[llvm-dev] What's the difference between llvm `resume` instruction and `__cxa_rethrow`?

[chuanqi.xcq via llvm-dev]

Hi,

   Recently I am checking the implementation of c++20 coroutine in clang and I am wondering if it is possible to mark its status as complete.
(Now the status is partial althought it's been a while that the clang's coroutine is used in production). 

   Here is the simple introduction, in the standard: https://eel.is/c++draft/dcl.fct.def.coroutine#5, it says both initial_suspend().await_ready() 
and initial_suspend().await_suspend() should be wrapped in `try` statement  and if there is an exception happens, it would be rethown by 
`throw;` statement. It may look like:

```C++
try {
   auto init_suspend = promise.init_suspend();
   if (!init_suspend.await_ready())
        init_suspend.await_suspend();
} catch (...) {
    throw;
}
```

    And the implementation didn't wrap them in `try...catch` statement. The code generated by clang would look like:

```LLVM
invoke promise.init_suspend()
    to label %cont unwind label %lpad

invoke init_suspend.await_ready()
    to label %cont1 unwind label %lpad1
invoke init_suspend.await_suspend()
    to label %cont2 unwind label %lpad2
...
%lpad:
   ...
   br label %eh.resume
%lpad1:
   ...
   br label %eh.resume
 %lpad2:
   ...
   br label %eh.resume
eh.resume:
   ....
   resume
```

   And I know that clang would generate `__cxa_rethrow` for `throw;`. I did some simple test locally, the behavior now looks 
good for me. (I could catch the exception in the caller of the coroutine). But I think it would be better to consult with the experts. 
I am wondering what's the difference and if it would be a block issue for conforming clang's implementation.

Thanks,
Chuanqi
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