[Ada] Fix the evaluation of access to packed array subscript

This change is relevant only for standard DWARF (as opposed to the GNAT
encodings extensions): at the time of writing it only makes a difference
with GCC patches that are to be integrated: see in particular
<https://gcc.gnu.org/ml/gcc-patches/2015-07/msg01364.html>.

Given the following Ada declarations:

    type Small is mod 2 ** 6;
    type Array_Type is array (0 .. 9) of Small
       with Pack;
    type Array_Access is access all Array_Type;

    A  : aliased Array_Type := (1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
    AA : constant Array_Type := A'Access;

Before this change, we would get the following GDB session:

    (gdb) print aa.all(2)
    $1 = 3
    (gdb) print aa(2)
    $2 = 16

This is wrong: both expression should yield the same value: 3.  The
problem is simply that the routine which handles accesses to arrays lack
general handling for packed arrays.  After this patch, we have the
expected output:

    (gdb) print aa.all(2)
    $1 = 3
    (gdb) print aa(2)
    $2 = 3

gdb/ChangeLog:

	* ada-lang.c (ada_value_ptr_subscript): Update the heading
	comment.  Handle packed arrays.

gdb/testsuite/ChangeLog:

	* gdb.ada/access_to_packed_array.exp: New testcase.
	* gdb.ada/access_to_packed_array/foo.adb: New file.
	* gdb.ada/access_to_packed_array/pack.adb: New file.
	* gdb.ada/access_to_packed_array/pack.ads: New file.

Tested on x86_64-linux, no regression.

1 files changed, 15 insertions, 2 deletions

diff --git a/gdb/ada-lang.c b/gdb/ada-lang.c
index d166d1c..49f3d9a 100644
--- a/gdb/ada-lang.c
+++ b/gdb/ada-lang.c
@@ -2757,14 +2757,27 @@ ada_value_subscript (struct value *arr, int arity, struct value **ind)
 
 /* Assuming ARR is a pointer to a GDB array, the value of the element
    of *ARR at the ARITY indices given in IND.
-   Does not read the entire array into memory.  */
+   Does not read the entire array into memory.
+
+   Note: Unlike what one would expect, this function is used instead of
+   ada_value_subscript for basically all non-packed array types.  The reason
+   for this is that a side effect of doing our own pointer arithmetics instead
+   of relying on value_subscript is that there is no implicit typedef peeling.
+   This is important for arrays of array accesses, where it allows us to
+   preserve the fact that the array's element is an array access, where the
+   access part os encoded in a typedef layer.  */
 
 static struct value *
 ada_value_ptr_subscript (struct value *arr, int arity, struct value **ind)
 {
   int k;
+  struct value *array_ind = ada_value_ind (arr);
   struct type *type
-    = check_typedef (value_enclosing_type (ada_value_ind (arr)));
+    = check_typedef (value_enclosing_type (array_ind));
+
+  if (TYPE_CODE (type) == TYPE_CODE_ARRAY
+      && TYPE_FIELD_BITSIZE (type, 0) > 0)
+    return value_subscript_packed (array_ind, arity, ind);
 
   for (k = 0; k < arity; k += 1)
     {