[llvm-dev] RFC: Add DWARF support for yaml2obj

[James Henderson via llvm-dev]

I think we have to be careful here. We might want flexibility to say "I
want to use a specific class" without having to specify the exact DW_FORM.
Sometimes, we might even end up in an ambiguous situation and not get the
result we want. For example, in DWARFv4, the DW_AT_high_pc attribute has
either a Constant or an Address class, which use completely different
forms, but if we have just "Value: 0x1234", which is it? In DWARFv3, it is
always an Address, if I remember correctly, so in that case, we might want
our default to be "Address". However, for DWARFv4 the compiler typically
emits DW_AT_high_pc using a Constant form, and most people might expect
that to be used instead.

I think having a different name for the tag might be a good thing to do,
with the name matching the different classes (of which there are 15 in
DWARFv5, possibly with some folded together like string/stroffsetsptr), but
it could be a little confusing as Pavel mentions. Alternatively, maybe we
could have Value and Form, where the Form can be a generic class name
instead of a specific DW_FORM value. This has the potential for ambiguity
still, but should be flexible enough at least.

James

On Fri, 22 May 2020 at 10:46, Pavel Labath via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On 22/05/2020 10:38, Xing GUO wrote:
> >  BTW, I think
> > the value of "DW_AT_decl_file" shall be "Str"? Can we map the values
> > of different types into same field?
> >
> > """
> > - Attr: DW_AT_decl_file
> >   Str: foo
> > """
> Yes, that is definitely possible. You just need to make the map calls
> conditional on the values of other attributes. Maybe something like this:
> IO.mapRequired("Attr", Attr);
> IO.mapOptional("Form", Form, getDefaultForm(Attr, Ctx.isSplitDwarf()
> /*or whatever*/));
> switch (getFormClass(Form)) {
> /* The cases could correspond to DWARF5 form classes, but maybe not
> completely.*/
> case String: IO.mapRequired("Value", Value.String);
> case Constant: IO.mapRequired("Value", Value.Int);
> case Block: IO.mapRequired("Value", Value.Bytes);
> ...
> }
>
>
> I think doing something like that would be reasonable, because i expect
> a relatively high number of "classes", and I think it might be tricky to
> remember which key to use for which value type.
>
> cheers,
> Pavel
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