[libcxx-commits] [clang] [libcxx] Elide suspension points via [[clang::coro_await_suspend_destroy]] (PR #152623)

[Chuanqi Xu via libcxx-commits]

================
@@ -401,36 +432,85 @@ static ReadySuspendResumeResult buildCoawaitCalls(Sema &S, VarDecl *CoroPromise,
     return Calls;
   }
   Expr *CoroHandle = CoroHandleRes.get();
+  Calls.UseAwaitSuspendDestroy = false;
   CallExpr *AwaitSuspend = cast_or_null<CallExpr>(
       BuildSubExpr(ACT::ACT_Suspend, "await_suspend", CoroHandle));
   if (!AwaitSuspend)
     return Calls;
+
+  // When this `await_suspend()` overload is annotated with
+  // `[[clang::coro_await_suspend_destroy]]`, do NOT call `await_suspend()` --
+  // instead call `await_suspend_destroy(Promise&)`.  This assumes that the
+  // `await_suspend()` is just a compatibility stub consisting of:
+  //     await_suspend_destroy(handle.promise());
+  //     handle.destroy();
+  // Users of the attribute must follow this contract.  Then, diagnostics from
+  // both `await_suspend` and `await_suspend_destroy` will get exposed.
+  CallExpr *PlainAwaitSuspend = nullptr;
+  if (FunctionDecl *AwaitSuspendCallee = AwaitSuspend->getDirectCallee()) {
+    if (AwaitSuspendCallee->hasAttr<CoroAwaitSuspendDestroyAttr>()) {
+      Calls.UseAwaitSuspendDestroy = true;
+      ExprResult PromiseRefRes =
+          buildPromiseRef(S, CoroPromise->getType(), Loc);
+      if (PromiseRefRes.isInvalid()) {
+        Calls.IsInvalid = true;
+        return Calls;
+      }
+      Expr *PromiseRef = PromiseRefRes.get();
+      PlainAwaitSuspend = AwaitSuspend;
+      AwaitSuspend = cast_or_null<CallExpr>(
+          BuildSubExpr(ACT::ACT_Suspend, "await_suspend_destroy", PromiseRef));
+      if (!AwaitSuspend)
+        return Calls;
+    }
+  }
----------------
ChuanqiXu9 wrote:

I think you get my points. 

To make this more clear: the reason why your patch can optimize some cases is, your patch skips `coro.await.suspend` intrinsics if the await_suspend function is marked as the attribute your proposed.

We can generalize this to a part:
- A semantical part. For example, the `coro_await_suspend_destroy` attribute and its semantics you proposed.
- Skip emitting `coro.await.suspend`. This is why we can optimize it. 

https://github.com/llvm/llvm-project/pull/152623