BitstreamWriter.h

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//===- BitstreamWriter.h - Low-level bitstream writer interface -*- 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
//
//===----------------------------------------------------------------------===//
//
// This header defines the BitstreamWriter class.  This class can be used to
// write an arbitrary bitstream, regardless of its contents.
//
//===----------------------------------------------------------------------===//
 
#ifndef LLVM_BITSTREAM_BITSTREAMWRITER_H
#define LLVM_BITSTREAM_BITSTREAMWRITER_H
 
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Bitstream/BitCodes.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <optional>
#include <vector>
 
namespace llvm {
 
class BitstreamWriter {
  /// Out - The buffer that keeps unflushed bytes.
  SmallVectorImpl<char> &Out;
 
  /// FS - The file stream that Out flushes to. If FS is nullptr, it does not
  /// support read or seek, Out cannot be flushed until all data are written.
  raw_fd_stream *FS;
 
  /// FlushThreshold - If FS is valid, this is the threshold (unit B) to flush
  /// FS.
  const uint64_t FlushThreshold;
 
  /// CurBit - Always between 0 and 31 inclusive, specifies the next bit to use.
  unsigned CurBit;
 
  /// CurValue - The current value. Only bits < CurBit are valid.
  uint32_t CurValue;
 
  /// CurCodeSize - This is the declared size of code values used for the
  /// current block, in bits.
  unsigned CurCodeSize;
 
  /// BlockInfoCurBID - When emitting a BLOCKINFO_BLOCK, this is the currently
  /// selected BLOCK ID.
  unsigned BlockInfoCurBID;
 
  /// CurAbbrevs - Abbrevs installed at in this block.
  std::vector<std::shared_ptr<BitCodeAbbrev>> CurAbbrevs;
 
  struct Block {
    unsigned PrevCodeSize;
    size_t StartSizeWord;
    std::vector<std::shared_ptr<BitCodeAbbrev>> PrevAbbrevs;
    Block(unsigned PCS, size_t SSW) : PrevCodeSize(PCS), StartSizeWord(SSW) {}
  };
 
  /// BlockScope - This tracks the current blocks that we have entered.
  std::vector<Block> BlockScope;
 
  /// BlockInfo - This contains information emitted to BLOCKINFO_BLOCK blocks.
  /// These describe abbreviations that all blocks of the specified ID inherit.
  struct BlockInfo {
    unsigned BlockID;
    std::vector<std::shared_ptr<BitCodeAbbrev>> Abbrevs;
  };
  std::vector<BlockInfo> BlockInfoRecords;
 
  void WriteWord(unsigned Value) {
    Value = support::endian::byte_swap<uint32_t, support::little>(Value);
    Out.append(reinterpret_cast<const char *>(&Value),
               reinterpret_cast<const char *>(&Value + 1));
  }
 
  uint64_t GetNumOfFlushedBytes() const { return FS ? FS->tell() : 0; }
 
  size_t GetBufferOffset() const { return Out.size() + GetNumOfFlushedBytes(); }
 
  size_t GetWordIndex() const {
    size_t Offset = GetBufferOffset();
    assert((Offset & 3) == 0 && "Not 32-bit aligned");
    return Offset / 4;
  }
 
  /// If the related file stream supports reading, seeking and writing, flush
  /// the buffer if its size is above a threshold.
  void FlushToFile() {
    if (!FS)
      return;
    if (Out.size() < FlushThreshold)
      return;
    FS->write((char *)&Out.front(), Out.size());
    Out.clear();
  }
 
public:
  /// Create a BitstreamWriter that writes to Buffer \p O.
  ///
  /// \p FS is the file stream that \p O flushes to incrementally. If \p FS is
  /// null, \p O does not flush incrementially, but writes to disk at the end.
  ///
  /// \p FlushThreshold is the threshold (unit M) to flush \p O if \p FS is
  /// valid. Flushing only occurs at (sub)block boundaries.
  BitstreamWriter(SmallVectorImpl<char> &O, raw_fd_stream *FS = nullptr,
                  uint32_t FlushThreshold = 512)
      : Out(O), FS(FS), FlushThreshold(FlushThreshold << 20), CurBit(0),
        CurValue(0), CurCodeSize(2) {}
 
  ~BitstreamWriter() {
    assert(CurBit == 0 && "Unflushed data remaining");
    assert(BlockScope.empty() && CurAbbrevs.empty() && "Block imbalance");
  }
 
  /// Retrieve the current position in the stream, in bits.
  uint64_t GetCurrentBitNo() const { return GetBufferOffset() * 8 + CurBit; }
 
  /// Retrieve the number of bits currently used to encode an abbrev ID.
  unsigned GetAbbrevIDWidth() const { return CurCodeSize; }
 
  //===--------------------------------------------------------------------===//
  // Basic Primitives for emitting bits to the stream.
  //===--------------------------------------------------------------------===//
 
  /// Backpatch a 32-bit word in the output at the given bit offset
  /// with the specified value.
  void BackpatchWord(uint64_t BitNo, unsigned NewWord) {
    using namespace llvm::support;
    uint64_t ByteNo = BitNo / 8;
    uint64_t StartBit = BitNo & 7;
    uint64_t NumOfFlushedBytes = GetNumOfFlushedBytes();
 
    if (ByteNo >= NumOfFlushedBytes) {
      assert((!endian::readAtBitAlignment<uint32_t, little, unaligned>(
                 &Out[ByteNo - NumOfFlushedBytes], StartBit)) &&
             "Expected to be patching over 0-value placeholders");
      endian::writeAtBitAlignment<uint32_t, little, unaligned>(
          &Out[ByteNo - NumOfFlushedBytes], NewWord, StartBit);
      return;
    }
 
    // If the byte offset to backpatch is flushed, use seek to backfill data.
    // First, save the file position to restore later.
    uint64_t CurPos = FS->tell();
 
    // Copy data to update into Bytes from the file FS and the buffer Out.
    char Bytes[9]; // Use one more byte to silence a warning from Visual C++.
    size_t BytesNum = StartBit ? 8 : 4;
    size_t BytesFromDisk = std::min(static_cast<uint64_t>(BytesNum), NumOfFlushedBytes - ByteNo);
    size_t BytesFromBuffer = BytesNum - BytesFromDisk;
 
    // When unaligned, copy existing data into Bytes from the file FS and the
    // buffer Out so that it can be updated before writing. For debug builds
    // read bytes unconditionally in order to check that the existing value is 0
    // as expected.
#ifdef NDEBUG
    if (StartBit)
#endif
    {
      FS->seek(ByteNo);
      ssize_t BytesRead = FS->read(Bytes, BytesFromDisk);
      (void)BytesRead; // silence warning
      assert(BytesRead >= 0 && static_cast<size_t>(BytesRead) == BytesFromDisk);
      for (size_t i = 0; i < BytesFromBuffer; ++i)
        Bytes[BytesFromDisk + i] = Out[i];
      assert((!endian::readAtBitAlignment<uint32_t, little, unaligned>(
                 Bytes, StartBit)) &&
             "Expected to be patching over 0-value placeholders");
    }
 
    // Update Bytes in terms of bit offset and value.
    endian::writeAtBitAlignment<uint32_t, little, unaligned>(Bytes, NewWord,
                                                             StartBit);
 
    // Copy updated data back to the file FS and the buffer Out.
    FS->seek(ByteNo);
    FS->write(Bytes, BytesFromDisk);
    for (size_t i = 0; i < BytesFromBuffer; ++i)
      Out[i] = Bytes[BytesFromDisk + i];
 
    // Restore the file position.
    FS->seek(CurPos);
  }
 
  void BackpatchWord64(uint64_t BitNo, uint64_t Val) {
    BackpatchWord(BitNo, (uint32_t)Val);
    BackpatchWord(BitNo + 32, (uint32_t)(Val >> 32));
  }
 
  void Emit(uint32_t Val, unsigned NumBits) {
    assert(NumBits && NumBits <= 32 && "Invalid value size!");
    assert((Val & ~(~0U >> (32-NumBits))) == 0 && "High bits set!");
    CurValue |= Val << CurBit;
    if (CurBit + NumBits < 32) {
      CurBit += NumBits;
      return;
    }
 
    // Add the current word.
    WriteWord(CurValue);
 
    if (CurBit)
      CurValue = Val >> (32-CurBit);
    else
      CurValue = 0;
    CurBit = (CurBit+NumBits) & 31;
  }
 
  void FlushToWord() {
    if (CurBit) {
      WriteWord(CurValue);
      CurBit = 0;
      CurValue = 0;
    }
  }
 
  void EmitVBR(uint32_t Val, unsigned NumBits) {
    assert(NumBits <= 32 && "Too many bits to emit!");
    uint32_t Threshold = 1U << (NumBits-1);
 
    // Emit the bits with VBR encoding, NumBits-1 bits at a time.
    while (Val >= Threshold) {
      Emit((Val & ((1 << (NumBits-1))-1)) | (1 << (NumBits-1)), NumBits);
      Val >>= NumBits-1;
    }
 
    Emit(Val, NumBits);
  }
 
  void EmitVBR64(uint64_t Val, unsigned NumBits) {
    assert(NumBits <= 32 && "Too many bits to emit!");
    if ((uint32_t)Val == Val)
      return EmitVBR((uint32_t)Val, NumBits);
 
    uint32_t Threshold = 1U << (NumBits-1);
 
    // Emit the bits with VBR encoding, NumBits-1 bits at a time.
    while (Val >= Threshold) {
      Emit(((uint32_t)Val & ((1 << (NumBits - 1)) - 1)) | (1 << (NumBits - 1)),
           NumBits);
      Val >>= NumBits-1;
    }
 
    Emit((uint32_t)Val, NumBits);
  }
 
  /// EmitCode - Emit the specified code.
  void EmitCode(unsigned Val) {
    Emit(Val, CurCodeSize);
  }
 
  //===--------------------------------------------------------------------===//
  // Block Manipulation
  //===--------------------------------------------------------------------===//
 
  /// getBlockInfo - If there is block info for the specified ID, return it,
  /// otherwise return null.
  BlockInfo *getBlockInfo(unsigned BlockID) {
    // Common case, the most recent entry matches BlockID.
    if (!BlockInfoRecords.empty() && BlockInfoRecords.back().BlockID == BlockID)
      return &BlockInfoRecords.back();
 
    for (BlockInfo &BI : BlockInfoRecords)
      if (BI.BlockID == BlockID)
        return &BI;
    return nullptr;
  }
 
  void EnterSubblock(unsigned BlockID, unsigned CodeLen) {
    // Block header:
    //    [ENTER_SUBBLOCK, blockid, newcodelen, <align4bytes>, blocklen]
    EmitCode(bitc::ENTER_SUBBLOCK);
    EmitVBR(BlockID, bitc::BlockIDWidth);
    EmitVBR(CodeLen, bitc::CodeLenWidth);
    FlushToWord();
 
    size_t BlockSizeWordIndex = GetWordIndex();
    unsigned OldCodeSize = CurCodeSize;
 
    // Emit a placeholder, which will be replaced when the block is popped.
    Emit(0, bitc::BlockSizeWidth);
 
    CurCodeSize = CodeLen;
 
    // Push the outer block's abbrev set onto the stack, start out with an
    // empty abbrev set.
    BlockScope.emplace_back(OldCodeSize, BlockSizeWordIndex);
    BlockScope.back().PrevAbbrevs.swap(CurAbbrevs);
 
    // If there is a blockinfo for this BlockID, add all the predefined abbrevs
    // to the abbrev list.
    if (BlockInfo *Info = getBlockInfo(BlockID))
      append_range(CurAbbrevs, Info->Abbrevs);
  }
 
  void ExitBlock() {
    assert(!BlockScope.empty() && "Block scope imbalance!");
    const Block &B = BlockScope.back();
 
    // Block tail:
    //    [END_BLOCK, <align4bytes>]
    EmitCode(bitc::END_BLOCK);
    FlushToWord();
 
    // Compute the size of the block, in words, not counting the size field.
    size_t SizeInWords = GetWordIndex() - B.StartSizeWord - 1;
    uint64_t BitNo = uint64_t(B.StartSizeWord) * 32;
 
    // Update the block size field in the header of this sub-block.
    BackpatchWord(BitNo, SizeInWords);
 
    // Restore the inner block's code size and abbrev table.
    CurCodeSize = B.PrevCodeSize;
    CurAbbrevs = std::move(B.PrevAbbrevs);
    BlockScope.pop_back();
    FlushToFile();
  }
 
  //===--------------------------------------------------------------------===//
  // Record Emission
  //===--------------------------------------------------------------------===//
 
private:
  /// EmitAbbreviatedLiteral - Emit a literal value according to its abbrev
  /// record.  This is a no-op, since the abbrev specifies the literal to use.
  template<typename uintty>
  void EmitAbbreviatedLiteral(const BitCodeAbbrevOp &Op, uintty V) {
    assert(Op.isLiteral() && "Not a literal");
    // If the abbrev specifies the literal value to use, don't emit
    // anything.
    assert(V == Op.getLiteralValue() &&
           "Invalid abbrev for record!");
  }
 
  /// EmitAbbreviatedField - Emit a single scalar field value with the specified
  /// encoding.
  template<typename uintty>
  void EmitAbbreviatedField(const BitCodeAbbrevOp &Op, uintty V) {
    assert(!Op.isLiteral() && "Literals should use EmitAbbreviatedLiteral!");
 
    // Encode the value as we are commanded.
    switch (Op.getEncoding()) {
    default: llvm_unreachable("Unknown encoding!");
    case BitCodeAbbrevOp::Fixed:
      if (Op.getEncodingData())
        Emit((unsigned)V, (unsigned)Op.getEncodingData());
      break;
    case BitCodeAbbrevOp::VBR:
      if (Op.getEncodingData())
        EmitVBR64(V, (unsigned)Op.getEncodingData());
      break;
    case BitCodeAbbrevOp::Char6:
      Emit(BitCodeAbbrevOp::EncodeChar6((char)V), 6);
      break;
    }
  }
 
  /// EmitRecordWithAbbrevImpl - This is the core implementation of the record
  /// emission code.  If BlobData is non-null, then it specifies an array of
  /// data that should be emitted as part of the Blob or Array operand that is
  /// known to exist at the end of the record. If Code is specified, then
  /// it is the record code to emit before the Vals, which must not contain
  /// the code.
  template <typename uintty>
  void EmitRecordWithAbbrevImpl(unsigned Abbrev, ArrayRef<uintty> Vals,
                                StringRef Blob, std::optional<unsigned> Code) {
    const char *BlobData = Blob.data();
    unsigned BlobLen = (unsigned) Blob.size();
    unsigned AbbrevNo = Abbrev-bitc::FIRST_APPLICATION_ABBREV;
    assert(AbbrevNo < CurAbbrevs.size() && "Invalid abbrev #!");
    const BitCodeAbbrev *Abbv = CurAbbrevs[AbbrevNo].get();
 
    EmitCode(Abbrev);
 
    unsigned i = 0, e = static_cast<unsigned>(Abbv->getNumOperandInfos());
    if (Code) {
      assert(e && "Expected non-empty abbreviation");
      const BitCodeAbbrevOp &Op = Abbv->getOperandInfo(i++);
 
      if (Op.isLiteral())
        EmitAbbreviatedLiteral(Op, *Code);
      else {
        assert(Op.getEncoding() != BitCodeAbbrevOp::Array &&
               Op.getEncoding() != BitCodeAbbrevOp::Blob &&
               "Expected literal or scalar");
        EmitAbbreviatedField(Op, *Code);
      }
    }
 
    unsigned RecordIdx = 0;
    for (; i != e; ++i) {
      const BitCodeAbbrevOp &Op = Abbv->getOperandInfo(i);
      if (Op.isLiteral()) {
        assert(RecordIdx < Vals.size() && "Invalid abbrev/record");
        EmitAbbreviatedLiteral(Op, Vals[RecordIdx]);
        ++RecordIdx;
      } else if (Op.getEncoding() == BitCodeAbbrevOp::Array) {
        // Array case.
        assert(i + 2 == e && "array op not second to last?");
        const BitCodeAbbrevOp &EltEnc = Abbv->getOperandInfo(++i);
 
        // If this record has blob data, emit it, otherwise we must have record
        // entries to encode this way.
        if (BlobData) {
          assert(RecordIdx == Vals.size() &&
                 "Blob data and record entries specified for array!");
          // Emit a vbr6 to indicate the number of elements present.
          EmitVBR(static_cast<uint32_t>(BlobLen), 6);
 
          // Emit each field.
          for (unsigned i = 0; i != BlobLen; ++i)
            EmitAbbreviatedField(EltEnc, (unsigned char)BlobData[i]);
 
          // Know that blob data is consumed for assertion below.
          BlobData = nullptr;
        } else {
          // Emit a vbr6 to indicate the number of elements present.
          EmitVBR(static_cast<uint32_t>(Vals.size()-RecordIdx), 6);
 
          // Emit each field.
          for (unsigned e = Vals.size(); RecordIdx != e; ++RecordIdx)
            EmitAbbreviatedField(EltEnc, Vals[RecordIdx]);
        }
      } else if (Op.getEncoding() == BitCodeAbbrevOp::Blob) {
        // If this record has blob data, emit it, otherwise we must have record
        // entries to encode this way.
 
        if (BlobData) {
          assert(RecordIdx == Vals.size() &&
                 "Blob data and record entries specified for blob operand!");
 
          assert(Blob.data() == BlobData && "BlobData got moved");
          assert(Blob.size() == BlobLen && "BlobLen got changed");
          emitBlob(Blob);
          BlobData = nullptr;
        } else {
          emitBlob(Vals.slice(RecordIdx));
        }
      } else {  // Single scalar field.
        assert(RecordIdx < Vals.size() && "Invalid abbrev/record");
        EmitAbbreviatedField(Op, Vals[RecordIdx]);
        ++RecordIdx;
      }
    }
    assert(RecordIdx == Vals.size() && "Not all record operands emitted!");
    assert(BlobData == nullptr &&
           "Blob data specified for record that doesn't use it!");
  }
 
public:
  /// Emit a blob, including flushing before and tail-padding.
  template <class UIntTy>
  void emitBlob(ArrayRef<UIntTy> Bytes, bool ShouldEmitSize = true) {
    // Emit a vbr6 to indicate the number of elements present.
    if (ShouldEmitSize)
      EmitVBR(static_cast<uint32_t>(Bytes.size()), 6);
 
    // Flush to a 32-bit alignment boundary.
    FlushToWord();
 
    // Emit literal bytes.
    assert(llvm::all_of(Bytes, [](UIntTy B) { return isUInt<8>(B); }));
    Out.append(Bytes.begin(), Bytes.end());
 
    // Align end to 32-bits.
    while (GetBufferOffset() & 3)
      Out.push_back(0);
  }
  void emitBlob(StringRef Bytes, bool ShouldEmitSize = true) {
    emitBlob(ArrayRef((const uint8_t *)Bytes.data(), Bytes.size()),
             ShouldEmitSize);
  }
 
  /// EmitRecord - Emit the specified record to the stream, using an abbrev if
  /// we have one to compress the output.
  template <typename Container>
  void EmitRecord(unsigned Code, const Container &Vals, unsigned Abbrev = 0) {
    if (!Abbrev) {
      // If we don't have an abbrev to use, emit this in its fully unabbreviated
      // form.
      auto Count = static_cast<uint32_t>(std::size(Vals));
      EmitCode(bitc::UNABBREV_RECORD);
      EmitVBR(Code, 6);
      EmitVBR(Count, 6);
      for (unsigned i = 0, e = Count; i != e; ++i)
        EmitVBR64(Vals[i], 6);
      return;
    }
 
    EmitRecordWithAbbrevImpl(Abbrev, ArrayRef(Vals), StringRef(), Code);
  }
 
  /// EmitRecordWithAbbrev - Emit a record with the specified abbreviation.
  /// Unlike EmitRecord, the code for the record should be included in Vals as
  /// the first entry.
  template <typename Container>
  void EmitRecordWithAbbrev(unsigned Abbrev, const Container &Vals) {
    EmitRecordWithAbbrevImpl(Abbrev, ArrayRef(Vals), StringRef(), std::nullopt);
  }
 
  /// EmitRecordWithBlob - Emit the specified record to the stream, using an
  /// abbrev that includes a blob at the end.  The blob data to emit is
  /// specified by the pointer and length specified at the end.  In contrast to
  /// EmitRecord, this routine expects that the first entry in Vals is the code
  /// of the record.
  template <typename Container>
  void EmitRecordWithBlob(unsigned Abbrev, const Container &Vals,
                          StringRef Blob) {
    EmitRecordWithAbbrevImpl(Abbrev, ArrayRef(Vals), Blob, std::nullopt);
  }
  template <typename Container>
  void EmitRecordWithBlob(unsigned Abbrev, const Container &Vals,
                          const char *BlobData, unsigned BlobLen) {
    return EmitRecordWithAbbrevImpl(Abbrev, ArrayRef(Vals),
                                    StringRef(BlobData, BlobLen), std::nullopt);
  }
 
  /// EmitRecordWithArray - Just like EmitRecordWithBlob, works with records
  /// that end with an array.
  template <typename Container>
  void EmitRecordWithArray(unsigned Abbrev, const Container &Vals,
                           StringRef Array) {
    EmitRecordWithAbbrevImpl(Abbrev, ArrayRef(Vals), Array, std::nullopt);
  }
  template <typename Container>
  void EmitRecordWithArray(unsigned Abbrev, const Container &Vals,
                           const char *ArrayData, unsigned ArrayLen) {
    return EmitRecordWithAbbrevImpl(
        Abbrev, ArrayRef(Vals), StringRef(ArrayData, ArrayLen), std::nullopt);
  }
 
  //===--------------------------------------------------------------------===//
  // Abbrev Emission
  //===--------------------------------------------------------------------===//
 
private:
  // Emit the abbreviation as a DEFINE_ABBREV record.
  void EncodeAbbrev(const BitCodeAbbrev &Abbv) {
    EmitCode(bitc::DEFINE_ABBREV);
    EmitVBR(Abbv.getNumOperandInfos(), 5);
    for (unsigned i = 0, e = static_cast<unsigned>(Abbv.getNumOperandInfos());
         i != e; ++i) {
      const BitCodeAbbrevOp &Op = Abbv.getOperandInfo(i);
      Emit(Op.isLiteral(), 1);
      if (Op.isLiteral()) {
        EmitVBR64(Op.getLiteralValue(), 8);
      } else {
        Emit(Op.getEncoding(), 3);
        if (Op.hasEncodingData())
          EmitVBR64(Op.getEncodingData(), 5);
      }
    }
  }
public:
 
  /// Emits the abbreviation \p Abbv to the stream.
  unsigned EmitAbbrev(std::shared_ptr<BitCodeAbbrev> Abbv) {
    EncodeAbbrev(*Abbv);
    CurAbbrevs.push_back(std::move(Abbv));
    return static_cast<unsigned>(CurAbbrevs.size())-1 +
      bitc::FIRST_APPLICATION_ABBREV;
  }
 
  //===--------------------------------------------------------------------===//
  // BlockInfo Block Emission
  //===--------------------------------------------------------------------===//
 
  /// EnterBlockInfoBlock - Start emitting the BLOCKINFO_BLOCK.
  void EnterBlockInfoBlock() {
    EnterSubblock(bitc::BLOCKINFO_BLOCK_ID, 2);
    BlockInfoCurBID = ~0U;
    BlockInfoRecords.clear();
  }
private:
  /// SwitchToBlockID - If we aren't already talking about the specified block
  /// ID, emit a BLOCKINFO_CODE_SETBID record.
  void SwitchToBlockID(unsigned BlockID) {
    if (BlockInfoCurBID == BlockID) return;
    SmallVector<unsigned, 2> V;
    V.push_back(BlockID);
    EmitRecord(bitc::BLOCKINFO_CODE_SETBID, V);
    BlockInfoCurBID = BlockID;
  }
 
  BlockInfo &getOrCreateBlockInfo(unsigned BlockID) {
    if (BlockInfo *BI = getBlockInfo(BlockID))
      return *BI;
 
    // Otherwise, add a new record.
    BlockInfoRecords.emplace_back();
    BlockInfoRecords.back().BlockID = BlockID;
    return BlockInfoRecords.back();
  }
 
public:
 
  /// EmitBlockInfoAbbrev - Emit a DEFINE_ABBREV record for the specified
  /// BlockID.
  unsigned EmitBlockInfoAbbrev(unsigned BlockID, std::shared_ptr<BitCodeAbbrev> Abbv) {
    SwitchToBlockID(BlockID);
    EncodeAbbrev(*Abbv);
 
    // Add the abbrev to the specified block record.
    BlockInfo &Info = getOrCreateBlockInfo(BlockID);
    Info.Abbrevs.push_back(std::move(Abbv));
 
    return Info.Abbrevs.size()-1+bitc::FIRST_APPLICATION_ABBREV;
  }
};
 
 
} // End llvm namespace
 
#endif