Re: [PATCH] Add extensions to dwarf2.def

[Cary Coutant <ccoutant@gmail.com>]

> I'm currently working on migrating debugging information for Ada from GNAT
> encodings to standard DWARF. At the moment, I have worked on two topics that
> I believe are not (completely) supported in standard DWARF:
>
>   - fixed point types with arbitrary scale factors;
>   - scalar types with biased representations.
>
> My goal is to submit an issue on dwarfstd.org in an attempt to introduce
> these extensions to the next DWARF standard. Before that, though, I would
> like to make sure that these extensions actually fit the need by having them
> supported both in GCC and GDB.
>
> The two attached patches make these extensions "public" so that no other
> vendor-specific tags/attributes conflict with them in the future. I cannot
> submit the patches that actually use these right now because I need first to
> port them from the 4.9 branch onto mainline (I hope I will be able to do
> this on early July).
>
> May I commit them?
>
> I also attached two documents that describe how to use these extensions. I
> guess this should go to the wiki just like for DW_AT_GNAT_descriptive_type
> (https://gcc.gnu.org/wiki/DW_AT_GNAT_descriptive_type). I will do this if
> the patches are integrated.
>
> Thank you in advance!
>
> include/
>         * dwarf2.def (DW_TAG_GNU_rational_constant): New tag.
>         (DW_AT_GNU_numerator, DW_AT_GNU_denominator): New attributes.

I don't think you really need a new TAG here -- DW_TAG_constant could
just as easily take DW_AT_GNU_numerator and DW_AT_GNU_denominator as
an alternative to DW_AT_const_value.

I'm not really sure why DW_AT_small was defined to refer to a
DW_TAG_constant DIE rather than just providing the constant as the
attribute value. It would seem more efficient, space-wise, to have a
DW_AT_scale attribute that would provide a multiplicative scale
factor, and an optional DW_AT_scale_divisor to provide the denominator
if necessary.

Another, perhaps far-fetched, alternative would be to introduce a new
form that would represent a rational constant as two unsigned LEB128
values, and allow that form for DW_AT_const_value and/or for
DW_AT_small.

For now, I'd suggest going with your proposal, except use the existing
DW_TAG_constant instead of a new TAG. (I.e., just add the two new
DW_AT_numerator and DW_AT_denominator attributes.)

> include/
>         * dwarf2.def (DW_AT_GNU_bias): New attribute.

This is OK. Looks like a good idea to me.

-cary