[RFA] (AArch64) wrong value returned by "finish" for HFA

[Joel Brobecker <brobecker@adacore.com>]

Hello,

Consider the gdb.ada/array_return.exp testcase, and in particular,
consider the following code...

   type Small_Float_Vector is array (1 .. 2) of Float;

   function Create_Small_Float_Vector return Small_Float_Vector is
   begin
      return (others => 4.25);
   end Create_Small_Float_Vector;

... which declares a type which is an array with 2 floats in it
(floats are 4 bytes on AArch64), trying to get GDB to print
the return value from that function does not work:

    (gdb) fin
    Run till exit from #0  pck.create_small_float_vector () at /[...]/pck.adb:15
    0x000000000000062c in p () at /[...]/p.adb:11
    11         Vector := Create_Small_Float_Vector;
    Value returned is $1 = (4.25, 0.0)
                                  ^^^
                                  |||

We expected the value shown to be:

    (gdb) fin
    Run till exit from #0  pck.create_small_float_vector () at /[...]/pck.adb:15
    0x000000000000062c in p () at /[...]/p.adb:11
    11         Vector := Create_Small_Float_Vector;
    Value returned is $1 = (4.25, 4.25)

Because the return type is an HFA, it is returned via the first two
SIMD registers. However, what happens is that the current implementation
fails to realize that this is an HFA, and therefore fetches the return
value from the wrong location. And the reason why it fails to realize
this is because it thinks that our array has 8 elements (HFAs have
a maximum of 4). Looking at aapcs_is_vfp_call_or_return_candidate_1,
where this is determined, we can easily see why (looks like a thinko):

        | case TYPE_CODE_ARRAY:
        | [...]
        |         struct type *target_type = TYPE_TARGET_TYPE (type);
        |         int count = aapcs_is_vfp_call_or_return_candidate_1
        |                       (target_type, fundamental_type);
        |
        |         if (count == -1)
        |           return count;
        |
  !! -> |         count *= TYPE_LENGTH (type);
        |           return count;

Here, we first determine the count for one element of our array,
and so we should then be multiplying that count by the number
of elements in our array (2 in our case). But instead, we multiply it
by the total size (8). As a result, we do not classify the return
type as an HFA, and thus pick the wrong location for fetching
the return value.

gdb/ChangeLog:

        * aarch64-tdep.c (aapcs_is_vfp_call_or_return_candidate_1):
        return the correct count for potential HFAs.

Tested on aarch64-linux, fixes:

    array_return.exp: value printed by finish of Create_Small_Float_Vector

OK to commit?

Thank you,
-- 
Joel

---
 gdb/aarch64-tdep.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index 0211930..51f1632 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -1212,7 +1212,7 @@ aapcs_is_vfp_call_or_return_candidate_1 (struct type *type,
 	    if (count == -1)
 	      return count;
 
-	    count *= TYPE_LENGTH (type);
+	    count *= (TYPE_LENGTH (type) / TYPE_LENGTH (target_type));
 	      return count;
 	  }
       }
-- 
2.1.4