[coroutines] Skip over passthrough operator co_await

[GorNishanov]

gentle ping

[rsmith]

Does it still make sense for us to have a UO_Coawait at all? As I recall, the only purpose it ever had was to represent a dependent co_await expression that couldn't yet be resolved to a CoawaitExpr. But now we have (and need!) a separate DependentCoawaitExpr node to store unqualified lookup results, it seems that the UnaryOperator portion of the representation serves no purpose (and as seen in this patch, it's getting in the way). Can we remove it?

Anyway, this change LGTM for now.

lib/CodeGen/CGCoroutine.cpp

146

paththrough -> passthrough

148–149

Indent.

[GorNishanov]

In D31627#761320, @rsmith wrote:

Does it still make sense for us to have a UO_Coawait at all? As I recall, the only purpose it ever had was to represent a dependent co_await expression that couldn't yet be resolved to a CoawaitExpr. But now we have (and need!) a separate DependentCoawaitExpr node to store unqualified lookup results, it seems that the UnaryOperator portion of the representation serves no purpose (and as seen in this patch, it's getting in the way). Can we remove it?

Anyway, this change LGTM for now.

Thank you for the review!
I'll see if I can remove UO_Coawait in a separate patch

[GorNishanov]

r303605 = b026b234d55d6b9fab849389b2085204f530af5d

lib/CodeGen/CGCoroutine.cpp

 //
 //  See llvm's docs/Coroutines.rst for more details.
 //
 static RValue emitSuspendExpression(CodeGenFunction &CGF, CGCoroData &Coro,
                                     CoroutineSuspendExpr const &S,
                                     AwaitKind Kind, AggValueSlot aggSlot,
                                     bool ignoreResult) {
   auto *E = S.getCommonExpr();
+
+  // Skip paththrough operator co_await (present when awaiting on an LValue).
+  if (auto *UO = dyn_cast<UnaryOperator>(E))
+  if (UO->getOpcode() == UO_Coawait)
+    E = UO->getSubExpr();
+
   auto Binder =
       CodeGenFunction::OpaqueValueMappingData::bind(CGF, S.getOpaqueValue(), E);
   auto UnbindOnExit = llvm::make_scope_exit([&] { Binder.unbind(CGF); });
 
   auto Prefix = buildSuspendPrefixStr(Coro, Kind);
   BasicBlock *ReadyBlock = CGF.createBasicBlock(Prefix + Twine(".ready"));
   BasicBlock *SuspendBlock = CGF.createBasicBlock(Prefix + Twine(".suspend"));
   BasicBlock *CleanupBlock = CGF.createBasicBlock(Prefix + Twine(".cleanup"));

test/CodeGenCoroutines/coro-await.cpp

extern "C" void TestOpAwait() {
   co_await MyInt(42);
   // CHECK: call void @_Zaw5MyInt(i32 42)
   // CHECK: call i32 @_ZN13ScalarAwaiter12await_resumeEv(%struct.ScalarAwaiter* %
 
   co_await MyAgg{};
   // CHECK: call void @_ZN5MyAggawEv(%struct.MyAgg* %
   // CHECK: call void @_ZN11AggrAwaiter12await_resumeEv(%struct.Aggr* sret %
 }
+
+// Verifies that we don't crash when awaiting on an lvalue.
+void AwaitLValue() {
+  suspend_always lval;
+  co_await lval;
+}