[clang] [llvm] [Clang][Coroutines] Improve CoroElide with [[clang::coro_structured_concurrency]] attribute for C++ (PR #94693)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

Thanks for the patch. I like it in the very high level. I did a quick scanning over the PR  and here is some comments:

- I feel the name containing `concurrency` is not proper. I don't feel it relates concurrency in any level.
- Every time we add or change IR related to coroutines, we need to update https://llvm.org/docs/Coroutines.html. So that we can understand the semantics of the proposed `llvm.coro.safe.elide` much easier.
- I'd like to add a new effect to the attribute to always inline (or an inline hint) every such callee function. Note that this won't be part of semantics but the implementation details.

For implementations,

- It looks like the argument of `llvm.coro.safe.elide` is the object. This is not good. The object is the concept in the higher level language but not the middle end language.
- What I prefer is to add a middle end function attribute (`must-coro-elide`) and apply this attribute and (always inline attribute) to the calls:
    - In the inliner, when we inlined a call with attribute `must-coro-elide`, we can add the attribute again to inlined intrinsics `llvm.coro.id`.
    - Then in the coro elider, we can decide whether or not to elide the specific corotuines by judging if the `llvm.coro.id` has the attribute very easily.

For FE implementation,
- To implement the semantics precisely, I think we have to touch the Parser and the Sema.  Otherwise, it may be pretty tricky to handle cases like `co_await foo(bar(...))` or `co_await (foo() + bar());`. It is hacky to implement this in CodeGen.

https://github.com/llvm/llvm-project/pull/94693