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:
> 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.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D132815/new/
https://reviews.llvm.org/D132815
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