Added benchmark for adding jobs to default actor when visiting a tree

benchmark/single-source/AsyncTree.swift

@@ -0,0 +1,77 @@
+//===--- AsyncTree.swift -------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2021 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import TestsUtils
+import Dispatch
+
+public var benchmarks: [BenchmarkInfo] {
+  guard #available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *) else {
+    return []
+  }
+  return [
+    BenchmarkInfo(
+      name: "AsyncTree.100",
+      runFunction: run_AsyncTree(treeSize: 100),
+      tags: [.concurrency]
+    ),
+    BenchmarkInfo(
+      name: "AsyncTree.5000",
+      runFunction: run_AsyncTree(treeSize: 5000),
+      tags: [.concurrency]
+    )
+  ]
+}
+
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
+private actor MyActor {
+  let g: DispatchGroup
+
+  init(_ g: DispatchGroup) {
+    self.g = g
+  }
+
+  func test(_ n: Int) {
+    let L = n / 2
+    let R = n - 1 - L
+
+    if L > 0 {
+      Task {
+        self.test(L)
+      }
+    }
+
+    if R > 0 {
+      Task {
+        self.test(R)
+      }
+    }
+
+    g.leave()
+  }
+}
+
+@available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *)
+private func run_AsyncTree(treeSize: Int) -> (Int) -> Void {
+  return { n in
+    for _ in 0..<n {
+      let g = DispatchGroup()
+      for _ in 0..<treeSize {
+        g.enter()
+      }
+      let actor = MyActor(g)
+      Task {
+        await actor.test(treeSize)
+      }
+      g.wait()
+    }
+  }
+}