[Lldb-commits] [PATCH] D96778: [lldb] Fix handling of `DW_AT_decl_file` according to D91014

Sat Feb 20 14:00:34 PST 2021

jankratochvil added a comment.

In D96778#2576927 <https://reviews.llvm.org/D96778#2576927>, @dblaikie wrote:

> What part of DWZ is specified by DWARFv5?

`DW_*_sup` being used by DWZ with DWARF-4 as `DW_*_GNU_*_alt` (and dwarfstd.org proposal <https://webcache.googleusercontent.com/search?q=cache:qSMqeKD7ej0J:www.dwarfstd.org/ShowIssue.php%3Fissue%3D120604.1+&cd=2&hl=en&ct=clnk&gl=cz&lr=lang_en> having it as `DW_*_alt`) for DWZ common files located in `/usr/lib/debug/.dwz/` where each file is for one rpm package sharing DIEs to multiple *.debug files of the same rpm package.
That is probably all but I cannot check more as www.dwarfstd.org <http://www.dwarfstd.org> is down now.

> (perhaps the original creation of partial units was motivated by DWZ?

Rather DWZ was motivated by partial units IMO.

> Which extensions would those be?

Those `DW_*_alt`/`DW_*_sup` ones.

> At a glance when looking at the small dwz example earlier I didn't notice any DWARF extensions.

This example is not using the DWZ common file (that makes sense only for >=2 `*.debug` files). Still `DW_TAG_partial_unit`, `DW_AT_import` etc. is used only by DWZ and AFAIK no other tool so it needs extra implementation in all the tools incl. LLDB.

> I'm not sure I follow the description here - when generating Clang ASTs from some DIE in LLDB, one would still have to potentially pass through multiple CUs to retrieve all the relevant types, especially under any form of LTO - are there particular features of DWZ that make this more/differently problematic?

If you have a reference to DIE then LLDB tries to ask for `DW_AT_language` of its CU. But if the DIE is in `DW_TAG_partial_unit` (PU) there is no `DW_AT_language` and you do not know which CU did use `DW_TAG_imported_unit->DW_AT_import` for that PU. Moreover the `DW_TAG_partial_unit` can be imported by two different CUs one having `DW_AT_language` as C and the other as C++. That is IMO a bug, each PU should have its `DW_AT_language` and then PU with different language would be separate.

Still such format change would not help much as `DWARFASTParser`, `CompilerDeclContext` and similar stuff needs to come from the parent CU as otherwise LLDB all breaks down when some DIEs (from `DW_AT_partial_unit` possibly even from DWZ common file) come from a different AST.

Repository:
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  https://reviews.llvm.org/D96778/new/

https://reviews.llvm.org/D96778