[PATCH] D51741: [coro]Pass rvalue reference for named local variable to return_value

Tanoy Sinha via Phabricator via cfe-commits cfe-commits at lists.llvm.org
Thu Sep 6 11:07:37 PDT 2018


tks2103 created this revision.
tks2103 added reviewers: modocache, GorNishanov.
Herald added a subscriber: cfe-commits.
tks2103 edited the summary of this revision.

Addressing https://bugs.llvm.org/show_bug.cgi?id=37265.

Implements [class.copy]/33 of coroutines TS

When the criteria for elision of a copy/move operation are met, but not
for an exception-declaration, and the object to be copied is designated by an lvalue, or when the
expression in a return or co_return statement is a (possibly parenthesized) id-expression that
names an object with automatic storage duration declared in the body or
parameter-declaration-clause of the innermost enclosing function or lambda-expression, overload resolution to select
the constructor for the copy or the return_value overload to call is first performed as if the object
were designated by an rvalue.


Repository:
  rC Clang

https://reviews.llvm.org/D51741

Files:
  lib/Sema/SemaCoroutine.cpp
  test/SemaCXX/coroutine-rvo.cpp


Index: test/SemaCXX/coroutine-rvo.cpp
===================================================================
--- /dev/null
+++ test/SemaCXX/coroutine-rvo.cpp
@@ -0,0 +1,69 @@
+// RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -stdlib=libc++ -std=c++1z -fcoroutines-ts -fsyntax-only
+
+namespace std::experimental {
+template <class Promise = void> struct coroutine_handle {
+  coroutine_handle() = default;
+  static coroutine_handle from_address(void *) noexcept;
+};
+
+template <> struct coroutine_handle<void> {
+  static coroutine_handle from_address(void *) noexcept;
+  coroutine_handle() = default;
+  template <class PromiseType>
+  coroutine_handle(coroutine_handle<PromiseType>) noexcept;
+};
+
+template <class... Args>
+struct void_t_imp {
+  using type = void;
+};
+template <class... Args>
+using void_t = typename void_t_imp<Args...>::type;
+
+template <class T, class = void>
+struct traits_sfinae_base {};
+
+template <class T>
+struct traits_sfinae_base<T, void_t<typename T::promise_type>> {
+  using promise_type = typename T::promise_type;
+};
+
+template <class Ret, class... Args>
+struct coroutine_traits : public traits_sfinae_base<Ret> {};
+}
+
+struct suspend_never {
+  bool await_ready() noexcept;
+  void await_suspend(std::experimental::coroutine_handle<>) noexcept;
+  void await_resume() noexcept;
+};
+
+struct MoveOnly {
+  MoveOnly() {};
+  MoveOnly(const MoveOnly&) = delete;
+  MoveOnly(MoveOnly&&) noexcept {};
+  ~MoveOnly() {};
+};
+
+template <typename T>
+struct task {
+  struct promise_type {
+    auto initial_suspend() { return suspend_never{}; }
+    auto final_suspend() { return suspend_never{}; }
+    auto get_return_object() { return task{}; }
+    static void unhandled_exception() {}
+    void return_value(T&& value) {}
+  };
+};
+
+task<MoveOnly> f() {
+  MoveOnly value;
+  co_return value;
+}
+
+int main() {
+  f();
+  return 0;
+}
+
+// expected-no-diagnostics
Index: lib/Sema/SemaCoroutine.cpp
===================================================================
--- lib/Sema/SemaCoroutine.cpp
+++ lib/Sema/SemaCoroutine.cpp
@@ -841,6 +841,16 @@
     E = R.get();
   }
 
+  // Move the return value if we can
+  if (E) {
+    InitializedEntity Entity =
+        InitializedEntity::InitializeResult(Loc, E->getType(), false);
+    ExprResult MoveResult =
+        this->PerformMoveOrCopyInitialization(Entity, nullptr, E->getType(), E);
+    if (MoveResult.get())
+      E = MoveResult.get();
+  }
+
   // FIXME: If the operand is a reference to a variable that's about to go out
   // of scope, we should treat the operand as an xvalue for this overload
   // resolution.


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