"tfind" across unavailable-stack frames.

Like when stepping, the current stack frame location is expected to be
printed as result of tfind command, if that results in moving to a
different function.  In tfind_1 we see:

  if (from_tty
      && (has_stack_frames () || traceframe_number >= 0))
    {
      enum print_what print_what;

      /* NOTE: in imitation of the step command, try to determine
         whether we have made a transition from one function to
         another.  If so, we'll print the "stack frame" (ie. the new
         function and it's arguments) -- otherwise we'll just show the
         new source line.  */

      if (frame_id_eq (old_frame_id,
                       get_frame_id (get_current_frame ())))
        print_what = SRC_LINE;
      else
        print_what = SRC_AND_LOC;

      print_stack_frame (get_selected_frame (NULL), 1, print_what, 1);
      do_displays ();
    }

However, when we haven't collected any registers in the tracepoint
(collect $regs), that doesn't actually work:

 (gdb) tstart
 (gdb) info tracepoints
 Num     Type           Disp Enb Address    What
 1       tracepoint     keep y   0x080483b7 in func0
                                            at ../.././../git/gdb/testsuite/gdb.trace/circ.c:28
         collect testload
     installed on target
 2       tracepoint     keep y   0x080483bc in func1
                                            at ../.././../git/gdb/testsuite/gdb.trace/circ.c:32
         collect testload
     installed on target
 (gdb) c
 Continuing.

 Breakpoint 3, end () at ../.././../git/gdb/testsuite/gdb.trace/circ.c:72
 72    }
 (gdb) tstop
 (gdb) tfind start
 Found trace frame 0, tracepoint 1
 #0  func0 () at ../.././../git/gdb/testsuite/gdb.trace/circ.c:28
 28    }
 (gdb) tfind
 Found trace frame 1, tracepoint 2
 32    }
 (gdb)

When we don't have info about the stack available
(UNWIND_UNAVAILABLE), frames end up with outer_frame_id as frame ID.
And in the scenario above, the issue is that both frames before and
after the second tfind (the frames for func0 an func1) have the same
id (outer_frame_id), so the frame_id_eq check returns false, even
though the frames were of different functions.  GDB knows that,
because the PC is inferred from the tracepoint's address, even if no
registers were collected.

To fix this, this patch adds support for frame ids with a valid code
address, but <unavailable> stack address, and then makes the unwinders
use that instead of the catch-all outer_frame_id for such frames.  The
frame_id_eq check in tfind_1 then automatically does the right thing
as expected.

I tested with --directory=gdb.trace/ , before/after the patch, and
compared the resulting gdb.logs, then adjusted the tests to expect the
extra output that came out.  Turns out that was only circ.exp, the
original test that actually brought this issue to light.

Tested on x86_64 Fedora 17, native and gdbserver.

gdb/
2013-12-17  Pedro Alves  <palves@redhat.com>

	* frame.h (enum frame_id_stack_status): New enum.
	(struct frame_id) <stack_addr>: Adjust comment.
	<stack_addr_p>: Delete field, replaced with ...
	<stack_status>: ... this new field.
	(frame_id_build_unavailable_stack): Declare.
	* frame.c (frame_addr_hash, fprint_field, outer_frame_id)
	(frame_id_build_special): Adjust.
	(frame_id_build_unavailable_stack): New function.
	(frame_id_build, frame_id_build_wild): Adjust.
	(frame_id_p, frame_id_eq, frame_id_inner): Adjust to take into
	account frames with unavailable stack.

	* amd64-tdep.c (amd64_frame_this_id)
	(amd64_sigtramp_frame_this_id, amd64_epilogue_frame_this_id): Use
	frame_id_build_unavailable_stack.
	* dwarf2-frame.c (dwarf2_frame_this_id): Likewise.
	* i386-tdep.c (i386_frame_this_id, i386_epilogue_frame_this_id)
	(i386_sigtramp_frame_this_id):  Likewise.

gdb/testsuite/
2013-12-17  Pedro Alves  <palves@redhat.com>

	* gdb.trace/circ.exp: Expect frame info to be printed when
	switching between frames with unavailable stack, but different
	functions.

1 files changed, 27 insertions, 3 deletions

diff --git a/gdb/frame.h b/gdb/frame.h
index f8d5bc1..b03f212 100644
--- a/gdb/frame.h
+++ b/gdb/frame.h
@@ -76,6 +76,23 @@ struct block;
 struct gdbarch;
 struct ui_file;
 
+/* Status of a given frame's stack.  */
+
+enum frame_id_stack_status
+{
+  /* Stack address is invalid.  E.g., this frame is the outermost
+     (i.e., _start), and the stack hasn't been setup yet.  */
+  FID_STACK_INVALID = 0,
+
+  /* Stack address is valid, and is found in the stack_addr field.  */
+  FID_STACK_VALID = 1,
+
+  /* Stack address is unavailable.  I.e., there's a valid stack, but
+     we don't know where it is (because memory or registers we'd
+     compute it from were not collected).  */
+  FID_STACK_UNAVAILABLE = -1
+};
+
 /* The frame object.  */
 
 struct frame_info;
@@ -97,8 +114,9 @@ struct frame_id
      function pointer register or stack pointer register.  They are
      wrong.
 
-     This field is valid only if stack_addr_p is true.  Otherwise, this
-     frame represents the null frame.  */
+     This field is valid only if frame_id.stack_status is
+     FID_STACK_VALID.  It will be 0 for other
+     FID_STACK_... statuses.  */
   CORE_ADDR stack_addr;
 
   /* The frame's code address.  This shall be constant through out the
@@ -129,7 +147,7 @@ struct frame_id
   CORE_ADDR special_addr;
 
   /* Flags to indicate the above fields have valid contents.  */
-  unsigned int stack_addr_p : 1;
+  ENUM_BITFIELD(frame_id_stack_status) stack_status : 2;
   unsigned int code_addr_p : 1;
   unsigned int special_addr_p : 1;
 
@@ -169,6 +187,12 @@ extern struct frame_id frame_id_build_special (CORE_ADDR stack_addr,
 					       CORE_ADDR code_addr,
 					       CORE_ADDR special_addr);
 
+/* Construct a frame ID representing a frame where the stack address
+   exists, but is unavailable.  CODE_ADDR is the frame's constant code
+   address (typically the entry point).  The special identifier
+   address is set to indicate a wild card.  */
+extern struct frame_id frame_id_build_unavailable_stack (CORE_ADDR code_addr);
+
 /* Construct a wild card frame ID.  The parameter is the frame's constant
    stack address (typically the outer-bound).  The code address as well
    as the special identifier address are set to indicate wild cards.  */