[llvm-dev] RFC: Add DWARF support for yaml2obj (original) (raw)

James Henderson via llvm-dev llvm-dev at lists.llvm.org
Fri May 22 03:28:06 PDT 2020

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 "DWATdeclfile" shall be "Str"? Can we map the values > of different types into same field? > > """ > - Attr: DWATdeclfile > 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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