Fix quadratic performance of the ListMerger in specific usage pattern

benchmark/CMakeLists.txt

@@ -40,6 +40,7 @@ set(SWIFT_BENCH_MODULES
     single-source/ArrayRemoveAll
     single-source/ArraySetElement
     single-source/ArraySubscript
+    single-source/AsyncTree
     single-source/BinaryFloatingPointConversionFromBinaryInteger
     single-source/BinaryFloatingPointProperties
     single-source/BitCount

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()
+    }
+  }
+}

benchmark/utils/main.swift

@@ -28,6 +28,7 @@ import ArrayOfRef
 import ArrayRemoveAll
 import ArraySetElement
 import ArraySubscript
+import AsyncTree
 import BinaryFloatingPointConversionFromBinaryInteger
 import BinaryFloatingPointProperties
 import BitCount
@@ -223,6 +224,7 @@ register(ArrayOfRef.benchmarks)
 register(ArrayRemoveAll.benchmarks)
 register(ArraySetElement.benchmarks)
 register(ArraySubscript.benchmarks)
+register(AsyncTree.benchmarks)
 register(BinaryFloatingPointConversionFromBinaryInteger.benchmarks)
 register(BinaryFloatingPointProperties.benchmarks)
 register(BitCount.benchmarks)

include/swift/ABI/MetadataValues.h

@@ -2511,6 +2511,32 @@ inline int descendingPriorityOrder(JobPriority lhs,
   return (lhs == rhs ? 0 : lhs > rhs ? -1 : 1);
 }
 
+enum { PriorityBucketCount = 5 };
+
+inline int getPriorityBucketIndex(JobPriority priority) {
+  // Any unknown priorities will be rounded up to a known one.
+  // Priorities higher than UserInteractive are clamped to UserInteractive.
+  // Jobs of unknown priorities will end up in the same bucket as jobs of a
+  // corresponding known priority. Within the bucket they will be sorted in
+  // FIFO order.
+  if (priority > JobPriority::UserInitiated) {
+    // UserInteractive and higher
+    return 0;
+  } else if (priority > JobPriority::Default) {
+    // UserInitiated
+    return 1;
+  } else if (priority > JobPriority::Utility) {
+    // Default
+    return 2;
+  } else if (priority > JobPriority::Background) {
+    // Utility
+    return 3;
+  } else {
+    // Background and lower
+    return 4;
+  }
+}
+
 inline JobPriority withUserInteractivePriorityDowngrade(JobPriority priority) {
   return (priority == JobPriority::UserInteractive) ? JobPriority::UserInitiated
                                                     : priority;

include/swift/Basic/HeaderFooterLayout.h

@@ -0,0 +1,39 @@
+//===--- HeaderFooterLayout.h -----------------------------------*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2024 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SWIFT_BASIC_HEADER_FOOTER_LAYOUT_H
+#define SWIFT_BASIC_HEADER_FOOTER_LAYOUT_H
+
+namespace swift {
+
+template <class Header, class Footer, size_t TotalSize>
+struct HeaderFooterLayoutPadding {
+private:
+  enum : ptrdiff_t {
+    maxFooterOffset = TotalSize - (ptrdiff_t)sizeof(Footer),
+    footerAlignment = (ptrdiff_t)alignof(Footer),
+    footerOffset = maxFooterOffset - (maxFooterOffset % footerAlignment),
+    size = footerOffset - (ptrdiff_t)sizeof(Header)
+  };
+  char padding[size];
+};
+
+template <class Header, class Footer, size_t TotalSize>
+struct HeaderFooterLayout
+    : Header,
+      HeaderFooterLayoutPadding<Header, Footer, TotalSize>,
+      Footer {};
+
+} // namespace swift
+
+#endif // SWIFT_BASIC_HEADER_FOOTER_LAYOUT_H
+