[PATCH] ARM: Do not use FP reg when on AAPCS

[Marius Muench <munch@eurecom.fr>]

Hi all,

GDB tries to dereference the frame pointer in arm_scan_prologue as a
last resort to create frame information.
However, the more recent AAPCS ABI does not make use of a frame pointer.

This patch checks whether the specified arm_abi is AAPCS before
dereferencing the "frame pointer". If so, just return as efforts to use
it for restoring frame information won't work.

Cheers,
Marius

gdb/ChangeLog
2018-10-23  Marius Muench  <marius.muench@eurecom.fr>

       * arm-tdeb.c (arm_scan_prologue): Don't dereference FP reg
       when on aapcs.
---
 gdb/arm-tdep.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/gdb/arm-tdep.c b/gdb/arm-tdep.c
index 53eee76926..859ce71d6a 100644
--- a/gdb/arm-tdep.c
+++ b/gdb/arm-tdep.c
@@ -1801,6 +1801,10 @@ arm_scan_prologue (struct frame_info *this_frame,
       CORE_ADDR frame_loc;
       ULONGEST return_value;

+      /* AAPCS does not use a frame register, so we can abort here. */
+      if (gdbarch_tdep (gdbarch)->arm_abi == ARM_ABI_AAPCS)
+          return;
+
       frame_loc = get_frame_register_unsigned (this_frame, ARM_FP_REGNUM);
       if (!safe_read_memory_unsigned_integer (frame_loc, 4, byte_order,
                                              &return_value))