[PATCH] Implement gdbarch_stack_frame_destroyed_p for aarch64

[Tom Tromey <tromey@adacore.com>]

The internal AdaCore testsuite has a test that checks that an
out-of-scope watchpoint is deleted.  This fails on some aarch64
configurations, reporting an extra stop:

    (gdb) continue
    Continuing.

    Thread 3 hit Watchpoint 2: result

    Old value = 64
    New value = 0
    0x0000000040021648 in pck.get_val (seed=0, off_by_one=false) at [...]/pck.adb:13
    13	   end Get_Val;

I believe what is happening here is that the variable is stored at:

    <efa>   DW_AT_location    : 2 byte block: 91 7c 	(DW_OP_fbreg: -4)

and the extra stop is reported just before a return, when the ldp
instruction is executed:

   0x0000000040021644 <+204>:	ldp	x29, x30, [sp], #48
   0x0000000040021648 <+208>:	ret

This instruction modifies the frame base calculation, and so the test
picks up whatever memory is pointed to in the callee frame.

Implementing the gdbarch hook gdbarch_stack_frame_destroyed_p fixes
this problem.

As usual with this sort of patch, it has passed internal testing, but
I don't have a good way to try it with dejagnu.  So, I don't know
whether some existing test covers this.  I suspect there must be one,
but it's also worth noting that this test passes for aarch64 in some
configurations -- I don't know what causes one to fail and another to
succeed.
---
 gdb/aarch64-tdep.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index b3efb3ebaff..5effa95d71b 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -3379,6 +3379,25 @@ aarch64_cannot_store_register (struct gdbarch *gdbarch, int regnum)
 	  || regnum == AARCH64_PAUTH_CMASK_REGNUM (tdep->pauth_reg_base));
 }
 
+/* Implement the stack_frame_destroyed_p gdbarch method.  */
+
+static int
+aarch64_stack_frame_destroyed_p (struct gdbarch *gdbarch, CORE_ADDR pc)
+{
+  CORE_ADDR func_start, func_end;
+  if (!find_pc_partial_function (pc, NULL, &func_start, &func_end))
+    return 0;
+
+  enum bfd_endian byte_order_for_code = gdbarch_byte_order_for_code (gdbarch);
+  uint32_t insn = read_memory_unsigned_integer (pc, 4, byte_order_for_code);
+
+  aarch64_inst inst;
+  if (aarch64_decode_insn (insn, &inst, 1, nullptr) != 0)
+    return 0;
+
+  return streq (inst.opcode->name, "ret");
+}
+
 /* Initialize the current architecture based on INFO.  If possible,
    re-use an architecture from ARCHES, which is a list of
    architectures already created during this debugging session.
@@ -3585,6 +3604,9 @@ aarch64_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)
   set_gdbarch_long_double_format (gdbarch, floatformats_ia64_quad);
   set_gdbarch_type_align (gdbarch, aarch64_type_align);
 
+  /* Detect whether PC is at a point where the stack has been destroyed.  */
+  set_gdbarch_stack_frame_destroyed_p (gdbarch, aarch64_stack_frame_destroyed_p);
+
   /* Internal <-> external register number maps.  */
   set_gdbarch_dwarf2_reg_to_regnum (gdbarch, aarch64_dwarf_reg_to_regnum);
 
-- 
2.34.1