avogelsgesang added inline comments.
================
Comment at: lldb/source/Plugins/Language/CPlusPlus/Coroutines.cpp:246-248
+ DataExtractor data(&promise_addr, sizeof(promise_addr),
+ process_sp->GetByteOrder(),
+ process_sp->GetAddressByteSize());
----------------
labath wrote:
> avogelsgesang wrote:
> > labath wrote:
> > > Have you checked there won't be a use-after-free problem here, given that
> > > this data extractor will refer to the stack object?
> > >
> > > To create persistent data, you need to use the DataBufferSP constructor,
> > > but I'm wondering if we couldn't fix this by creating the (non-pointer)
> > > object using the `CreateValueObjectFromAddress` function, as above, but
> > > then actually use valobj->AddressOf as the synthetic child.
> > >
> > > I am also somewhat surprised that we need to use the GetAddressOf trick
> > > here, as this seems to indicate that the coroutine contains (in the
> > > proper C "subobject" kind of way) the promise object. That's not
> > > necessarily wrong, but it makes me think we may be "breaking the cycle"
> > > at the wrong place.
> > Thanks for taking a look!
> >
> > > To create persistent data, you need to use the DataBufferSP constructor
> >
> > good point, I will keep this in mind as a fallback in case we don't decide
> > to follow any of the other directions you hinted at.
> >
> > > wondering if we couldn't fix this by creating the (non-pointer) object
> > > using the CreateValueObjectFromAddress function, as above, but then
> > > actually use valobj->AddressOf as the synthetic child
> >
> > Good idea! I will give it a try
> >
> >
> > > [...] as this seems to indicate that the coroutine contains (in the
> > > proper C "subobject" kind of way) the promise object. That's not
> > > necessarily wrong, but it makes me think we may be "breaking the cycle"
> > > at the wrong place.
> >
> > The physical layout of this is:
> > ```
> > // in the standard library
> > template<typename promise_type>
> > struct exception_handle<promise_type> {
> > __coro_frame<promise_type>* __hdl; // <--- this is the pointer we read
> > with `GetCoroFramePtrFromHandle`
> > };
> >
> > // compiler-generated coroutine frame. Generated ad-hoc per coroutine
> > struct __coro_frame<promise_type> {
> > // The ABI guaranteees that hose two pointers are always the first two
> > pointers in the struct.
> > void (*resume)(void*); // function pointer for type erasure
> > void (*destroy)(void*); // function pointer for type erasure
> > // Next comes our promise type. This is under the control of the
> > program author
> > promise_type promise;
> > // Next comes any compiler-generated, internal state which gets
> > persisted across suspension points.
> > // The functions pointed to by `resume`/`destroy` know how to
> > interpret this part of the coroutine frame.
> > int __suspension_point_id;
> > double __some_internal_state;
> > std::string __some_other_internal_state;
> > ....
> > };
> > ```
> >
> > The programmer (i.e., most likely the user of this pretty-printer), wrote
> > only the "promise" explicitly in his code. Everything else is
> > compiler-generated. As such, the lldb-user will usually look for the
> > "promise" first, and I would like to make it easy to find it, by exposing
> > it as a top-level children of the `exception_handle` instead of hiding it
> > inside a sub-child.
> > As such, the lldb-user will usually look for the "promise" first, and I
> > would like to make it easy to find it, by exposing it as a top-level
> > children of the `exception_handle` instead of hiding it inside a sub-child.
>
> That makes sense. And I think that's a good argument for automatically
> "dereferencing" that object (i.e., status quo). That said, it's not fully
> clear to me how do we end up looping here. I take it the promise object
> contains a (compiler-generated ?) pointer to another __coro_frame object?
> What would happen if we turned *that* into a pointer?
> I take it the promise object contains a [...] pointer to another __coro_frame
> object?
yes
> (compiler-generated ?)
no
We end up looping if the user explicitly puts an `coroutine_handle` inside
`promise_type`. You can find an example of this in
https://clang.llvm.org/docs/DebuggingCoroutines.html#get-the-asynchronous-stack,
in particular
```
struct promise_type {
// [...] shortened for readability
std::coroutine_handle<> continuation = std::noop_coroutine();
int result = 0;
};
```
In asynchronous programming, it is common to "chain continuations" by putting a
`coroutine_handle` into the `promise_type`. This coroutine_handle inside the
promise_type gives my asynchronous coroutine the answer to the question "where
should I continue next, after finishing the asychronous operation modelled by
my own coroutine?".
In normal situations (i.e. in the absence of bugs inside the debugged program),
those coroutine_handle's should not form cycles. But I guess there could also
be other use cases for coroutines where coroutine_handle cycles might be
useful...
> What would happen if we turned *that* into a pointer?
The `promise_type` is a user-written struct, so I guess that I have little
leverage whether it contains a `coroutine_handle` by value or by pointer? And
turning an object which the user wrote as "by value" into a "by pointer"
representation in the debugger would be pretty surprising...
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D132815/new/
https://reviews.llvm.org/D132815
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