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

[Gaius Mulley <gaius@glam.ac.uk>]

Jim Blandy <jimb@codesourcery.com> writes:

> 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.

Hi Jim,

thank you for the various pointers (re: dwarf2 opcodes, subranges and
unbounded arrays - I also note your --fixme-- in the dwarf2read code)
- I wonder whether it might be better to hold off applying the patch
and attempt to solve the problem properly using the COMPUTE_BOUND
method explained above.  I'm willing to attempt this - its always more
satisfying solving a problem correctly :-) and in theory this method
might be useful for Pascal as well.  It should also mean ISO M2
multidimensional unbounded arrays will be easier to implement.

regards,
Gaius