[all-commits] [llvm/llvm-project] 558db7: [LLDB] Devirtualize coroutine promise types for `s...

[Adrian Vogelsgesang via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 558db7787005348e2efaabb628ec36f1c461a741
      https://github.com/llvm/llvm-project/commit/558db7787005348e2efaabb628ec36f1c461a741
  Author: Adrian Vogelsgesang <avogelsgesang at salesforce.com>
  Date:   2022-11-11 (Fri, 11 Nov 2022)

  Changed paths:
    M lldb/source/Plugins/Language/CPlusPlus/Coroutines.cpp
    M lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/coroutine_handle/TestCoroutineHandle.py
    M lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/coroutine_handle/main.cpp

  Log Message:
  -----------
  [LLDB] Devirtualize coroutine promise types for `std::coroutine_handle`

This commit teaches the `std::coroutine_handle` pretty-printer to
devirtualize type-erased promise types. This is particularly useful to
resonstruct call stacks, either of asynchronous control flow or of
recursive invocations of `std::generator`. For the example recently
introduced by https://reviews.llvm.org/D132451, printing the `__promise`
variable now shows

```
(std::__coroutine_traits_sfinae<task, void>::promise_type) __promise = {
  continuation = coro frame = 0x555555562430 {
    resume = 0x0000555555556310 (a.out`task detail::chain_fn<1>() at llvm-nested-example.cpp:66)
    destroy = 0x0000555555556700 (a.out`task detail::chain_fn<1>() at llvm-nested-example.cpp:66)
    promise = {
      continuation = coro frame = 0x5555555623e0 {
        resume = 0x0000555555557070 (a.out`task detail::chain_fn<2>() at llvm-nested-example.cpp:66)
        destroy = 0x0000555555557460 (a.out`task detail::chain_fn<2>() at llvm-nested-example.cpp:66)
        promise = {
          ...
        }
      }
      result = 0
    }
  }
  result = 0
}
```

(shortened to keep the commit message readable) instead of

```
(std::__coroutine_traits_sfinae<task, void>::promise_type) __promise = {
  continuation = coro frame = 0x555555562430 {
    resume = 0x0000555555556310 (a.out`task detail::chain_fn<1>() at llvm-nested-example.cpp:66)
    destroy = 0x0000555555556700 (a.out`task detail::chain_fn<1>() at llvm-nested-example.cpp:66)
  }
  result = 0
}
```

Note how the new debug output reveals the complete asynchronous call
stack: our own function resumes `chain_fn<1>` which in turn will resume
`chain_fn<2>` and so on. Thereby this change allows users of lldb to
inspect the logical coroutine call stack without using any custom debug
scripts (although the display is still a bit clumsy. It would be nicer
to also integrate this into lldb's backtrace feature, but I don't know
how to do so)

The devirtualization currently works by introspecting the function
pointed to by the `destroy` pointer. (The `resume` pointer is not worth
much, given that for the final suspend point `resume` is set to a
nullptr. We have to use the `destroy` pointer instead.) We then look
for a `__promise` variable inside the `destroy` function. This
`__promise` variable is synthetically generated by LLVM, and looking at
its type reveals the type-erased promise_type.

This approach only works for clang-generated code, though. While gcc
also adds a `_Coro_promise` variable to the `resume` function, it does
not do so for the `destroy` function. However, we can't use the `resume`
function, as it will be reset to a nullptr at the final suspension
point. For the time being, I am happy with de-virtualization only working
for clang. A follow-up commit will further improve devirtualization and
also expose the variables spilled to the coroutine frame. As part of
this, I will also revisit gcc support.

Differential Revision: https://reviews.llvm.org/D132624