Add a default-actor bit to actor class descriptors.

Author: rjmccall

include/swift/ABI/Metadata.h

@@ -4288,6 +4288,10 @@ class TargetClassDescriptor final
     return FieldOffsetVectorOffset;
   }
 
+  bool isDefaultActor() const {
+    return this->getTypeContextDescriptorFlags().class_isDefaultActor();
+  }
+
   bool hasVTable() const {
     return this->getTypeContextDescriptorFlags().class_hasVTable();
   }

include/swift/ABI/MetadataValues.h

@@ -1385,6 +1385,14 @@ class TypeContextDescriptorFlags : public FlagSet<uint16_t> {
 
     // Type-specific flags:
 
+    /// Set if the class is a default actor class.  Note that this is
+    /// based on the best knowledge available to the class; actor
+    /// classes with resilient superclassess might be default actors
+    /// without knowing it.
+    ///
+    /// Only meaningful for class descriptors.
+    Class_IsDefaultActor = 8,
+
     /// The kind of reference that this class makes to its resilient superclass
     /// descriptor.  A TypeReferenceKind.
     ///
@@ -1464,6 +1472,9 @@ class TypeContextDescriptorFlags : public FlagSet<uint16_t> {
   FLAGSET_DEFINE_FLAG_ACCESSORS(Class_AreImmediateMembersNegative,
                                 class_areImmediateMembersNegative,
                                 class_setAreImmediateMembersNegative)
+  FLAGSET_DEFINE_FLAG_ACCESSORS(Class_IsDefaultActor,
+                                class_isDefaultActor,
+                                class_setIsDefaultActor)
 
   FLAGSET_DEFINE_FIELD_ACCESSORS(Class_ResilientSuperclassReferenceKind,
                                  Class_ResilientSuperclassReferenceKind_width,

lib/IRGen/GenMeta.cpp

@@ -1619,6 +1619,10 @@ namespace {
 
         if (MetadataLayout->hasResilientSuperclass())
           flags.class_setHasResilientSuperclass(true);
+
+        if (getType()->isDefaultActor(IGM.getSwiftModule(),
+                                      ResilienceExpansion::Maximal))
+          flags.class_setIsDefaultActor(true);
       }
 
       if (ResilientSuperClassRef) {

test/IRGen/async/Inputs/resilient_actor.swift

@@ -0,0 +1,13 @@
+open actor ResilientBaseActor {
+  public init() {}
+}
+
+@_fixed_layout
+open actor FixedSubclassOfResilientBaseActor : ResilientBaseActor {
+  public override init() {}
+}
+
+@_fixed_layout
+open actor FixedBaseActor {
+  public init() {}
+}

test/IRGen/async/default_actor.swift

@@ -0,0 +1,46 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-module -enable-experimental-concurrency -enable-library-evolution -emit-module-path=%t/resilient_actor.swiftmodule -module-name=resilient_actor %S/Inputs/resilient_actor.swift
+// RUN: %target-swift-frontend -I %t -emit-ir -enable-experimental-concurrency -enable-library-evolution %s | %IRGenFileCheck %s
+// REQUIRES: concurrency
+
+// CHECK: @"$s13default_actor1ACMn" = hidden constant
+//   0x81000050: 0x01000000 IsDefaultActor
+// CHECK-SAME: i32 -2130706352,
+
+// CHECK: @"$s13default_actor1BCMn" = hidden constant
+//   0x62010050: 0x02000000 IndirectTypeDescriptor + 0x01000000 IsDefaultActor
+// CHECK-SAME: i32 1644232784,
+
+// CHECK: @"$s13default_actor1CCMn" = hidden constant
+//   0x62010050: 0x02000000 IndirectTypeDescriptor + 0x01000000 IsDefaultActor
+// CHECK-SAME: i32 1644232784,
+
+// CHECK: @"$s13default_actor1DCMn" = hidden constant
+//   0x63010050: 0x02000000 IndirectTypeDescriptor + 0x01000000 IsDefaultActor
+// CHECK-SAME: i32 1661010000,
+
+import resilient_actor
+
+// CHECK-LABEL: define hidden swiftcc void @"$s13default_actor1ACfD"(%T13default_actor1AC* swiftself %0)
+// CHECK-NOT: ret void
+// CHECK:     call swiftcc void @swift_deallocObject(
+// CHECK:     ret void
+actor A {}
+
+// CHECK-LABEL: define hidden swiftcc void @"$s13default_actor1BCfD"(%T13default_actor1BC* swiftself %0)
+// CHECK-NOT: ret void
+// CHECK:     call swiftcc void @swift_deallocObject(
+// CHECK:     ret void
+actor B : ResilientBaseActor {}
+
+// CHECK-LABEL: define hidden swiftcc void @"$s13default_actor1CCfD"(%T13default_actor1CC* swiftself %0)
+// CHECK-NOT: ret void
+// CHECK:     call swiftcc void @swift_deallocObject(
+// CHECK:     ret void
+actor C : FixedSubclassOfResilientBaseActor {}
+
+// CHECK-LABEL: define hidden swiftcc void @"$s13default_actor1DCfD"(%T13default_actor1DC* swiftself %0)
+// CHECK-NOT: ret void
+// CHECK:     call swiftcc void @swift_deallocObject(
+// CHECK:     ret void
+actor D : FixedBaseActor {}