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diff --git a/clang-tools-extra/clang-doc/BitcodeWriter.h b/clang-tools-extra/clang-doc/BitcodeWriter.h
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+++ b/clang-tools-extra/clang-doc/BitcodeWriter.h
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+//===--  BitcodeWriter.h - ClangDoc Bitcode Writer --------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements a writer for serializing the clang-doc internal
+// representation to LLVM bitcode. The writer takes in a stream and emits the
+// generated bitcode to that stream.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_DOC_BITCODEWRITER_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_DOC_BITCODEWRITER_H
+
+#include "Representation.h"
+#include "clang/AST/AST.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Bitcode/BitstreamWriter.h"
+#include <initializer_list>
+#include <vector>
+
+namespace clang {
+namespace doc {
+
+// Current version number of clang-doc bitcode.
+// Should be bumped when removing or changing BlockIds, RecordIds, or
+// BitCodeConstants, though they can be added without breaking it.
+static const unsigned VersionNumber = 1;
+
+struct BitCodeConstants {
+  static constexpr unsigned RecordSize = 16U;
+  static constexpr unsigned SignatureBitSize = 8U;
+  static constexpr unsigned SubblockIDSize = 4U;
+  static constexpr unsigned BoolSize = 1U;
+  static constexpr unsigned IntSize = 16U;
+  static constexpr unsigned StringLengthSize = 16U;
+  static constexpr unsigned FilenameLengthSize = 16U;
+  static constexpr unsigned LineNumberSize = 16U;
+  static constexpr unsigned ReferenceTypeSize = 8U;
+  static constexpr unsigned USRLengthSize = 6U;
+  static constexpr unsigned USRBitLengthSize = 8U;
+};
+
+// New Ids need to be added to both the enum here and the relevant IdNameMap in
+// the implementation file.
+enum BlockId {
+  BI_VERSION_BLOCK_ID = llvm::bitc::FIRST_APPLICATION_BLOCKID,
+  BI_NAMESPACE_BLOCK_ID,
+  BI_ENUM_BLOCK_ID,
+  BI_TYPE_BLOCK_ID,
+  BI_FIELD_TYPE_BLOCK_ID,
+  BI_MEMBER_TYPE_BLOCK_ID,
+  BI_RECORD_BLOCK_ID,
+  BI_FUNCTION_BLOCK_ID,
+  BI_COMMENT_BLOCK_ID,
+  BI_FIRST = BI_VERSION_BLOCK_ID,
+  BI_LAST = BI_COMMENT_BLOCK_ID
+};
+
+// New Ids need to be added to the enum here, and to the relevant IdNameMap and
+// initialization list in the implementation file.
+#define INFORECORDS(X) X##_USR, X##_NAME, X##_NAMESPACE
+
+enum RecordId {
+  VERSION = 1,
+  INFORECORDS(FUNCTION),
+  FUNCTION_DEFLOCATION,
+  FUNCTION_LOCATION,
+  FUNCTION_PARENT,
+  FUNCTION_ACCESS,
+  FUNCTION_IS_METHOD,
+  COMMENT_KIND,
+  COMMENT_TEXT,
+  COMMENT_NAME,
+  COMMENT_DIRECTION,
+  COMMENT_PARAMNAME,
+  COMMENT_CLOSENAME,
+  COMMENT_SELFCLOSING,
+  COMMENT_EXPLICIT,
+  COMMENT_ATTRKEY,
+  COMMENT_ATTRVAL,
+  COMMENT_ARG,
+  TYPE_REF,
+  FIELD_TYPE_REF,
+  FIELD_TYPE_NAME,
+  MEMBER_TYPE_REF,
+  MEMBER_TYPE_NAME,
+  MEMBER_TYPE_ACCESS,
+  INFORECORDS(NAMESPACE),
+  INFORECORDS(ENUM),
+  ENUM_DEFLOCATION,
+  ENUM_LOCATION,
+  ENUM_MEMBER,
+  ENUM_SCOPED,
+  INFORECORDS(RECORD),
+  RECORD_DEFLOCATION,
+  RECORD_LOCATION,
+  RECORD_TAG_TYPE,
+  RECORD_PARENT,
+  RECORD_VPARENT,
+  RI_FIRST = VERSION,
+  RI_LAST = RECORD_VPARENT
+};
+
+static constexpr unsigned BlockIdCount = BI_LAST - BI_FIRST + 1;
+static constexpr unsigned RecordIdCount = RI_LAST - RI_FIRST + 1;
+
+#undef INFORECORDS
+
+class ClangDocBitcodeWriter {
+public:
+  ClangDocBitcodeWriter(llvm::BitstreamWriter &Stream) : Stream(Stream) {
+    emitHeader();
+    emitBlockInfoBlock();
+    emitVersionBlock();
+  }
+
+#ifndef NDEBUG // Don't want explicit dtor unless needed.
+  ~ClangDocBitcodeWriter() {
+    // Check that the static size is large-enough.
+    assert(Record.capacity() > BitCodeConstants::RecordSize);
+  }
+#endif
+
+  // Block emission of different info types.
+  void emitBlock(const NamespaceInfo &I);
+  void emitBlock(const RecordInfo &I);
+  void emitBlock(const FunctionInfo &I);
+  void emitBlock(const EnumInfo &I);
+  void emitBlock(const TypeInfo &B);
+  void emitBlock(const FieldTypeInfo &B);
+  void emitBlock(const MemberTypeInfo &B);
+  void emitBlock(const CommentInfo &B);
+
+private:
+  class AbbreviationMap {
+    llvm::DenseMap<unsigned, unsigned> Abbrevs;
+
+  public:
+    AbbreviationMap() : Abbrevs(RecordIdCount) {}
+
+    void add(RecordId RID, unsigned AbbrevID);
+    unsigned get(RecordId RID) const;
+  };
+
+  class StreamSubBlockGuard {
+    llvm::BitstreamWriter &Stream;
+
+  public:
+    StreamSubBlockGuard(llvm::BitstreamWriter &Stream_, BlockId ID)
+        : Stream(Stream_) {
+      // NOTE: SubBlockIDSize could theoretically be calculated on the fly,
+      // based on the initialization list of records in each block.
+      Stream.EnterSubblock(ID, BitCodeConstants::SubblockIDSize);
+    }
+
+    StreamSubBlockGuard() = default;
+    StreamSubBlockGuard(const StreamSubBlockGuard &) = delete;
+    StreamSubBlockGuard &operator=(const StreamSubBlockGuard &) = delete;
+
+    ~StreamSubBlockGuard() { Stream.ExitBlock(); }
+  };
+
+  // Emission of validation and overview blocks.
+  void emitHeader();
+  void emitVersionBlock();
+  void emitRecordID(RecordId ID);
+  void emitBlockID(BlockId ID);
+  void emitBlockInfoBlock();
+  void emitBlockInfo(BlockId BID, const std::initializer_list<RecordId> &RIDs);
+
+  // Emission of individual record types.
+  void emitRecord(StringRef Str, RecordId ID);
+  void emitRecord(const SymbolID &Str, RecordId ID);
+  void emitRecord(const Location &Loc, RecordId ID);
+  void emitRecord(const Reference &Ref, RecordId ID);
+  void emitRecord(bool Value, RecordId ID);
+  void emitRecord(int Value, RecordId ID);
+  void emitRecord(unsigned Value, RecordId ID);
+  bool prepRecordData(RecordId ID, bool ShouldEmit = true);
+
+  // Emission of appropriate abbreviation type.
+  void emitAbbrev(RecordId ID, BlockId Block);
+
+  // Static size is the maximum length of the block/record names we're pushing
+  // to this + 1. Longest is currently `MemberTypeBlock` at 15 chars.
+  SmallVector<uint32_t, BitCodeConstants::RecordSize> Record;
+  llvm::BitstreamWriter &Stream;
+  AbbreviationMap Abbrevs;
+};
+
+} // namespace doc
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_DOC_BITCODEWRITER_H