Add arm epilogue unwinder

Nowadays, GDB can't unwind successfully from epilogue on arm,

 (gdb) bt
 #0  0x76ff65a2 in shr1 () from /home/yao/Source/gnu/build/gdb/testsuite/gdb.reverse/shr1.sl
 #1  0x0000869e in main () at /home/yao/Source/gnu/build/gdb/testsuite/../../../binutils-gdb/gdb/testsuite/gdb.reverse/solib-reverse.c:34
 Backtrace stopped: previous frame inner to this frame (corrupt stack?)

(gdb) disassemble shr1
Dump of assembler code for function shr1:
   ....
   0x76ff659a <+10>:	adds	r7, #12
   0x76ff659c <+12>:	mov	sp, r7
   0x76ff659e <+14>:	ldr.w	r7, [sp], #4
   0x76ff65a2 <+18>:	bx	lr
End of assembler dump.

in this case, prologue unwinder is used.  It analyzes the prologue and
get the offsets of saved registers to SP.  However, in epilogue, the
SP has been restored, prologue unwinder gets the registers from the
wrong address, and even the frame id is wrong.

In reverse debugging, this case (program stops at the last instruction
of function) happens quite frequently due to the reverse execution.
There are many test fails due to missing epilogue unwinder.

This adds epilogue unwinder, but the frame cache is still get by
prologue unwinder except that SP is fixed up separately, because SP
is restored in epilogue.

This patch fixes many fails in solib-precsave.exp, and solib-reverse.exp.

gdb:

2016-03-30  Yao Qi  <yao.qi@linaro.org>

	* arm-tdep.c: (arm_make_epilogue_frame_cache): New function.
	(arm_epilogue_frame_this_id): New function.
	(arm_epilogue_frame_prev_register): New function.
	(arm_epilogue_frame_sniffer): New function.
	(arm_epilogue_frame_unwind): New.
	(arm_gdbarch_init): Append unwinder arm_epilogue_frame_unwind.

gdb/ChangeLog

@@ -1,3 +1,12 @@
+2016-03-30  Yao Qi  <yao.qi@linaro.org>
+
+	* arm-tdep.c: (arm_make_epilogue_frame_cache): New function.
+	(arm_epilogue_frame_this_id): New function.
+	(arm_epilogue_frame_prev_register): New function.
+	(arm_epilogue_frame_sniffer): New function.
+	(arm_epilogue_frame_unwind): New.
+	(arm_gdbarch_init): Append unwinder arm_epilogue_frame_unwind.
+
 2016-03-30  Yao Qi  <yao.qi@linaro.org>
 
 	* arm-tdep.c (arm_stack_frame_destroyed_p): Rename it ...

gdb/arm-tdep.c

@@ -2669,6 +2669,112 @@ struct frame_unwind arm_exidx_unwind = {
   arm_exidx_unwind_sniffer
 };
 
+static struct arm_prologue_cache *
+arm_make_epilogue_frame_cache (struct frame_info *this_frame)
+{
+  struct arm_prologue_cache *cache;
+  CORE_ADDR sp;
+  int reg;
+
+  cache = FRAME_OBSTACK_ZALLOC (struct arm_prologue_cache);
+  cache->saved_regs = trad_frame_alloc_saved_regs (this_frame);
+
+  /* Still rely on the offset calculated from prologue.  */
+  arm_scan_prologue (this_frame, cache);
+
+  /* Since we are in epilogue, the SP has been restored.  */
+  cache->prev_sp = get_frame_register_unsigned (this_frame, ARM_SP_REGNUM);
+
+  /* Calculate actual addresses of saved registers using offsets
+     determined by arm_scan_prologue.  */
+  for (reg = 0; reg < gdbarch_num_regs (get_frame_arch (this_frame)); reg++)
+    if (trad_frame_addr_p (cache->saved_regs, reg))
+      cache->saved_regs[reg].addr += cache->prev_sp;
+
+  return cache;
+}
+
+/* Implementation of function hook 'this_id' in
+   'struct frame_uwnind' for epilogue unwinder.  */
+
+static void
+arm_epilogue_frame_this_id (struct frame_info *this_frame,
+			    void **this_cache,
+			    struct frame_id *this_id)
+{
+  struct arm_prologue_cache *cache;
+  CORE_ADDR pc, func;
+
+  if (*this_cache == NULL)
+    *this_cache = arm_make_epilogue_frame_cache (this_frame);
+  cache = (struct arm_prologue_cache *) *this_cache;
+
+  /* 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.  */
+  pc = get_frame_pc (this_frame);
+  func = get_frame_func (this_frame);
+  if (func == NULL)
+    func = pc;
+
+  (*this_id) = frame_id_build (cache->prev_sp, pc);
+}
+
+/* Implementation of function hook 'prev_register' in
+   'struct frame_uwnind' for epilogue unwinder.  */
+
+static struct value *
+arm_epilogue_frame_prev_register (struct frame_info *this_frame,
+				  void **this_cache, int regnum)
+{
+  struct arm_prologue_cache *cache;
+
+  if (*this_cache == NULL)
+    *this_cache = arm_make_epilogue_frame_cache (this_frame);
+  cache = (struct arm_prologue_cache *) *this_cache;
+
+  return arm_prologue_prev_register (this_frame, this_cache, regnum);
+}
+
+static int arm_stack_frame_destroyed_p_1 (struct gdbarch *gdbarch,
+					  CORE_ADDR pc);
+static int thumb_stack_frame_destroyed_p (struct gdbarch *gdbarch,
+					  CORE_ADDR pc);
+
+/* Implementation of function hook 'sniffer' in
+   'struct frame_uwnind' for epilogue unwinder.  */
+
+static int
+arm_epilogue_frame_sniffer (const struct frame_unwind *self,
+			    struct frame_info *this_frame,
+			    void **this_prologue_cache)
+{
+  if (frame_relative_level (this_frame) == 0)
+    {
+      struct gdbarch *gdbarch = get_frame_arch (this_frame);
+      CORE_ADDR pc = get_frame_pc (this_frame);
+
+      if (arm_frame_is_thumb (this_frame))
+	return thumb_stack_frame_destroyed_p (gdbarch, pc);
+      else
+	return arm_stack_frame_destroyed_p_1 (gdbarch, pc);
+    }
+  else
+    return 0;
+}
+
+/* Frame unwinder from epilogue.  */
+
+static const struct frame_unwind arm_epilogue_frame_unwind =
+{
+  NORMAL_FRAME,
+  default_frame_unwind_stop_reason,
+  arm_epilogue_frame_this_id,
+  arm_epilogue_frame_prev_register,
+  NULL,
+  arm_epilogue_frame_sniffer,
+};
+
 /* Recognize GCC's trampoline for thumb call-indirect.  If we are in a
    trampoline, return the target PC.  Otherwise return 0.
 
@@ -9285,6 +9391,7 @@ arm_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
   frame_unwind_append_unwinder (gdbarch, &arm_stub_unwind);
   dwarf2_append_unwinders (gdbarch);
   frame_unwind_append_unwinder (gdbarch, &arm_exidx_unwind);
+  frame_unwind_append_unwinder (gdbarch, &arm_epilogue_frame_unwind);
   frame_unwind_append_unwinder (gdbarch, &arm_prologue_unwind);
 
   /* Now we have tuned the configuration, set a few final things,