================
@@ -9270,6 +9270,93 @@ Example:
}];
}
+def CoroAwaitSuspendDestroyDoc : Documentation {
+ let Category = DocCatDecl;
+ let Content = [{
+
+The ``[[clang::coro_await_suspend_destroy]]`` attribute may be applied to a C++
+coroutine awaiter type. When this attribute is present, the awaiter must
+implement ``void await_suspend_destroy(Promise&)``. If ``await_ready()``
+returns ``false`` at a suspension point, ``await_suspend_destroy`` will be
+called directly, bypassing the ``await_suspend(std::coroutine_handle<...>)``
+method. The coroutine being suspended will then be immediately destroyed.
+
+Logically, the new behavior is equivalent to this standard code:
+
+.. code-block:: c++
+
+ void await_suspend_destroy(YourPromise&) { ... }
+ void await_suspend(auto handle) {
+ await_suspend_destroy(handle.promise());
+ handle.destroy();
+ }
+
+This enables `await_suspend_destroy()` usage in portable awaiters — just add a
+stub ``await_suspend()`` as above. Without ``coro_await_suspend_destroy``
+support, the awaiter will behave nearly identically, with the only difference
+being heap allocation instead of stack allocation for the coroutine frame.
+
+This attribute exists to optimize short-circuiting coroutines—coroutines whose
+suspend points are either (i) trivial (like ``std::suspend_never``), or (ii)
+short-circuiting (like a ``co_await`` that can be expressed in regular control
+flow as):
+
+.. code-block:: c++
+
+ T val;
+ if (awaiter.await_ready()) {
+ val = awaiter.await_resume();
+ } else {
+ awaiter.await_suspend();
+ return /* value representing the "execution short-circuited" outcome */;
+ }
+
+The benefits of this attribute are:
+ - **Avoid heap allocations for coro frames**: Allocating short-circuiting
----------------
snarkmaster wrote:
I understand the mechanism to be this:
- If all `co_await` / `co_yield` awaiters use `await_suspend_destroy()`, that
leaves the coro with just 2 suspend points: initial & final.
- If those suspend points have `await_ready() { return true; }` like
`suspend_never`, the remaining two suspends get cut out somewhere in the middle
end.
- At that point, there are no suspend intrinsics left for the escape analysis,
and heap elision kicks in.
If we were able to elide `suspend_never` earlier, I suspect that
short-circuiting coros could optimize even more like plain functions.
Can you please clarify how you'd like me to edit this portion of the **docs**?
My reasoning seems a bit too low-level to be user-relevant. I feel like if a
user cares about allocs, they would just look at the generated code for their
application...
https://github.com/llvm/llvm-project/pull/152623
_______________________________________________
cfe-commits mailing list
cfe-commits@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
