[Clang][LLVM][Coroutines] Prevent __coro_gro from outliving __promise

clang/lib/CodeGen/CGCoroutine.cpp

@@ -535,6 +535,11 @@ struct GetReturnObjectManager {
     Builder.CreateStore(Builder.getFalse(), GroActiveFlag);
 
     GroEmission = CGF.EmitAutoVarAlloca(*GroVarDecl);
+    auto *GroAlloca = dyn_cast_or_null<llvm::AllocaInst>(
+        GroEmission.getOriginalAllocatedAddress().getPointer());
+    assert(GroAlloca && "expected alloca to be emitted");
+    GroAlloca->setMetadata(llvm::LLVMContext::MD_coro_outside_frame,
+                           llvm::MDNode::get(CGF.CGM.getLLVMContext(), {}));
 
     // Remember the top of EHStack before emitting the cleanup.
     auto old_top = CGF.EHStack.stable_begin();

clang/test/CodeGenCoroutines/coro-gro.cpp

@@ -30,12 +30,13 @@ void doSomething() noexcept;
 int f() {
   // CHECK: %[[RetVal:.+]] = alloca i32
   // CHECK: %[[GroActive:.+]] = alloca i1
+  // CHECK: %[[CoroGro:.+]] = alloca %struct.GroType, {{.*}} !coro.outside.frame ![[OutFrameMetadata:.+]]
 
   // CHECK: %[[Size:.+]] = call i64 @llvm.coro.size.i64()
   // CHECK: call noalias noundef nonnull ptr @_Znwm(i64 noundef %[[Size]])
   // CHECK: store i1 false, ptr %[[GroActive]]
   // CHECK: call void @_ZNSt16coroutine_traitsIiJEE12promise_typeC1Ev(
-  // CHECK: call void @_ZNSt16coroutine_traitsIiJEE12promise_type17get_return_objectEv(
+  // CHECK: call void @_ZNSt16coroutine_traitsIiJEE12promise_type17get_return_objectEv({{.*}} %[[CoroGro]]
   // CHECK: store i1 true, ptr %[[GroActive]]
 
   Cleanup cleanup;
@@ -104,3 +105,5 @@ invoker g() {
   // CHECK: call void @_ZN7invoker15invoker_promise17get_return_objectEv({{.*}} %[[AggRes]]
   co_return;
 }
+
+// CHECK: ![[OutFrameMetadata]] = !{}

llvm/docs/LangRef.rst

@@ -6691,6 +6691,22 @@ sections that the user does not want removed after linking.
   ...
   !0 = !{}
 
+'``coro.outside.frame``' Metadata
+^^^^^^^^^^^^^^^^^^^^^^
+
+``coro.outside.frame`` metadata may be attached to an alloca instruction to
+to signify that it shouldn't be promoted to the coroutine frame, useful for
+filtering allocas out by the frontend when emitting internal control mechanisms.
+Additionally, this metadata is only used as a flag, so the associated
+node must be empty.
+
+.. code-block:: text
+
+  %__coro_gro = alloca %struct.GroType, align 1, !coro.outside.frame !0
+
+  ...
+  !0 = !{}
+
 '``unpredictable``' Metadata
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

llvm/include/llvm/IR/FixedMetadataKinds.def

@@ -50,3 +50,4 @@ LLVM_FIXED_MD_KIND(MD_callsite, "callsite", 35)
 LLVM_FIXED_MD_KIND(MD_kcfi_type, "kcfi_type", 36)
 LLVM_FIXED_MD_KIND(MD_pcsections, "pcsections", 37)
 LLVM_FIXED_MD_KIND(MD_DIAssignID, "DIAssignID", 38)
+LLVM_FIXED_MD_KIND(MD_coro_outside_frame, "coro.outside.frame", 39)

llvm/lib/Transforms/Coroutines/CoroFrame.cpp

@@ -2804,6 +2804,11 @@ static void collectFrameAlloca(AllocaInst *AI, coro::Shape &Shape,
   if (AI == Shape.SwitchLowering.PromiseAlloca)
     return;
 
+  // The __coro_gro alloca should outlive the promise, make sure we
+  // keep it outside the frame.
+  if (MDNode *MD = AI->getMetadata(LLVMContext::MD_coro_outside_frame))
+    return;
+
   // The code that uses lifetime.start intrinsic does not work for functions
   // with loops without exit. Disable it on ABIs we know to generate such
   // code.