[RFA] Bail out of Thumb unwinding if there is no symbol

[Daniel Jacobowitz <drow@false.org>]

If find_pc_partial_function fails, prologue_start won't have been
initialized.  The previous effect of this code was to try to disassemble at
0 (by luck, the variable was unset).  This patch just stops unwinding when
we're stuck in a Thumb function without a symbol.

The astute may notice that it's a bit tricky to get to this situation
with current GDB; if we don't have symbols, how do we know that we're
Thumb?  Well, there's a couple ways to do it, including another
patch that I'm about to post.

This seems like an improvement over the current situation.  Lightly tested;
OK?

-- 
Daniel Jacobowitz
CodeSourcery

2006-02-20  Daniel Jacobowitz  <dan@codesourcery.com>

	* arm-tdep.c (thumb_scan_prologue): Don't try to analyze
	the function at zero if we have no symbols.

Index: src/gdb/arm-tdep.c
===================================================================
--- src.orig/gdb/arm-tdep.c	2006-02-20 16:11:02.000000000 -0500
+++ src/gdb/arm-tdep.c	2006-02-20 16:34:39.000000000 -0500
@@ -468,9 +468,9 @@ thumb_scan_prologue (CORE_ADDR prev_pc, 
 	prologue_end = sal.end;		/* (probably means no prologue)  */
     }
   else
-    /* We're in the boondocks: allow for 
-       16 pushes, an add, and "mv fp,sp".  */
-    prologue_end = prologue_start + 40;
+    /* We're in the boondocks: we have no idea where the start of the
+       function is.  */
+    return;
 
   prologue_end = min (prologue_end, prev_pc);