[rfa/mips] Stop backtraces when we've lost the PC

[Daniel Jacobowitz <drow@false.org>]

Here's an updated version of a little hack I've been using since GDB 6.0.
If we are in a nested normal frame, i.e. something whose next frame is a
function that it called in the normal way, and we didn't find a saved PC,
we're going to be stuck in a loop.  We might have been able to figure out
the frame size, but not where the return address was stored; as the comment
says, this happens in glibc's clone function.  Of course the problem there
is that it _doesn't_ save $ra in the normal fashion; it won't return.

Without this patch schedlock.exp falls apart, because backtraces continue
forever printing "clone()" on every line.

OK?  Or a better way to do this?

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer

2004-03-06  Daniel Jacobowitz  <drow@mvista.com>

	* mips-tdep.c (mips_mdebug_frame_this_id): Terminate unwinding if
	we haven't found a saved PC.

Index: mips-tdep.c
===================================================================
RCS file: /big/fsf/rsync/src-cvs/src/gdb/mips-tdep.c,v
retrieving revision 1.283
diff -u -p -r1.283 mips-tdep.c
--- mips-tdep.c	17 Feb 2004 15:21:21 -0000	1.283
+++ mips-tdep.c	6 Mar 2004 22:52:29 -0000
@@ -1672,6 +1672,24 @@ mips_mdebug_frame_this_id (struct frame_
 {
   struct mips_frame_cache *info = mips_mdebug_frame_cache (next_frame,
 							   this_cache);
+
+  /* If the return address is not saved for two frames in a row,
+     then we are probably hosed.  Not necessarily - it's possible to
+     write working assembly that violates this rule - but we can't
+     backtrace through that either.  Eventually MIPS will support
+     DWARF2 unwind information, allowing assembly programmers to
+     avoid this problem.
+
+     One place this check triggers is in the GNU/Linux clone syscall
+     wrapper.  */
+  if (frame_relative_level (next_frame) >= 0
+      && get_frame_type (next_frame) == NORMAL_FRAME
+      && !trad_frame_addr_p (info->saved_regs, NUM_REGS + PC_REGNUM))
+    {
+      (*this_id) = null_frame_id;
+      return;
+    }
+
   (*this_id) = frame_id_build (info->base, frame_func_unwind (next_frame));
 }