[Dwarf-discuss] Coroutines

[Adrian Prantl]

> On Feb 27, 2024, at 10:32 AM, Kyle Huey <khuey@pernos.co> wrote:
> 
> On Wed, Feb 21, 2024 at 9:12 AM Adrian Prantl <aprantl@apple.com <mailto:aprantl@apple.com>> wrote:
>> Can you clarify what kind of information you are interested in? Are you talking about representing variables inside of coroutines, function pointers to coroutines, linetables for coroutines, ...?
>> 
>> LLDB's Swift plugin for example supports Swift async functions (which are effectively coroutines) but does so without adding any new extensions to DWARF. Here's a recent talk from last year's LLVM dev meeting on that topic: https://www.youtube.om/watch?v=g4fei6Vl7o8 <https://www.youtube.com/watch?v=g4fei6Vl7o8>
> 
> The async control flow parts of this (backtraces and stepping) are what I am particularly interested in. Variables seem straightforward since DWARF location expressions are already generic enough to let you point to heap allocated storage for variables that survive across yield points. Your talk is a good demo of the behavior I'd like to achieve (for Rust specifically). A few technical questions:
> 
> How do you "identify" a particular chain of funclet invocations that go together? I assume this is done based on the pointer identity of the async_context.

Each async_context has a continuation pointer pointing to the next funclet that is going to be executed when the current coroutine returns.

> And the logic to understand what the async_context is and where it's found at function entry points is hardcoded into the Swift plugin?

Yes, there is a Swift-specific unwinder plugin in LLDB that does this. There is also a custom stepping plan there.

> 
> How do you handle pointer reuse on async_contexts?

If I'm understanding the question correctly: there is a heap async_context object for every continuation that is on the schedule at any time. I don't think we can prevent a later continuation to be allocated in the same place as a previous one, but since they can never appear at the same time, this also isn't a problem.

-- adrian

> 
> - Kyle
>  
>> -- adrian
>> 
>> 
>> 
>> 
>> 
>> > On Feb 20, 2024, at 11:47 AM, Kyle Huey via Dwarf-discuss <dwarf-discuss@lists.dwarfstd.org <mailto:dwarf-discuss@lists.dwarfstd.org>> wrote:
>> > 
>> > Has anyone proposed or discussed DWARF structures to represent coroutines or similar constructs? I skimmed the open issues list and didn't see anything relevant.
>> > 
>> > - Kyle
>> 

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