Rationalize "backtrace" command line parsing

The backtrace command has peculiar command-line parsing.  In
particular, it splits the command line, then loops over the arguments.
If it sees a word it recognizes, like "full", it effectively drops
this word from the argument vector.  Then, it pastes together the
remaining arguments, passing them on to backtrace_command_1, which in
turn passes the resulting string to parse_and_eval_long.

The documentation doesn't mention the parse_and_eval_long at all, so
it is a bit of a hidden feature that you can "bt 3*2".  The strange
algorithm above also means you can "bt 3 * no-filters 2" and get 6
frames...

This patch changes backtrace's command line parsing to be a bit more
rational.  Now, special words like "full" are only recognized at the
start of the command.

This also updates the documentation to describe the various bt options
individually.

gdb/ChangeLog
2018-03-26  Tom Tromey  <tom@tromey.com>

	* stack.c (backtrace_command): Rewrite command line parsing.

gdb/doc/ChangeLog
2018-03-26  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Backtrace): Describe options individually.

4 files changed, 57 insertions, 73 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 47e6cff..532826c 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2018-03-26  Tom Tromey  <tom@tromey.com>
+
+	* stack.c (backtrace_command): Rewrite command line parsing.
+
 2018-03-26  Simon Marchi  <simon.marchi@ericsson.com>
 
 	* dwarf2read.c (DEF_VEC_I(offset_type)): Remove.
diff --git a/gdb/doc/ChangeLog b/gdb/doc/ChangeLog
index 2441f15..413e456 100644
--- a/gdb/doc/ChangeLog
+++ b/gdb/doc/ChangeLog
@@ -1,3 +1,7 @@
+2018-03-26  Tom Tromey  <tom@tromey.com>
+
+	* gdb.texinfo (Backtrace): Describe options individually.
+
 2018-03-19  Tom Tromey  <tom@tromey.com>
 
 	* observer.texi: Remove.
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 74e0fdb..28254c9 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -7307,45 +7307,43 @@ frame (frame zero), followed by its caller (frame one), and on up the
 stack.
 
 @anchor{backtrace-command}
-@table @code
 @kindex backtrace
 @kindex bt @r{(@code{backtrace})}
-@item backtrace
-@itemx bt
-Print a backtrace of the entire stack: one line per frame for all
-frames in the stack.
-
-You can stop the backtrace at any time by typing the system interrupt
-character, normally @kbd{Ctrl-c}.
-
-@item backtrace @var{n}
-@itemx bt @var{n}
-Similar, but print only the innermost @var{n} frames.
-
-@item backtrace -@var{n}
-@itemx bt -@var{n}
-Similar, but print only the outermost @var{n} frames.
-
-@item backtrace full
-@itemx bt full
-@itemx bt full @var{n}
-@itemx bt full -@var{n}
-Print the values of the local variables also.  As described above,
-@var{n} specifies the number of frames to print.
-
-@item backtrace no-filters
-@itemx bt no-filters
-@itemx bt no-filters @var{n}
-@itemx bt no-filters -@var{n}
-@itemx bt no-filters full
-@itemx bt no-filters full @var{n}
-@itemx bt no-filters full -@var{n}
+To print a backtrace of the entire stack, use the @code{backtrace}
+command, or its alias @code{bt}.  This command will print one line per
+frame for frames in the stack.  By default, all stack frames are
+printed.  You can stop the backtrace at any time by typing the system
+interrupt character, normally @kbd{Ctrl-c}.
+
+@table @code
+@item backtrace [@var{args}@dots{}]
+@itemx bt [@var{args}@dots{}]
+Print the backtrace of the entire stack.  The optional @var{args} can
+be one of the following:
+
+@table @code
+@item @var{n}
+@itemx @var{n}
+Print only the innermost @var{n} frames, where @var{n} is a positive
+number.
+
+@item -@var{n}
+@itemx -@var{n}
+Print only the outermost @var{n} frames, where @var{n} is a positive
+number.
+
+@item full
+Print the values of the local variables also.  This can be combined
+with a number to limit the number of frames shown.
+
+@item no-filters
 Do not run Python frame filters on this backtrace.  @xref{Frame
 Filter API}, for more information.  Additionally use @ref{disable
 frame-filter all} to turn off all frame filters.  This is only
 relevant when @value{GDBN} has been configured with @code{Python}
 support.
 @end table
+@end table
 
 @kindex where
 @kindex info stack
diff --git a/gdb/stack.c b/gdb/stack.c
index aad8fcd..13af659 100644
--- a/gdb/stack.c
+++ b/gdb/stack.c
@@ -1850,61 +1850,39 @@ backtrace_command_1 (const char *count_exp, int show_locals, int no_filters,
 static void
 backtrace_command (const char *arg, int from_tty)
 {
-  int fulltrace_arg = -1, arglen = 0, argc = 0, no_filters  = -1;
-  int user_arg = 0;
+  bool fulltrace = false;
+  bool filters = true;
 
-  std::string reconstructed_arg;
   if (arg)
     {
-      char **argv;
-      int i;
+      bool done = false;
 
-      gdb_argv built_argv (arg);
-      argv = built_argv.get ();
-      argc = 0;
-      for (i = 0; argv[i]; i++)
+      while (!done)
 	{
-	  unsigned int j;
+	  const char *save_arg = arg;
+	  std::string this_arg = extract_arg (&arg);
 
-	  for (j = 0; j < strlen (argv[i]); j++)
-	    argv[i][j] = TOLOWER (argv[i][j]);
+	  if (this_arg.empty ())
+	    break;
 
-	  if (no_filters < 0 && subset_compare (argv[i], "no-filters"))
-	    no_filters = argc;
+	  if (subset_compare (this_arg.c_str (), "no-filters"))
+	    filters = false;
+	  else if (subset_compare (this_arg.c_str (), "full"))
+	    fulltrace = true;
 	  else
 	    {
-	      if (fulltrace_arg < 0 && subset_compare (argv[i], "full"))
-		fulltrace_arg = argc;
-	      else
-		{
-		  user_arg++;
-		  arglen += strlen (argv[i]);
-		}
-	    }
-	  argc++;
-	}
-      arglen += user_arg;
-      if (fulltrace_arg >= 0 || no_filters >= 0)
-	{
-	  if (arglen > 0)
-	    {
-	      for (i = 0; i < argc; i++)
-		{
-		  if (i != fulltrace_arg && i != no_filters)
-		    {
-		      reconstructed_arg += argv[i];
-		      reconstructed_arg += " ";
-		    }
-		}
-	      arg = reconstructed_arg.c_str ();
+	      /* Not a recognized argument, so stop.  */
+	      arg = save_arg;
+	      done = true;
 	    }
-	  else
-	    arg = NULL;
 	}
+
+      if (*arg == '\0')
+	arg = NULL;
     }
 
-  backtrace_command_1 (arg, fulltrace_arg >= 0 /* show_locals */,
-		       no_filters >= 0 /* no frame-filters */, from_tty);
+  backtrace_command_1 (arg, fulltrace /* show_locals */,
+		       !filters /* no frame-filters */, from_tty);
 }
 
 /* Iterate over the local variables of a block B, calling CB with