Handling of empty Ada ranges with a negative upper bound.

Handling of empty Ada ranges with a negative upper bound.

Consider the following variable declaration:

    type Array_Type is array (Integer range <>) of Integer;
    Var: Array_Type (0 .. -1);

"ptype var" prints the wrong upper bound for that array:

    (gdb) ptype var
    type = array (0 .. 4294967295) of integer

The debugging info for the type of variable "Var" is as follow:

  <2><cf>: Abbrev Number: 13 (DW_TAG_structure_type)
     <d0>   DW_AT_name        : foo__var___PAD
  <3><db>: Abbrev Number: 14 (DW_TAG_member)
     <dc>   DW_AT_name        : F
     <e0>   DW_AT_type        : <0xa5>

This is just an artifact from code generation, which is just
a wrapper that we should ignore. The real type is the type of
field "F" in that PAD type, which is described as:

  <2><a5>: Abbrev Number: 10 (DW_TAG_array_type)
     <a6>   DW_AT_name        : foo__TvarS
  <3><b6>: Abbrev Number: 11 (DW_TAG_subrange_type)
     <b7>   DW_AT_type        : <0xc1>
     <bb>   DW_AT_lower_bound : 0
     <bc>   DW_AT_upper_bound : 0xffffffff

Trouble occurs because DW_AT_upper_bound is encoded using
a DW_FORM_data4, which is ambiguous regarding signedness.
In that case, dwarf2read.c::dwarf2_get_attr_constant_value
reads the value as unsigned, which is not what we want
in this case.

As it happens, we already have code dealing with this situation
in dwarf2read.c::read_subrange_type which checks whether
the subrange's type is signed or not, and if it is, fixes
the bound's value by sign-extending it:

  if (high.kind == PROP_CONST
      && !TYPE_UNSIGNED (base_type) && (high.data.const_val & negative_mask))
    high.data.const_val |= negative_mask;

Unfortunately, what happens in our case is that the base type
of the array's subrange type is marked as being unsigned, and
so we never get to apply the sign extension. Following the DWARF
trail, the range's base type is described as another subrange type...

  <2><c1>: Abbrev Number: 12 (DW_TAG_subrange_type)
     <c7>   DW_AT_name        : foo__TTvarSP1___XDLU_0__1m
     <cb>   DW_AT_type        : <0x2d>

... whose base type is, (finally), a basic type (signed):

  <1><2d>: Abbrev Number: 2 (DW_TAG_base_type)
     <2e>   DW_AT_byte_size   : 4
     <2f>   DW_AT_encoding    : 5        (signed)
     <30>   DW_AT_name        : integer

The reason why GDB thinks that foo__TTvarSP1___XDLU_0__1m
(the base type of the array's range type) is an unsigned type
is found in gdbtypes.c::create_range_type.  We consider that
a range type is unsigned iff its lower bound is >= 0:

  if (low_bound->kind == PROP_CONST && low_bound->data.const_val >= 0)
    TYPE_UNSIGNED (result_type) = 1;

That is normally sufficient, as one would expect the upper bound to
always be greater or equal to the lower bound. But Ada actually
allows the declaration of empty range types where the upper bound
is less than the lower bound. In this case, the upper bound is
negative, so we should not be marking the type as unsigned.

This patch fixes the issue by simply checking the upper bound as well
as the lower bound, and clears the range type's unsigned flag when
it is found to be constant and negative.

gdb/ChangeLog:

        * gdbtypes.c (create_range_type): Unset RESULT_TYPE's
        flag_unsigned if HIGH_BOUND is constant and negative.

gdb/testsuite/ChangeLog:

        * gdb.ada/n_arr_bound: New testcase.

Tested on x86_64-linux.