[PATCH 2/2] gdb: New maintenance command to print XML target description

[Andrew Burgess <andrew.burgess@embecosm.com>]

This commit adds a new maintenance command that dumps the current
target description as an XML document.  This is a maintenance command
as I currently only see this being useful for GDB developers, or for
people debugging a new remote target.

By default the command will print whatever the current target
description is, whether this was delivered by the remote, loaded by
the user from a file, or if it is a built in target within GDB.

The command can also take an optional filename argument.  In this case
GDB loads a target description from the file, and then reprints it.
This could be useful for testing GDB's parsing of target descriptions,
or to check that GDB can successfully parse a particular XML
description.

It is worth noting that the XML description printed will not be an
exact copy of the document fed into GDB.  For example this minimal
input file:

  <target>
    <feature name="abc">
      <reg name="r1" bitsize="32"/>
    </feature>
  </target>

Will produce this output:

  (gdb) maint print xml-tdesc path/to/file.xml
  <?xml version="1.0"?>
  <!DOCTYPE target SYSTEM "gdb-target.dtd">
  <target>
    <feature name="abc">
      <reg name="r1" bitsize="32" type="int" regnum="0"/>
    </feature>
  </target>

Notice that GDB filled in both the 'type' and 'regnum' fields of the
<reg>.  I think this is actually a positive as it means we get to
really understand how GDB processed the document, if GDB made some
assumptions that differ to those the user expected then hopefully this
will bring those issues to the users attention.

As for implementation I initially started writing a new tdesc visitor
class that produced XML, but after remembering to check gdbsupport/ I
found such a class already existed.

However, that class produced XML which seems designed for a machine to
read, with only minimal indentation.  With a bit of tweaking I was
able to restructure things so I could inherit from the existing XML
producer and add indentation.

During this tweaking I did make two changes to the original XML
producing class, these are:

  1. The <architecture>...</architecture> tags are now not produced if
  the architecture name is NULL.

  2. The <osabi>...</osabi> tags get a newline at the end.

gdbsupport/ChangeLog:

	* tdesc.cc (print_xml_feature::visit_pre): Use add_line to add
	output content, and call indent as needed in all overloaded
	variants.
	(print_xml_feature::visit_post): Likewise.
	(print_xml_feature::visit): Likewise.
	(print_xml_feature::add_line): Two new overloaded functions.
	* tdesc.h (print_xml_feature::indent): Declare new member
	function.
	(print_xml_feature::add_line): Two new overloaded member
	functions.

gdb/ChangeLog:

	* target-descriptions.c (tdesc_architecture_name): Protect against
	NULL pointer dereference.
	(class gdb_print_xml_feature): New class.
	(maint_print_xml_tdesc_cmd): New function.
	(_initialize_target_descriptions): Register new 'maint print
	xml-tdesc' command and give it the filename completer.
	* NEWS: Mention new 'maint print xml-tdesc' command.

gdb/testsuite/ChangeLog:

	* gdb.xml/maint-xml-dump-01.xml: New file.
	* gdb.xml/maint-xml-dump-02.xml: New file.
	* gdb.xml/maint-xml-dump.exp: New file.

gdb/doc/ChangeLog:

	* gdb.texinfo (Maintenance Commands): Document new 'maint print
	xml-desc' command.
---
 gdb/ChangeLog                               |  10 ++
 gdb/NEWS                                    |   6 +
 gdb/doc/ChangeLog                           |   5 +
 gdb/doc/gdb.texinfo                         |   9 ++
 gdb/target-descriptions.c                   |  90 +++++++++++++-
 gdb/testsuite/ChangeLog                     |   6 +
 gdb/testsuite/gdb.xml/maint-xml-dump-01.xml |  10 ++
 gdb/testsuite/gdb.xml/maint-xml-dump-02.xml |  27 +++++
 gdb/testsuite/gdb.xml/maint-xml-dump.exp    | 124 ++++++++++++++++++++
 gdbsupport/ChangeLog                        |  13 ++
 gdbsupport/tdesc.cc                         |  99 +++++++++++-----
 gdbsupport/tdesc.h                          |  15 +++
 12 files changed, 383 insertions(+), 31 deletions(-)
 create mode 100644 gdb/testsuite/gdb.xml/maint-xml-dump-01.xml
 create mode 100644 gdb/testsuite/gdb.xml/maint-xml-dump-02.xml
 create mode 100644 gdb/testsuite/gdb.xml/maint-xml-dump.exp

diff --git a/gdb/NEWS b/gdb/NEWS
index 9ef85ab3ca7..491209b2772 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -66,6 +66,12 @@ tui new-layout NAME WINDOW WEIGHT [WINDOW WEIGHT]...
   Define a new TUI layout, specifying its name and the windows that
   will be displayed.
 
+maintenance print xml-tdesc [FILE]
+  Prints the current target description as an XML document.  If the
+  optional FILE is provided (which is an XML target description) then
+  the target description is read from FILE into GDB, and then
+  reprinted.
+
 * New targets
 
 GNU/Linux/RISC-V (gdbserver)	riscv*-*-linux*
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 64181628495..add14856f45 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -38397,6 +38397,15 @@
 built again.  This command is used by developers after they add or
 modify XML target descriptions.
 
+@kindex maint print xml-tdesc @r{[}@var{file}@r{]}
+@item maint print xml-tdesc
+Print the target description (@pxref{Target Descriptions}) as an XML
+file.  By default print the target description for the current target,
+but if the optional argument @var{file} is provided, then that file is
+read in by GDB and then used to produce the description.  The
+@var{file} should be an XML document, of the form described in
+@ref{Target Description Format}.
+
 @kindex maint check xml-descriptions
 @item maint check xml-descriptions @var{dir}
 Check that the target descriptions dynamically created by @value{GDBN}
diff --git a/gdb/target-descriptions.c b/gdb/target-descriptions.c
index 55ea416d69a..f86ffdeb518 100644
--- a/gdb/target-descriptions.c
+++ b/gdb/target-descriptions.c
@@ -639,7 +639,9 @@ tdesc_architecture (const struct target_desc *target_desc)
 const char *
 tdesc_architecture_name (const struct target_desc *target_desc)
 {
-  return target_desc->arch->printable_name;
+  if (target_desc->arch != NULL)
+    return target_desc->arch->printable_name;
+  return NULL;
 }
 
 /* Return the OSABI associated with this target description, or
@@ -1717,6 +1719,87 @@ maint_print_c_tdesc_cmd (const char *args, int from_tty)
     }
 }
 
+/* Class to format the target description XML with nesting.  */
+
+class gdb_print_xml_feature : public print_xml_feature
+{
+public:
+  /* Constructor.  */
+
+  gdb_print_xml_feature (std::string *arg)
+    :print_xml_feature (arg)
+  { /* Nothing.  */ }
+
+  /* Override this from the parent so we can add the compatibility
+     information.   */
+
+  void visit_pre (const target_desc *e) override
+  {
+    print_xml_feature::visit_pre (e);
+    for (const bfd_arch_info_type *compatible : e->compatible)
+      add_line ("<compatible>%s</compatible>", compatible->printable_name);
+  };
+
+protected:
+
+  /* Pull in all copies of add_line.  */
+
+  using print_xml_feature::add_line;
+
+  /* Override this version of add_line to add white space for indentation
+     at the start of the line.  */
+
+  void add_line (const std::string &str) override
+  {
+    std::string tmp = string_printf ("%*s", m_depth, "");
+    tmp += str;
+    print_xml_feature::add_line (tmp);
+  }
+
+  /* Increase our nesting by ADJUST.  We use two spaces for every level of
+     indentation.  */
+
+  void indent (int adjust) override
+  {
+    m_depth += (adjust * 2);
+  }
+
+private:
+
+  /* The current level of indentation.  */
+  int m_depth = 0;
+};
+
+/* Implement the maintenance print xml-tdesc command.  */
+
+static void
+maint_print_xml_tdesc_cmd (const char *args, int from_tty)
+{
+  const struct target_desc *tdesc;
+
+  if (args == NULL)
+    {
+      /* Use the global target-supplied description, not the current
+	 architecture's.  This lets a GDB for one architecture generate C
+	 for another architecture's description, even though the gdbarch
+	 initialization code will reject the new description.  */
+      tdesc = current_target_desc;
+    }
+  else
+    {
+      /* Use the target description from the XML file.  */
+      tdesc = file_read_description_xml (args);
+    }
+
+  if (tdesc == NULL)
+    error (_("There is no target description to print."));
+
+  std::string buf;
+  gdb_print_xml_feature v (&buf);
+  tdesc->accept (v);
+  puts_unfiltered (buf.c_str ());
+}
+
 namespace selftests {
 
 /* A reference target description, used for testing (see record_xml_tdesc).  */
@@ -1854,6 +1937,11 @@ Print the current target description as a C source file."),
 	   &maintenanceprintlist);
   set_cmd_completer (cmd, filename_completer);
 
+  cmd = add_cmd ("xml-tdesc", class_maintenance, maint_print_xml_tdesc_cmd, _("\
+Print the current target description as an XML file."),
+		 &maintenanceprintlist);
+  set_cmd_completer (cmd, filename_completer);
+
   cmd = add_cmd ("xml-descriptions", class_maintenance,
 		 maintenance_check_xml_descriptions, _("\
 Check equality of GDB target descriptions and XML created descriptions.\n\
diff --git a/gdb/testsuite/gdb.xml/maint-xml-dump-01.xml b/gdb/testsuite/gdb.xml/maint-xml-dump-01.xml
new file mode 100644
index 00000000000..dd4f9251676
--- /dev/null
+++ b/gdb/testsuite/gdb.xml/maint-xml-dump-01.xml
@@ -0,0 +1,10 @@
+<?xml version="1.0"?>
+<!-- This is a comment before DOCTYPE -->
+<!DOCTYPE target SYSTEM "gdb-target.dtd">
+<!-- This is a comment after DOCTYPE -->
+<target>
+  <feature name="abc">
+    <!-- The following is a register. -->
+    <reg name="r1" bitsize="32"/>	<!-- <reg name="r1" bitsize="32" type="int" regnum="0"/> -->
+  </feature>
+</target>
diff --git a/gdb/testsuite/gdb.xml/maint-xml-dump-02.xml b/gdb/testsuite/gdb.xml/maint-xml-dump-02.xml
new file mode 100644
index 00000000000..c192294ca10
--- /dev/null
+++ b/gdb/testsuite/gdb.xml/maint-xml-dump-02.xml
@@ -0,0 +1,27 @@
+<target>
+  <osabi>Solaris</osabi>
+  <feature name="abc">
+    <vector id="foo" type="int32" count="4"/>
+    <reg name="foo" bitsize="16" />	<!-- <reg name="foo" bitsize="16" type="int" regnum="0"/> -->
+  </feature>
+  <feature name="def.xyz">
+    <struct id="my_struct">
+      <field name="field1" type="int8"/>
+      <field name="field2" type="int16"/>
+      <field name="field3" type="int8"/>
+    </struct>
+    <struct id="bit_field" size="8">
+      <field name="bits1" start="0" end="3" type="int8"/>
+      <field name="bits2" start="4" end="6" type="int8"/>
+      <field name="bits3" start="7" end="7"/>	<!-- <field name="bits3" start="7" end="7" type="bool"/> -->
+    </struct>
+    <flags id="my_flags" size="8">
+      <field name="flg1" start="0" end="0"/>	<!-- <field name="flg1" start="0" end="0" type="bool"/> -->
+      <field name="flg2" start="1" end="1"/>	<!-- <field name="flg2" start="1" end="1" type="bool"/> -->
+      <field name="flg3" start="2" end="6"/>	<!-- <field name="flg3" start="2" end="6" type="uint64"/> -->
+      <field name="flg4" start="7" end="7"/>	<!-- <field name="flg4" start="7" end="7" type="bool"/> -->
+    </flags>
+    <reg name="r1" bitsize="8" type="my_flags"/>	<!-- <reg name="r1" bitsize="8" type="my_flags" regnum="1"/> -->
+    <reg name="r2" bitsize="8" type="bit_field"/>	<!-- <reg name="r2" bitsize="8" type="bit_field" regnum="2"/> -->
+  </feature>
+</target>
diff --git a/gdb/testsuite/gdb.xml/maint-xml-dump.exp b/gdb/testsuite/gdb.xml/maint-xml-dump.exp
new file mode 100644
index 00000000000..163996450eb
--- /dev/null
+++ b/gdb/testsuite/gdb.xml/maint-xml-dump.exp
@@ -0,0 +1,124 @@
+# Copyright 2020 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 the 'maint print xml-tdesc' command.  This file picks up every
+# XML file matching the pattern maint-xml-dump-*.xml (in the same
+# directory as this script) and passes each in turn to the command
+# 'maint print xml-tdesc'.
+#
+# The expected output is generated by parsing the input XML file.  The
+# rules for changing an XML file into the expected output are:
+#
+# 1. Blank lines, and lines starting with a comment are stripped from
+#    the expected output.
+#
+# 2. The <?xml ... ?> and <!DOCTYPE ...> entities are optional,
+#    suitable defaults will be added if these lines are missing from
+#    the input file.
+#
+# 3. A trailing comment on a line will replace the expected output for
+#    that line but with the indentation of the line preserved.  So
+#    this (The '|' marks the start of the line):
+#    |    <reg name="r1" bitsize="32"/>  <!-- <reg name="r1" bitsize="32" type="int" regnum="0"/> -->
+#    Will actually look for the following output:
+#    |    <reg name="r1" bitsize="32" type="int" regnum="0"/>
+#
+# 4. Indentation of lines will be preserved so your input file needs
+#    to follow the expected indentation.
+if {[gdb_skip_xml_test]} {
+    unsupported "maint-xml-dump.exp"
+    return -1
+}
+
+gdb_start
+
+# Read the XML file FILENAME and produce an output pattern that should
+# match what GDB produces with the 'maint print xml-desc' command.
+proc build_pattern { filename } {
+    set pattern {}
+
+    set xml_version_line {<?xml version="1.0"?>}
+    set doc_type_line {<!DOCTYPE target SYSTEM "gdb-target.dtd">}
+
+    set linenum 0
+    set ifd [open "$filename" r]
+    while {[gets $ifd line] >= 0} {
+	incr linenum
+
+	# The <?xml .... ?> tag can only appear as the first line in
+	# the file.  If it is not present then add one to the expected
+	# output now.
+	if {$linenum == 1} {
+	    if {![regexp {^<\?xml} $line]} {
+		set pattern [string_to_regexp $xml_version_line]
+		set xml_version_line ""
+	    }
+	}
+
+	# If we have not yet seen a DOCTYPE line, then maybe we should
+	# be adding one?  If we find <target> then add a default
+	# DOCTYPE line, otherwise, if the XML file includes a DOCTYPE
+	# line, use that.
+	if {$doc_type_line != "" } {
+	    if {[regexp {^[ \t]*<target>} $line]} {
+		set pattern [multi_line $pattern \
+				 [string_to_regexp $doc_type_line]]
+		set doc_type_line ""
+	    } elseif {[regexp {^[ \t]*<!DOCTYPE } $line]} {
+		set doc_type_line ""
+	    }
+	}
+
+	if {[regexp {^[ \t]*<!--} $line]} {
+	    # Comment line, ignore it.
+	} elseif {[regexp {^[ \t]+$} $line]} {
+	    # Blank line, ignore it.
+	} elseif {[regexp {^([ \t]*).*<!-- (.*) -->$} $line \
+		       matches grp1 grp2]} {
+	    set pattern [multi_line \
+			     $pattern \
+			     [string_to_regexp "$grp1$grp2"]]
+	} else {
+	    set pattern [multi_line \
+			     $pattern \
+			     [string_to_regexp $line]]
+	}
+    }
+    close $ifd
+
+    # Due handling the <?xml ...?> tags we can end up with a stray
+    # '\r\n' at the start of the output pattern.  Remove it here.
+    if {[string range $pattern 0 1] == "\r\n"} {
+	set pattern [string range $pattern 2 end]
+    }
+
+    return $pattern
+}
+
+# Run over every test XML file and check the output.
+foreach filename [lsort [glob $srcdir/$subdir/maint-xml-dump-*.xml]] {
+    set pattern [build_pattern $filename]
+
+    if {[is_remote host]} {
+	set test_path [remote_download host $filename]
+    } else {
+	set test_path $filename
+    }
+
+    verbose -log "Looking for:\n$pattern"
+
+    gdb_test "maint print xml-tdesc $test_path" \
+	"$pattern" "check [file tail $filename]"
+}
diff --git a/gdbsupport/tdesc.cc b/gdbsupport/tdesc.cc
index aaea8e0d8a8..cc11d3013c3 100644
--- a/gdbsupport/tdesc.cc
+++ b/gdbsupport/tdesc.cc
@@ -294,12 +294,14 @@ tdesc_add_enum_value (tdesc_type_with_fields *type, int value,
 
 void print_xml_feature::visit_pre (const tdesc_feature *e)
 {
-  string_appendf (*m_buffer, "<feature name=\"%s\">\n", e->name.c_str ());
+  add_line ("<feature name=\"%s\">", e->name.c_str ());
+  indent (1);
 }
 
 void print_xml_feature::visit_post (const tdesc_feature *e)
 {
-  string_appendf (*m_buffer, "</feature>\n");
+  indent (-1);
+  add_line ("</feature>");
 }
 
 void print_xml_feature::visit (const tdesc_type_builtin *t)
@@ -309,8 +311,8 @@ void print_xml_feature::visit (const tdesc_type_builtin *t)
 
 void print_xml_feature::visit (const tdesc_type_vector *t)
 {
-  string_appendf (*m_buffer, "<vector id=\"%s\" type=\"%s\" count=\"%d\"/>\n",
-		  t->name.c_str (), t->element_type->name.c_str (), t->count);
+  add_line ("<vector id=\"%s\" type=\"%s\" count=\"%d\"/>",
+	    t->name.c_str (), t->element_type->name.c_str (), t->count);
 }
 
 void print_xml_feature::visit (const tdesc_type_with_fields *t)
@@ -319,7 +321,9 @@ void print_xml_feature::visit (const tdesc_type_with_fields *t)
 
   gdb_assert (t->kind >= TDESC_TYPE_STRUCT && t->kind <= TDESC_TYPE_ENUM);
 
-  string_appendf (*m_buffer,
+  std::string tmp;
+
+  string_appendf (tmp,
 		  "<%s id=\"%s\"", types[t->kind - TDESC_TYPE_STRUCT],
 		  t->name.c_str ());
 
@@ -328,33 +332,37 @@ void print_xml_feature::visit (const tdesc_type_with_fields *t)
     case TDESC_TYPE_STRUCT:
     case TDESC_TYPE_FLAGS:
       if (t->size > 0)
-	string_appendf (*m_buffer, " size=\"%d\"", t->size);
-      string_appendf (*m_buffer, ">\n");
+	string_appendf (tmp, " size=\"%d\"", t->size);
+      string_appendf (tmp, ">");
+      add_line (tmp);
 
       for (const tdesc_type_field &f : t->fields)
 	{
-	  string_appendf (*m_buffer, "  <field name=\"%s\" ", f.name.c_str ());
-	  if (f.start == -1)
-	    string_appendf (*m_buffer, "type=\"%s\"/>\n",
-			    f.type->name.c_str ());
-	  else
-	    string_appendf (*m_buffer, "start=\"%d\" end=\"%d\"/>\n", f.start,
+	  tmp.clear ();
+	  string_appendf (tmp, "  <field name=\"%s\"", f.name.c_str ());
+	  if (f.start != -1)
+	    string_appendf (tmp, " start=\"%d\" end=\"%d\"", f.start,
 			    f.end);
+	  string_appendf (tmp, " type=\"%s\"/>",
+			  f.type->name.c_str ());
+	  add_line (tmp);
 	}
       break;
 
     case TDESC_TYPE_ENUM:
-      string_appendf (*m_buffer, ">\n");
+      string_appendf (tmp, ">");
+      add_line (tmp);
       for (const tdesc_type_field &f : t->fields)
-	string_appendf (*m_buffer, "  <field name=\"%s\" start=\"%d\"/>\n",
-			f.name.c_str (), f.start);
+	add_line ("  <field name=\"%s\" start=\"%d\"/>",
+		  f.name.c_str (), f.start);
       break;
 
     case TDESC_TYPE_UNION:
-      string_appendf (*m_buffer, ">\n");
+      string_appendf (tmp, ">");
+      add_line (tmp);
       for (const tdesc_type_field &f : t->fields)
-	string_appendf (*m_buffer, "  <field name=\"%s\" type=\"%s\"/>\n",
-			f.name.c_str (), f.type->name.c_str ());
+	add_line ("  <field name=\"%s\" type=\"%s\"/>",
+		  f.name.c_str (), f.type->name.c_str ());
       break;
 
     default:
@@ -362,40 +370,71 @@ void print_xml_feature::visit (const tdesc_type_with_fields *t)
 	     t->name.c_str ());
     }
 
-  string_appendf (*m_buffer, "</%s>\n", types[t->kind - TDESC_TYPE_STRUCT]);
+  add_line ("</%s>", types[t->kind - TDESC_TYPE_STRUCT]);
 }
 
 void print_xml_feature::visit (const tdesc_reg *r)
 {
-  string_appendf (*m_buffer,
+  std::string tmp;
+
+  string_appendf (tmp,
 		  "<reg name=\"%s\" bitsize=\"%d\" type=\"%s\" regnum=\"%ld\"",
 		  r->name.c_str (), r->bitsize, r->type.c_str (),
 		  r->target_regnum);
 
   if (r->group.length () > 0)
-    string_appendf (*m_buffer, " group=\"%s\"", r->group.c_str ());
+    string_appendf (tmp, " group=\"%s\"", r->group.c_str ());
 
   if (r->save_restore == 0)
-    string_appendf (*m_buffer, " save-restore=\"no\"");
+    string_appendf (tmp, " save-restore=\"no\"");
 
-  string_appendf (*m_buffer, "/>\n");
+  string_appendf (tmp, "/>");
+
+  add_line (tmp);
 }
 
 void print_xml_feature::visit_pre (const target_desc *e)
 {
 #ifndef IN_PROCESS_AGENT
-  string_appendf (*m_buffer, "<?xml version=\"1.0\"?>\n");
-  string_appendf (*m_buffer, "<!DOCTYPE target SYSTEM \"gdb-target.dtd\">\n");
-  string_appendf (*m_buffer, "<target>\n<architecture>%s</architecture>\n",
-		  tdesc_architecture_name (e));
+  add_line ("<?xml version=\"1.0\"?>");
+  add_line ("<!DOCTYPE target SYSTEM \"gdb-target.dtd\">");
+  add_line ("<target>");
+  indent (1);
+  if (tdesc_architecture_name (e))
+    add_line ("<architecture>%s</architecture>",
+	      tdesc_architecture_name (e));
 
   const char *osabi = tdesc_osabi_name (e);
   if (osabi != nullptr)
-    string_appendf (*m_buffer, "<osabi>%s</osabi>", osabi);
+    add_line ("<osabi>%s</osabi>", osabi);
 #endif
 }
 
 void print_xml_feature::visit_post (const target_desc *e)
 {
-  string_appendf (*m_buffer, "</target>\n");
+  indent (-1);
+  add_line ("</target>");
+}
+
+/* See gdbsupport/tdesc.h.  */
+
+void
+print_xml_feature::add_line (const std::string &str)
+{
+  string_appendf (*m_buffer, "%s", str.c_str ());
+  string_appendf (*m_buffer, "\n");
+}
+
+/* See gdbsupport/tdesc.h.  */
+
+void
+print_xml_feature::add_line (const char *fmt, ...)
+{
+  std::string tmp;
+
+  va_list ap;
+  va_start (ap, fmt);
+  string_vappendf (tmp, fmt, ap);
+  va_end (ap);
+  add_line (tmp);
 }
diff --git a/gdbsupport/tdesc.h b/gdbsupport/tdesc.h
index 3b1f1f57ff8..009710e736b 100644
--- a/gdbsupport/tdesc.h
+++ b/gdbsupport/tdesc.h
@@ -401,6 +401,21 @@ class print_xml_feature : public tdesc_element_visitor
   void visit (const tdesc_type_with_fields *type) override;
   void visit (const tdesc_reg *reg) override;
 
+protected:
+
+  /* Called with a positive value of ADJUST  we move inside an element, for
+     example inside <target>, and with a negative value when we leave the
+     element.  In this class this function does nothing, but a sub-class
+     can override this to track the current level of nesting.  */
+  virtual void indent (int adjust)
+  { /* Nothing.  */ }
+
+  /* Functions to add lines to the output buffer M_BUFFER.  Each of these
+     functions appends a newline, so don't include one the strings being
+     sent.  */
+  virtual void add_line (const std::string &str);
+  virtual void add_line (const char *fmt, ...);
+
 private:
   std::string *m_buffer;
 };
-- 
2.25.4