Using multiple insertion points to ensure all jobs are always inserted in O(1)

Fully separated unprocessed jobs and processed jobs
Reverse jobs after updating status to minimise contention

Author: nickolas-pohilets

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
+