[all-commits] [llvm/llvm-project] 52a9ba: [clang][DebugInfo] Revert to printing canonical ty...

[Michael Buch via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 52a9ba7ca4fb9427706c28bb3ca15f7a56eecf3f
      https://github.com/llvm/llvm-project/commit/52a9ba7ca4fb9427706c28bb3ca15f7a56eecf3f
  Author: Michael Buch <michaelbuch12 at gmail.com>
  Date:   2024-10-02 (Wed, 02 Oct 2024)

  Changed paths:
    M clang/lib/CodeGen/CGDebugInfo.cpp
    M clang/test/CodeGenCXX/debug-info-alias.cpp

  Log Message:
  -----------
  [clang][DebugInfo] Revert to printing canonical typenames for template aliases (#110767)

This was originally added in https://reviews.llvm.org/D142268 to have
LLDB display variable typenames that benefit from suppressing defaulted
template arguments.

We currently represent template aliases as `DW_AT_typedef`s instead of
`DW_TAG_template_alias`. This means for types like:
```
template <class _Tp>
using __remove_cv_t = __remove_cv(_Tp);

template <class _Tp>
using remove_cv_t = __remove_cv_t<_Tp>;

template<typename T>
class optional {
  using value_type = T;
  remove_cv_t<value_type> __val_;
}
```
we would generate DWARF like:
```
0x0000274f:       DW_TAG_typedef
                    DW_AT_type  (0x0000000000002758 "__remove_cv_t<value_type>")
                    DW_AT_name  ("remove_cv_t<value_type>")

```

This is an actual libc++ type layout introduced in
https://github.com/llvm/llvm-project/pull/110355, and uncovered a
shortcoming of LLDB's data-formatter infrastructure, where we cache
formatters on the contents of `DW_AT_name` (which currently wouldn't be
a fully resolved typename for template specializations).

To unblock the libc++ change, I think we can revert this without much
fallout.

Then we have two options for follow-up (or do both):
1. reland this but adjust the LLDB formatter cache so it doesn't cache
formatters for template specializations
2. implement support for `DW_TAG_template_alias` in LLDB (and make Clang
generate them by default).



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