labath added a comment.
In D144181#4142295 <https://reviews.llvm.org/D144181#4142295>, @Michael137
wrote:
> We thought a bit about what it would take to link a constructor declaration
> DIE to the various definitions (e.g., via a
> `DW_AT_LLVM_complete_ctor_linkage_name` or `DW_AT_LLVM_complete_ctor_ref`).
> The issue with this is that it would mess with type uniquing. E.g., what if
> two different CUs each had a constructor declaration but differed only in
> which definitions they linked to? Even if dsymutil were to be able to merge
> the declarations into a single one, LLDB would still have to support the case
> where two constructor DIEs for the same type point to different definitions
> depending on how they were used in the corresponding object files.
>
> One could instead have some new attribute on the various constructor
> definitions specifying which constructor type it is, and then implement
> @pavel's suggestion of attaching multiple `asm` labels to the
> `CXXConstructorDecl`. To support that in LLDB we'd have to do a lookup in the
> DWARF index and filter appropriately.
What about doing both? Have the declaration DIE specify the linkage names of
both structor types (regardless of whether they are used in a particular CU --
just like with "normal" functions), and then have the definition DIEs specify
which flavour of the structor are they defining?
> But it's unclear to me whether this is much better than the proposed patch. I
> pinged the libcxx people regarding their policy of using abi-tags on
> structures/namespaces
Yea.. I don't know. I don't really like it because it's still very much a
secret clang-lldb handshake that will not work with other compilers or
debuggers. But maybe it's better because it's not a three-way libc++-clang-lldb
handshake and has a chance of working with libraries using abi tags in a
different way than libc++?
I'm just the peanut gallery here, so feel free to overrule me.
> Some more findings on the Clang side: a very naive way to support multiple
> `asm` labels is to adjust the `ManglingContext::mangleName` to be aware of
> multiple `asm` labels and pick the correct one based on the `GlobalDecl`
> we're mangling. But we'd need to somehow match `CtorType` to the
> `AsmLabelAttr` in question. Which is possibly doable by encoding the
> `CtorType` into the label. E.g., `asm("C2<mangled_name>")` (?) Though that
> seems hacky
A bit, though if the code change itself is simple/localized, then we might be
able to get away with a new attribute, or an extension of the existing one
(`A() __attribute((full_structor_asm("fooC1"), base_structor_asm("fooC2"))` or
something -- it's not like people will be actually typing this code anyway...)
Repository:
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https://reviews.llvm.org/D144181/new/
https://reviews.llvm.org/D144181
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