Patchwork [FYI,4/6,Ada] array of variant record subscripting

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Submitter Joel Brobecker
Date April 21, 2015, 3:44 p.m.
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Download mbox | patch
Permalink /patch/6344/
State New
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Joel Brobecker - April 21, 2015, 3:44 p.m.
Consider the following (Ada) array...

   A1 : Array_Type := (1 => (I => 0, S => <>),
                       2 => (I => 1, S => "A"),
                       3 => (I => 2, S => "AB"));

... where Array_Type is declared as follow:

   subtype Small_Type is Integer range 0 .. 10;
   type Record_Type (I : Small_Type := 0) is record
      S : String (1 .. I);
   end record;
   type Array_Type is array (Integer range <>) of Record_Type;

Trying to print the value of each element individually does not
always work. Printing the value of the first one does:

(gdb) p a1(1)
    $1 = (i => 0, s => "")

But printing the value of the subsequent ones often does not.
For instance:

    (gdb) p a1(2)
    $2 = (i => 1, s => "")  <<<--- s should be "A"
    (gdb) p a1(3)
    $3 = (i => 2, s => "")  <<<--- s should be "AB"

I traced the problem to ada_value_primitive_packed_val,
which is trying to perform the array subscripting by
extracting the value of the corresponding array element
into a buffer where the contents is now byte-aligned.

The element type that ada_value_primitive_packed_val gets passed
is a dynamic type. As it happens, that dynamic type can get resolved
thanks to:

      v = value_at (type, value_address (obj));
      type = value_type (v);

However, obj represents the array, so the address given in the call
to value_at represents the value of the first element. As a result,
the solution of component S's upper bound always gets resolved based
on the value of component I in the  first element of the array, whose
value is 0, thus leading to GDB mistakely resolving the element type
where S's upper bound is always 0.

The proper fix would be to systematically resolve the element type
first. But, this requires us to extract-and-realign the element's
value so as to be able to pass it as "valaddr" to resolve_dynamic_type.
In the meantime, it's easy to make the situation a little better by
passing "value_address (obj) + offset" as the object address. This
only works when BIT_OFFSET is nul, but that should be the case when
the element type is anything but a scalar, which seems to be the only
situation where it seems important to resolve the type now. And we're
not that worse off otherwise.

But we'll try to find a better solution in a separate patch.


        * ada-lang.c (ada_value_primitive_packed_val): Use a more
        correct address in call to value_at.  Adjust call to
        value_address accordingly.
 gdb/ada-lang.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)


diff --git a/gdb/ada-lang.c b/gdb/ada-lang.c
index 9926cfb..d71a243 100644
--- a/gdb/ada-lang.c
+++ b/gdb/ada-lang.c
@@ -2417,10 +2417,10 @@  ada_value_primitive_packed_val (struct value *obj, const gdb_byte *valaddr,
   else if (VALUE_LVAL (obj) == lval_memory && value_lazy (obj))
-      v = value_at (type, value_address (obj));
+      v = value_at (type, value_address (obj) + offset);
       type = value_type (v);
       bytes = (unsigned char *) alloca (len);
-      read_memory (value_address (v) + offset, bytes, len);
+      read_memory (value_address (v), bytes, len);