--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/lld/MachO/ConcatOutputSection.h	Tue Jun 08 06:07:14 2021 +0900
@@ -0,0 +1,87 @@
+//===- ConcatOutputSection.h ------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLD_MACHO_MERGED_OUTPUT_SECTION_H
+#define LLD_MACHO_MERGED_OUTPUT_SECTION_H
+
+#include "InputSection.h"
+#include "OutputSection.h"
+#include "lld/Common/LLVM.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/MapVector.h"
+
+namespace lld {
+namespace macho {
+
+class Defined;
+
+// Linking multiple files will inevitably mean resolving sections in different
+// files that are labeled with the same segment and section name. This class
+// contains all such sections and writes the data from each section sequentially
+// in the final binary.
+class ConcatOutputSection : public OutputSection {
+public:
+  explicit ConcatOutputSection(StringRef name)
+      : OutputSection(ConcatKind, name) {}
+
+  const InputSection *firstSection() const { return inputs.front(); }
+  const InputSection *lastSection() const { return inputs.back(); }
+
+  // These accessors will only be valid after finalizing the section
+  uint64_t getSize() const override { return size; }
+  uint64_t getFileSize() const override { return fileSize; }
+
+  void addInput(InputSection *input);
+  void finalize() override;
+  bool needsThunks() const;
+  uint64_t estimateStubsInRangeVA(size_t callIdx) const;
+
+  void writeTo(uint8_t *buf) const override;
+
+  std::vector<InputSection *> inputs;
+  std::vector<InputSection *> thunks;
+
+  static bool classof(const OutputSection *sec) {
+    return sec->kind() == ConcatKind;
+  }
+
+private:
+  void mergeFlags(InputSection *input);
+
+  size_t size = 0;
+  uint64_t fileSize = 0;
+};
+
+// We maintain one ThunkInfo per real function.
+//
+// The "active thunk" is represented by the sym/isec pair that
+// turns-over during finalize(): as the call-site address advances,
+// the active thunk goes out of branch-range, and we create a new
+// thunk to take its place.
+//
+// The remaining members -- bools and counters -- apply to the
+// collection of thunks associated with the real function.
+
+struct ThunkInfo {
+  // These denote the active thunk:
+  Defined *sym = nullptr;       // private-extern symbol for active thunk
+  InputSection *isec = nullptr; // input section for active thunk
+
+  // The following values are cumulative across all thunks on this function
+  uint32_t callSiteCount = 0;  // how many calls to the real function?
+  uint32_t callSitesUsed = 0;  // how many call sites processed so-far?
+  uint32_t thunkCallCount = 0; // how many call sites went to thunk?
+  uint8_t sequence = 0;        // how many thunks created so-far?
+};
+
+extern llvm::DenseMap<Symbol *, ThunkInfo> thunkMap;
+
+} // namespace macho
+} // namespace lld
+
+#endif