Add $_as_string convenience function

This patch is a follow-up to "Add printf format specifier for printing
enumerator":

  https://sourceware.org/ml/gdb-patches/2016-02/msg00144.html

Instead of having a solution specific to the printf command, Pedro
suggested adding a general purpose function $_as_string() that would
cover this use case and more.

So, in order to print the textual label of an enum, one can use:

  (gdb) printf "Visiting node of type %s\n", $_as_string(node)
  Visiting node of type NODE_INTEGER

gdb/ChangeLog:

	* data-directory/Makefile.in (PYTHON_FILE_LIST): Install
	gdb/function/as_string.py.
	* python/lib/gdb/function/as_string.py: New file.
	* NEWS: Mention the new $_as_string function.

gdb/testsuite/ChangeLog:

	* gdb.python/py-as-string.exp: New file.
	* gdb.python/py-as-string.c: New file.

gdb/doc/ChangeLog:

	* gdb.texinfo (Convenience Functions): Document $_as_string.

gdb/ChangeLog

@@ -1,3 +1,10 @@
+2016-03-10  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	* data-directory/Makefile.in (PYTHON_FILE_LIST): Install
+	gdb/function/as_string.py.
+	* python/lib/gdb/function/as_string.py: New file.
+	* NEWS: Mention the new $_as_string function.
+
 2016-03-09  Jose E. Marchesi  <jose.marchesi@oracle.com>
 
 	* target.h: Fix doc string of target_can_use_hardware_watchpoint.

gdb/NEWS

@@ -3,6 +3,10 @@
 
 *** Changes since GDB 7.11
 
+* New Python-based convenience function $_as_string(val), which returns
+  the textual representation of a value.  This function is especially
+  useful to obtain the text label of an enum value.
+
 * Intel MPX bound violation handling.
 
    Segmentation faults caused by a Intel MPX boundary violation

gdb/data-directory/Makefile.in

@@ -74,6 +74,7 @@ PYTHON_FILE_LIST = \
 	gdb/command/prompt.py \
 	gdb/command/explore.py \
 	gdb/function/__init__.py \
+	gdb/function/as_string.py \
 	gdb/function/caller_is.py \
 	gdb/function/strfns.py \
 	gdb/printer/__init__.py \

gdb/doc/ChangeLog

@@ -1,3 +1,7 @@
+2016-03-10  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	* gdb.texinfo (Convenience Functions): Document $_as_string.
+
 2016-02-23  Doug Evans  <dje@google.com>
 
 	* gdb.texinfo (Skipping Over Functions and Files): Document new

gdb/doc/gdb.texinfo

@@ -10760,6 +10760,19 @@ checks all stack frames from the immediate caller to the frame specified
 by @var{number_of_frames}, whereas @code{$_caller_matches} only checks the
 frame specified by @var{number_of_frames}.
 
+@item $_as_string(@var{value})
+@findex $_as_string@r{, convenience function}
+Return the string representation of @var{value}.
+
+This function is useful to obtain the textual label (enumerator) of an
+enumeration value.  For example, assuming the variable @var{node} is of
+an enumerated type:
+
+@smallexample
+(gdb) printf "Visiting node of type %s\n", $_as_string(node)
+Visiting node of type NODE_INTEGER
+@end smallexample
+
 @end table
 
 @value{GDBN} provides the ability to list and get help on

gdb/python/lib/gdb/function/as_string.py

@@ -0,0 +1,39 @@
+# Copyright (C) 2016 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/>.
+
+import gdb
+
+
+class _AsString(gdb.Function):
+    """Return the string representation of a value.
+
+Usage:
+  $_as_string(value)
+
+Arguments:
+
+  value: A gdb.Value.
+
+Returns:
+  The string representation of the value.
+"""
+
+    def __init__(self):
+        super(_AsString, self).__init__("_as_string")
+
+    def invoke(self, val):
+        return str(val)
+
+_AsString()

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2016-03-10  Simon Marchi  <simon.marchi@polymtl.ca>
+
+	* gdb.python/py-as-string.exp: New file.
+	* gdb.python/py-as-string.c: New file.
+
 2016-03-09  Pedro Alves  <palves@redhat.com>
 
 	* gdb.threads/attach-into-signal.exp: Adjust to "Program received

gdb/testsuite/gdb.python/py-as-string.c

@@ -0,0 +1,32 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2016 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/>.  */
+
+enum EnumType {
+  ENUM_VALUE_A,
+  ENUM_VALUE_B,
+  ENUM_VALUE_C,
+  ENUM_VALUE_D,
+};
+
+static enum EnumType enum_valid = ENUM_VALUE_B;
+static enum EnumType enum_invalid = 20;
+
+int
+main ()
+{
+  return 0;
+}

gdb/testsuite/gdb.python/py-as-string.exp

@@ -0,0 +1,40 @@
+# Copyright (C) 2016 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/>.
+
+# This file is part of the GDB testsuite.  It tests the convenience
+# functions in as_string.py.
+
+load_lib gdb-python.exp
+
+standard_testfile
+
+if { [prepare_for_testing ${testfile}.exp ${testfile} ${srcfile}] } {
+    return -1
+}
+
+if ![runto_main] {
+    fail "Couldn't run to main."
+    return 0
+}
+
+if { [skip_python_tests] } { continue }
+
+proc test_as_string { } {
+    gdb_test "p \$_as_string(2)" "\"2\""
+    gdb_test "p \$_as_string(enum_valid)" "\"ENUM_VALUE_B\""
+    gdb_test "p \$_as_string(enum_invalid)" "\"20\""
+}
+
+test_as_string