[Concurrency] Give Actor enqueue(partialTask:) its own ptrauth discriminator.

For actor class's implementations of `enqueue(partialTask:)`, use a
fixed ptrauth discriminator. Paired with the fixed vtable location,
this allows one to invoke this operation on any native actor instance
without knowing the specific type.

Author: DougGregor

include/swift/ABI/MetadataValues.h

@@ -1177,6 +1177,9 @@ namespace SpecialPointerAuthDiscriminators {
 
   /// Resilient class stub initializer callback
   const uint16_t ResilientClassStubInitCallback = 0xC671;
+
+  /// Actor enqueue(partialTask:).
+  const uint16_t ActorEnqueuePartialTask = 0x8f3d;
 }
 
 /// The number of arguments that will be passed directly to a generic

lib/IRGen/GenPointerAuth.cpp

@@ -400,6 +400,17 @@ PointerAuthEntity::getDeclDiscriminator(IRGenModule &IGM) const {
     assert(!constant.isForeign &&
            "discriminator for foreign declaration not supported yet!");
 
+    // Special case: methods that are witnesses to Actor.enqueue(partialTask:)
+    // have their own descriminator, which is shared across all actor classes.
+    if (constant.hasFuncDecl()) {
+      auto func = dyn_cast<FuncDecl>(constant.getFuncDecl());
+      if (func->isActorEnqueuePartialTaskWitness()) {
+        cache = IGM.getSize(
+            Size(SpecialPointerAuthDiscriminators::ActorEnqueuePartialTask));
+        return cache;
+      }
+    }
+
     auto mangling = constant.mangle();
     cache = getDiscriminatorForString(IGM, mangling);
     return cache;

test/IRGen/ptrauth-actor.swift

@@ -0,0 +1,24 @@
+// RUN: %swift -swift-version 5 -target arm64e-apple-macos11.0 -parse-stdlib %s -emit-ir -disable-llvm-optzns -enable-experimental-concurrency -o - | %FileCheck %s
+
+// REQUIRES: CODEGENERATOR=ARM
+// REQUIRES: concurrency
+// REQUIRES: CPU=arm64e
+// REQUIRES: OS=macosx
+
+import _Concurrency
+import Swift
+
+public actor class A1 {
+  var x: Int = 17
+}
+
+open actor class A3<T>: Actor {
+  open func f() { }
+}
+
+// enqueue(partialTask:) has the same discriminator across all classes.
+// CHECK: s4main2A1C7enqueue11partialTasky12_Concurrency012PartialAsyncE0V_tF.ptrauth{{.*}}i64 36669
+
+// CHECK: @"$s4main2A3CyxG12_Concurrency5ActorAaeFP7enqueue11partialTaskyAE012PartialAsyncG0V_tFTW"
+// CHECK-NOT: ret void
+// CHECK: call i64 @llvm.ptrauth.blend.i64(i64 %{{[0-9]+}}, i64 36669)