Add optional argument to command "maint prints c-tdesc"

Nowadays, we need two steps to print c files for xml target description,
that is, 1) read xml target description in, update the current tdesc,
2) visit the current tdesc, print the c file.  It is unnecessary to
involve in current tdesc, and some validations in each gdbarch are
performed unnecessarily, which will reject some target descriptions if
they are missing some mandatory feature.

This patch adds an optional argument to "maint print c-tdesc", which
is an XML file target description, so that we can combine the two
steps above into one step, and don't have to involve in global current
tdesc.

gdb:

2017-07-26  Yao Qi  <yao.qi@linaro.org>

	* NEWS: Mention it.
	* features/Makefile (%.c: %.xml): Pass the xml file name to
	command "maint print c-tdesc".
	* target-descriptions.c (maint_print_c_tdesc_cmd): Get file
	name from 'arg'.

gdb/doc:

2017-07-26  Yao Qi  <yao.qi@linaro.org>

	* gdb.texinfo (Maintenance Commands): Document optional
	argument of "maint print c-tdesc".

gdb/ChangeLog

@@ -1,3 +1,11 @@
+2017-07-26  Yao Qi  <yao.qi@linaro.org>
+
+	* NEWS: Mention it.
+	* features/Makefile (%.c: %.xml): Pass the xml file name to
+	command "maint print c-tdesc".
+	* target-descriptions.c (maint_print_c_tdesc_cmd): Get file
+	name from 'arg'.
+
 2017-07-26  Yao Qi  <yao.qi@linaro.org>
 
 	* target-descriptions.c (target_desc): Add ctor and dtor.  Do

gdb/NEWS

@@ -17,6 +17,9 @@
 QStartupWithShell
   Indicates whether the inferior must be started with a shell or not.
 
+* The "maintenance print c-tdesc" command now takes an optional
+  argument which is the file name of XML target description.
+
 * New commands
 
 set debug separate-debug-file

gdb/doc/ChangeLog

@@ -1,3 +1,8 @@
+2017-07-26  Yao Qi  <yao.qi@linaro.org>
+
+	* gdb.texinfo (Maintenance Commands): Document optional
+	argument of "maint print c-tdesc".
+
 2017-07-18  Yao Qi  <yao.qi@linaro.org>
 
 	* gdb.texinfo (Maintenance Commands): Improve the doc to

gdb/doc/gdb.texinfo

@@ -34687,12 +34687,16 @@ checksum.
 Print the entire architecture configuration.  The optional argument
 @var{file} names the file where the output goes.
 
-@kindex maint print c-tdesc
+@kindex maint print c-tdesc @r{[}@var{file}@r{]}
 @item maint print c-tdesc
-Print the current target description (@pxref{Target Descriptions}) as
-a C source file.  The created source file is built into @value{GDBN}
-when @value{GDBN} is built again.  This command is used by developers
-after they add or modify XML target descriptions.
+Print the target description (@pxref{Target Descriptions}) as
+a C source file.  By default, the target description is for the current
+target, but if the optional argument @var{file} is provided, that file
+is used to produce the description.  The @var{file} should be an XML
+document, of the form described in @ref{Target Description Format}.
+The created source file is built into @value{GDBN} when @value{GDBN} is
+built again.  This command is used by developers after they add or
+modify XML target descriptions.
 
 @kindex maint print dummy-frames
 @item maint print dummy-frames

gdb/features/Makefile

@@ -254,8 +254,7 @@ $(outdir)/%.dat: %.xml number-regs.xsl sort-regs.xsl gdbserver-regs.xsl
 
 cfiles: $(CFILES)
 %.c: %.xml
-	$(GDB) -nx -q -batch \
-	  -ex "set tdesc filename $<" -ex 'maint print c-tdesc' > $@.tmp
+	$(GDB) -nx -q -batch -ex 'maint print c-tdesc $<' > $@.tmp
 	sh ../../move-if-change $@.tmp $@
 
 # Other dependencies.

gdb/target-descriptions.c

@@ -1729,18 +1729,29 @@ maint_print_c_tdesc_cmd (char *args, int from_tty)
   int ix, ix2, ix3;
   int printed_field_type = 0;
 
-  /* 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;
+  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;
+      filename = target_description_filename;
+    }
+  else
+    {
+      /* Use the target description from the XML file.  */
+      filename = args;
+      tdesc = file_read_description_xml (filename);
+    }
+
   if (tdesc == NULL)
     error (_("There is no target description to print."));
 
-  if (target_description_filename == NULL)
+  if (filename == NULL)
     error (_("The current target description did not come from an XML file."));
 
-  filename = lbasename (target_description_filename);
+  filename = lbasename (filename);
   function = (char *) alloca (strlen (filename) + 1);
   for (inp = filename, outp = function; *inp != '\0'; inp++)
     if (*inp == '.')