[PATCH] [ARM] minor opt in thumb_stack_frame_destroyed_p

[Yao Qi <qiyaoltc@gmail.com>]

thumb_stack_frame_destroyed_p scans the instructions from PC to the
end of the function, but if PC is far from the end of pc, we don't
have to scan, because PC should be in epilogue if it is still
far from the end of the function.  The criterion I use here is 16
bytes, which is more than 4 instructions.

Regression tested on aarch64-linux with mutli-arch debug.  I'll push
it in.

gdb:

2016-04-15  Yao Qi  <yao.qi@linaro.org>

	* arm-tdep.c (thumb_stack_frame_destroyed_p): Return zero if
	PC is far from the end of function.
---
 gdb/ChangeLog  | 5 +++++
 gdb/arm-tdep.c | 8 ++++++++
 2 files changed, 13 insertions(+)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 09937a9..e85023e 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2016-04-15  Yao Qi  <yao.qi@linaro.org>
+
+	* arm-tdep.c (thumb_stack_frame_destroyed_p): Return zero if
+	PC is far from the end of function.
+
 2016-04-14  Pedro Alves  <palves@redhat.com>
 
 	* cli/cli-cmds.c (alias_usage_error): New function.
diff --git a/gdb/arm-tdep.c b/gdb/arm-tdep.c
index fb698a4..02e66e5 100644
--- a/gdb/arm-tdep.c
+++ b/gdb/arm-tdep.c
@@ -3135,6 +3135,14 @@ thumb_stack_frame_destroyed_p (struct gdbarch *gdbarch, CORE_ADDR pc)
   if (!find_pc_partial_function (pc, NULL, &func_start, &func_end))
     return 0;
 
+  if (func_end - pc > 4 * 4)
+    {
+      /* There shouldn't be more than four instructions in epilogue.
+	 If PC is still 16 bytes away from FUNC_END, it isn't in
+	 epilogue.  */
+      return 0;
+    }
+
   /* The epilogue is a sequence of instructions along the following lines:
 
     - add stack frame size to SP or FP
-- 
1.9.1