dwarflint semantic DIE checks

[Roland McGrath <roland at redhat dot com>]

> I put some time into that over the week.  The code now includes maps of 
> expected attributes (per DIE tag), and expected forms (per attribute form).

You should not need the latter, at least not done in that way.  The
dwarf::location_attr interfaces already encode that knowledge, in
dwarf::attr_value::what_space (c++/values.cc).  Actually the knowledge is
in c++/dwarf-knowledge.cc, which you might use directly.  (That file is
where I intended to have tables of expected children tags, etc. too.)

> Attributes are checked by recursive iteration of the DIE tree.   That 
> approach will be necessary for checking expected children, but 
> attributes, I think, would better be checked from abbreviations.  But I 
> don't think that <dwarf> provides support for that.

The only benefit to checking at the abbrev granularity is avoiding
repetition.  I don't think it's really worth the trouble for that.  In the
diagnostics, you want to cite the particular affected DIEs anyway.  Rather,
high-level checks use high-level interfaces.  Abbrevs are a low-level
encoding detail; at the "semantic level" they just don't matter at all.

> Attribute form is not checked, because there's no way to obtain that 
> from <dwarf>.  So the table is here, but it's not being used currently.

Don't check for "expected form".  Check for "expected value space".
attr->what_space () returns a space or throws an exception for ambiguity.
(The exception types still need to be worked out, but for now catching any
exception from what_space () as meaning "unexpected" is fine.)  The decoder
is liberal in cases without ambiguity, so you still want to check the
value_space returned against the expected_value_space () mask.
But you can let it entirely take care of the name+form->value_space mapping.


Thanks,
Roland