name, size, "%s#%d", parent_name, ordinal);
+	  return name;
+	}
+      if (die->tag == DW_TAG_formal_parameter)
+	{
+	  if (!attr || attr_name == NULL)
+	    return NULL;
+	  break;
+	}
+      /* DWARF does not actually require template tags to have a name.  */
       if (attr_name == nullptr)
 	return unnamed_template_tag_name (die, cu);
       /* FALLTHROUGH.  */