[Ada] Crash when trying to set value of packed array element

[Ada] Crash when trying to set value of packed array element

Consider the following declaration:

   type Small is new Integer range 0 .. 2 ** 4 - 1;
   type Simple_Array is array (1 .. 4) of Small;
   pragma Pack (Simple_Array);

   SA : Simple_Array := (1, 2, 3, 4);

Trying to change the value of one of the elements in the packed array
causes the debugger to crash:

    (gdb) set sa(3) := 9
    [1]    4880 segmentation fault  gdb -q foo

The circumstances leading to the crash are as follow:

   . ada_evaluate_subexp creates a value corresponding to "sa(3)".

   . ada_evaluate_subexp then tries to assign 9 to this value, and
     for this calls value_assign (via ada_value_assign).

   . Because the array is packed, the destination value is 3 bits long,
     and as a result, value_assign uses the parent to determine that
     element byte address and offset:

      | if (value_bitsize (toval))
      |   {
      |     struct value *parent = value_parent (toval);
      |
      |     changed_addr = value_address (parent) + value_offset (toval);

The destination value (corresponding to "sa(3)") was incorrectly created
by ada-lang.c:ada_value_primitive_packed_val, because the "parent" was
left as NULL. So, when we try to dereference it to get the parent address,
GDB crashed.

The first part of the fix therefore consists in setting that field.
This required the addition of a new "setter" in value.[hc].  It fixes
the crash, but is still not sufficient for the assignment to actually
work.

The second part of the problem came from the fact that value_assign
seems to expect the "child"'s address to be equal to the parent's address,
with the difference being the offset. Unfortunately, this requirement was
not followed by ada_value_primitive_packed_val, so the second part of
the fix consisted in fixing that.

Still, this was not sufficient, because it caused a regression when
trying to perform an aggregate assignment of a packed array of packed
record.  The key element here is the nesting of packed entities.
Looking at the way ada_value_primitive_packed_val creates the value
of each sub-component, one can see that the value's offset is set
to the offset compared to the start of the parent. This was meant to
match what value_primitive_field does as well.

So, with our array of records, if the record offset was 2, and if
the field we're interested in that record is at offset 1, the record
value's offset would be set to 2, and the field value's offset would
be set to 1. But the address for both values would be left to the
array's address. This is where things start breaking down, because
the value_address function for our field value would return the
address of the array + 1, instead of + 3.

This is what causes the final issue, here, because ada-lang.c's
value_assign_to_component needs to compute the offset of the
subcomponent compared to the top-level aggregate's start address
(the array in our case). And it does so by subtracting the array's
address from the sub-component's address.  When you have two levels
of packed components, and the mid-level component is at an offset of
the top-level component, things didn't work, because the component's
address was miscomputed (the parent's offset is missing).

The fix consists is fixing value_address to match the work done by
value_primitive_field (where we ignore the parent's offset).

gdb/ChangeLog:

        * value.h (set_value_parent): Add declaration.
        * value.c (set_value_parent): New function.
        (value_address): If VALUE->PARENT is not NULL, then use it as
        the base address instead of VALUE->LOCATION.address.
        * ada-lang.c (ada_value_primitive_packed_val): Keep V's address
        the same as OBJ's address.  Adjust V's offset accordingly.
        Set V's parent.

gdb/testsuite/ChangeLog:

        * gdb.ada/set_pckd_arr_elt: New testcase.