[PATCH] Relax ARM prologue unwinder assumption

[Luis Machado <lgustavo at codesourcery dot com>]

Just recently i ran into a problem with a bare metal ARM target where 
GDB would not allow some registers to be changed after the PC was 
manually set to some other value.

In reality the target had just started and one of its cores, out of 
four, was running the program, but the other ones were in some random state.

The PC of one of the other 3 cores was then adjusted to point to a known 
function address.

GDB's reaction to this change is to invalidate the regcache and frame 
and build a brand new chain and cache, while trying to retain the 
previously selected frame - #0 in this case.

What i noticed is that GDB was selecting frame #1 instead of #0 due to 
unfortunate coincidences with both frames' SP's being 0. And we can't 
modify some registers on non-innermost frames for obvious reasons.

Here's a brief log of what happens:


[Switching to thread 2 (Thread 2)]
#0  0x0093ff10 in ?? ()
(gdb) set $pc=functioncore2
(gdb) bt
#0  functioncore2 () at test.c:32
#1  0x0000fc44 in ?? ()
(gdb) frame
#1  0x0000fc44 in ?? ()
(gdb) set $sp=0x2030
Attempt to assign to an unmodifiable value.

I tracked this problem down to this old 
(https://sourceware.org/ml/gdb-patches/2003-08/msg00526.html) piece of 
code in arm-tdep.c:arm_prologue_this_id:

  /* If we've hit a wall, stop.  */
  if (cache->prev_sp == 0)
    return;

Due to the SP being 0 for this specific core, GDB returns early and does 
not set the frame's PC to the new value. That means we have a frame with 
PC==0x0 and SP==0x0, which ends up matching frame #1 in our case. But 
this is obviously wrong.

I looked up the patch that introduced this chunk of code and did not 
find any reasonable explanation for this check. Though it may make sense 
for non-bare metal targets, a bare-metal board attached to a probe can 
be stopped at any random place, so we should be able to set its 
registers freely without worrying about unwinding assumptions. This is 
generic code after all. I understand a valid workaround is to make sure 
the proper frame is selected, but GDB should be smart enough not to 
confuse things. Therefore this "wall" described in the comment seems too 
strict to be in generic code.

The attached patch removes this restriction and does not cause any 
regressions for ARM bare metal, but i'd like feedback.
2015-02-05  Luis Machado  <lgustavo@codesourcery.com>

	* arm-tdep.c (arm_prologue_this_id): Remove check for the stack
	pointer being 0.

diff --git a/gdb/arm-tdep.c b/gdb/arm-tdep.c
index 8e9552a..771cbeb 100644
--- a/gdb/arm-tdep.c
+++ b/gdb/arm-tdep.c
@@ -2042,10 +2042,6 @@ arm_prologue_this_id (struct frame_info *this_frame,
   if (pc <= gdbarch_tdep (get_frame_arch (this_frame))->lowest_pc)
     return;
 
-  /* If we've hit a wall, stop.  */
-  if (cache->prev_sp == 0)
-    return;
-
   /* Use function start address as part of the frame ID.  If we cannot
      identify the start address (due to missing symbol information),
      fall back to just using the current PC.  */