Re: [PATCH] Unbounded array support implemented (for Modula-2)

[Jim Blandy <jimb@codesourcery.com>]

I'd like to see the tests go in, too.  I'm not sure the approach they
take is one we'd like to follow in future code, though.

I'm inferring from the test and from the Modula-2 code in GDB that
the debugging information GCC emits for unbounded arrays claims that
they're a structure of the form:

    struct {
       char *_m2_contents;
       int _m2_high;
    };

Then, GDB's Modula-2 code recognizes this structure as a special case,
and treats it as an array.

It seems to me the tests should go ahead and require a Modula-2
compiler, compile a real Modula-2 program, and exit with 'unsupported'
if there's no Modula-2 compiler present.

It's a shame that the debugging info doesn't actually treat these
Modula-2 arrays as arrays.  The 'proper' DWARF representation would be
a DW_TAG_array_type die with a DW_TAG_subrange_type for the variable
index whose a DW_AT_upper_bound attribute is some DWARF expression
like:

      DW_OP_push_object_address DW_OP_lit4 DW_OP_plus DW_OP_deref

to compute the upper bound of the array.

Then GDB would need to learn to understand these things.  In a
quick-and-dirty implementation, the DWARF reader would turn the DWARF
above into arrays whose index subrange types used a new
BOUND_BY_DESCRIPTOR value for 'enum array_bound_type', ignoring the
DWARF expression.  Then language-specific code would have to handle
those as a special case.

The best implementation would be to have a new BOUND_COMPUTED style of
bound, and have the 'loc' union in 'struct field' point to a structure
like this:

    struct computed_bound {
      /* Return the value of a computed bound.  V is the array value whose
         bound we're computing; CB is a pointer to this structure.  */
      LONGEST (*compute_bound (struct computed_bound *cb, struct value *v));

      /* Data to be used by the COMPUTE_BOUND function.  */
      void *data;
    };

The DWARF reader would create these when it sees arrays with upper
bounds that are expressions, making DATA point to the expression and
COMPUTE_BOUND point to a function that uses dwarf2expr.h to evaluate
the expression.

Then the generic GDB array code could handle Modula-2 variable arrays
(or anything else that the DWARF described accurately).

This isn't exactly a one-weekend project, though.