gdb: new 'maint print frame-id' command

When debugging a certain class of GDB bug, I often end up wanting to
know what GDB thinks the frame-id is in a particular frame.  It's
not too hard to pull this from some debug output, but I thought it
might be nice if there was a maintenance command that could tell us.

This commit adds 'maint print frame-id' which prints the frame-id of
the currently selected frame.  You can also pass a frame level number
to find the frame-id for a specific frame.

There's a new test too.

gdb/NEWS

@@ -93,6 +93,10 @@ show style disassembler symbol
   The 'disassembler address' and 'disassembler symbol' styles are
   aliases for the 'address' and 'function' styles respectively.
 
+maintenance print frame-id [ LEVEL ]
+  Print GDB's internal frame-id for the frame at LEVEL.  If LEVEL is
+  not given, then print the frame-id for the currently selected frame.
+
 * Changed commands
 
 maintenance info line-table

gdb/doc/gdb.texinfo

@@ -40142,6 +40142,23 @@ The program being debugged stopped while in a function called from GDB.
 
 Takes an optional file parameter.
 
+@kindex maint print frame-id
+@item maint print frame-id
+@itemx maint print frame-id @var{level}
+Print @value{GDBN}'s internal frame-id for the frame at relative
+@var{level}, or for the currently selected frame when @var{level} is
+not given.
+
+If used, @var{level} should be an integer, as displayed in the
+@command{backtrace} output.
+
+@smallexample
+(@value{GDBP}) maint print frame-id
+frame-id for frame #0: @{stack=0x7fffffffac70,code=0x0000000000401106,!special@}
+(@value{GDBP}) maint print frame-id 2
+frame-id for frame #2: @{stack=0x7fffffffac90,code=0x000000000040111c,!special@}
+@end smallexample
+
 @kindex maint print registers
 @kindex maint print raw-registers
 @kindex maint print cooked-registers

gdb/frame.c

@@ -3116,6 +3116,30 @@ the rest of the stack trace."),
   },
 };
 
+/* Implement the 'maintenance print frame-id' command.  */
+
+static void
+maintenance_print_frame_id (const char *args, int from_tty)
+{
+  struct frame_info *frame;
+
+  /* Use the currently selected frame, or select a frame based on the level
+     number passed by the user.  */
+  if (args == nullptr)
+    frame = get_selected_frame ("No frame selected");
+  else
+    {
+      int level = value_as_long (parse_and_eval (args));
+      frame = find_relative_frame (get_current_frame (), &level);
+    }
+
+  /* Print the frame-id.  */
+  gdb_assert (frame != nullptr);
+  gdb_printf ("frame-id for frame #%d: %s\n",
+	      frame_relative_level (frame),
+	      get_frame_id (frame).to_string ().c_str ());
+}
+
 void _initialize_frame ();
 void
 _initialize_frame ()
@@ -3160,4 +3184,8 @@ When non-zero, frame specific internal debugging is enabled."),
 			   NULL,
 			   show_frame_debug,
 			   &setdebuglist, &showdebuglist);
+
+  add_cmd ("frame-id", class_maintenance, maintenance_print_frame_id,
+	   _("Print the current frame-id."),
+	   &maintenanceprintlist);
 }

gdb/testsuite/gdb.base/maint-print-frame-id.c

@@ -0,0 +1,40 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2022 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/>.  */
+
+int
+foo (void)
+{
+  return 0;
+}
+
+int
+bar (void)
+{
+  return foo ();
+}
+
+int
+baz (void)
+{
+  return bar ();
+}
+
+int
+main (void)
+{
+  return baz ();
+}

gdb/testsuite/gdb.base/maint-print-frame-id.exp

@@ -0,0 +1,68 @@
+# Copyright 2022 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 frame-id' command.
+
+standard_testfile
+
+if {[prepare_for_testing "failed to prepare" $testfile \
+	 $srcfile {debug}]} {
+    return -1
+}
+
+if ![runto_main] then {
+    return -1
+}
+
+gdb_breakpoint foo
+gdb_continue_to_breakpoint "run to foo"
+
+proc get_frame_id { level } {
+    set id "**unknown**"
+
+    gdb_test_multiple "maint print frame-id ${level}" "" {
+	-re "^maint print frame-id\[^\r\n\]+\r\n" {
+	    exp_continue
+	}
+
+	-wrap -re "^frame-id for frame #\[0-9\]+: (\[^\r\n\]+)" {
+	    set id $expect_out(1,string)
+	    pass $gdb_test_name
+	}
+    }
+
+    return $id
+}
+
+# Get the frame-id for each frame using the frame level.
+array set ids {}
+with_test_prefix "get id by level" {
+    for { set i 0 } { $i < 4 } { incr i } {
+	set ids($i) [get_frame_id $i]
+    }
+}
+
+# Now get the frame-id of the currently selected frame, and check it
+# matches the frame-id we got earlier.  Then move up the stack,
+# selecting a new frame.
+for { set i 0 } { $i < 4 } { incr i } {
+    with_test_prefix "frame $i is current" {
+	set id [get_frame_id ""]
+	gdb_assert { [string equal $id $ids($i)] } \
+	    "check frame-id matches"
+    }
+    gdb_test "up" ".*" \
+	"move up from frame $i"
+}