On Mon, Feb 27, 2023 at 4:41 PM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
>
> On Mon, 2023-02-27 at 13:13 -0800, Andrii Nakryiko wrote:
> > On Wed, Feb 22, 2023 at 11:51 AM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
> > >
> > > On Wed, 2023-02-22 at 10:11 -0800, Alexei Starovoitov wrote:
> > > [...]
> > > > > > > What do you think about something like "debug_type_tag" or
> > > > > > > "debug_type_annotation" (and a similar update for the decl tags)?
> > > > > > > The translation into BTF records would be the same, but the DWARF info
> > > > > > > would stand on its own without being tied to BTF.
> > > > > > >
> > > > > > > (Naming is a bit tricky since terms like 'tag' are already in use by
> > > > > > > DWARF, e.g. "type tag" in the context of DWARF DIEs makes me think of
> > > > > > > DW_TAG_xxxx_type...)
> > > > > > >
> > > > > > > As far as I understand, early proposals for the tags were more generic
> > > > > > > but the LLVM reviewers wished for something more specific due to the
> > > > > > > relatively limited use of the tags at the time. Now that the tags and
> > > > > > > their DWARF format have matured I think a good case can be made to
> > > > > > > make these generic. We'd be happy to help push for such change.
> > > > > >
> > > > > > On the other hand, BTF is a thing we are using this annotation for.
> > > > > > Any other tool can reuse DW_TAG_LLVM_annotation, but it will need a
> > > > > > way to distinguish it's annotations from BTF annotations. And this can
> > > > > > be done by using a different DW_AT_name. So, it seems logical to
> > > > > > retain "btf" in the DW_AT_name. What do you think?
> > > > >
> > > > > OK I can understand keeping it BTF specific.
> > > > >
> > > > > Other than that, I don't come up with any significantly different idea
> > > > > than to use the ":v2" suffix, so let's go with "btf_type_tag:v2"?
> > > >
> > > > I don't like v2 suffix either.
> > > > Please come up with something else.
> > >
> > > Nothing particularly good comes to mind:
> > > - btf_type_tag:wrapper
> > > - btf_type_tag:outer
> > > - btf_type_tag:own
> > > - exterior_btf_type_tag
> > > - outer_btf_tag
> > > - btf_type_prefix
> > > - btf_type_qualifier (as in const/volatile)
> > >
> > > Or might as well use btf_type_tag:gcc, as you suggested earlier,
> > > but it is as confusing as the others.
> >
> > btf.type_tag or btf:type_tag or btf/type_tag (you get the idea, it's
> > "BTF scoped")?
>
> `btf/type_tag` is nice but might be somewhat confusing when DWARF is inspected:
> - both old-style and new-style tags would be present in DWARF for some
> time for backwards compatibility;
> - old-style tag has name "btf_type_tag".
old-style tag will be deprecated and removed eventually, so I'd
optimize for the new-style naming, as that's what we'll be dealing
with the most going forward
>
> Thus, the following C code:
>
> #define __tag1 __attribute__((btf_type_tag("tag1")))
> #define __tag2 __attribute__((btf_type_tag("tag2")))
>
> int __tag1 * __tag2 g;
>
> Would be encoded in DWARF as:
>
> 0x29: DW_TAG_pointer_type
> DW_AT_type (0x35 "int")
>
> 0x2e: DW_TAG_LLVM_annotation
> DW_AT_name ("btf/type_tag:")
> DW_AT_const_value ("tag2")
>
> 0x31: DW_TAG_LLVM_annotation
> DW_AT_name ("btf_type_tag")
> DW_AT_const_value ("tag1")
>
> 0x34: NULL
>
> 0x35: DW_TAG_base_type
> DW_AT_name ("int")
> DW_AT_encoding (DW_ATE_signed)
> DW_AT_byte_size (0x04)
>
> 0x39: DW_TAG_LLVM_annotation
> DW_AT_name ("btf/type_tag:")
> DW_AT_const_value ("tag1")
>
> 0x3c: NULL
>
> Which is not very helpful.
>
> In my opinion "btf_type_tag:v2" is the least confusing option, but if
> Alexei does not like it, let's use "btf_type_tag:parent" and move on.
>
> Thanks,
> Eduard
>
