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diff --git a/include/llvm/Bitstream/BitstreamReader.h b/include/llvm/Bitstream/BitstreamReader.h
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+++ b/include/llvm/Bitstream/BitstreamReader.h
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+//===- BitstreamReader.h - Low-level bitstream reader 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 BitstreamReader class.  This class can be used to
+// read an arbitrary bitstream, regardless of its contents.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_BITSTREAM_BITSTREAMREADER_H
+#define LLVM_BITSTREAM_BITSTREAMREADER_H
+
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/Bitstream/BitCodes.h"
+#include "llvm/Support/Endian.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/MathExtras.h"
+#include "llvm/Support/MemoryBuffer.h"
+#include <algorithm>
+#include <cassert>
+#include <climits>
+#include <cstddef>
+#include <cstdint>
+#include <memory>
+#include <string>
+#include <utility>
+#include <vector>
+
+namespace llvm {
+
+/// This class maintains the abbreviations read from a block info block.
+class BitstreamBlockInfo {
+public:
+  /// 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::string Name;
+    std::vector<std::pair<unsigned, std::string>> RecordNames;
+  };
+
+private:
+  std::vector<BlockInfo> BlockInfoRecords;
+
+public:
+  /// If there is block info for the specified ID, return it, otherwise return
+  /// null.
+  const BlockInfo *getBlockInfo(unsigned BlockID) const {
+    // Common case, the most recent entry matches BlockID.
+    if (!BlockInfoRecords.empty() && BlockInfoRecords.back().BlockID == BlockID)
+      return &BlockInfoRecords.back();
+
+    for (unsigned i = 0, e = static_cast<unsigned>(BlockInfoRecords.size());
+         i != e; ++i)
+      if (BlockInfoRecords[i].BlockID == BlockID)
+        return &BlockInfoRecords[i];
+    return nullptr;
+  }
+
+  BlockInfo &getOrCreateBlockInfo(unsigned BlockID) {
+    if (const BlockInfo *BI = getBlockInfo(BlockID))
+      return *const_cast<BlockInfo*>(BI);
+
+    // Otherwise, add a new record.
+    BlockInfoRecords.emplace_back();
+    BlockInfoRecords.back().BlockID = BlockID;
+    return BlockInfoRecords.back();
+  }
+};
+
+/// This represents a position within a bitstream. There may be multiple
+/// independent cursors reading within one bitstream, each maintaining their
+/// own local state.
+class SimpleBitstreamCursor {
+  ArrayRef<uint8_t> BitcodeBytes;
+  size_t NextChar = 0;
+
+public:
+  /// This is the current data we have pulled from the stream but have not
+  /// returned to the client. This is specifically and intentionally defined to
+  /// follow the word size of the host machine for efficiency. We use word_t in
+  /// places that are aware of this to make it perfectly explicit what is going
+  /// on.
+  using word_t = size_t;
+
+private:
+  word_t CurWord = 0;
+
+  /// This is the number of bits in CurWord that are valid. This is always from
+  /// [0...bits_of(size_t)-1] inclusive.
+  unsigned BitsInCurWord = 0;
+
+public:
+  static const constexpr size_t MaxChunkSize = sizeof(word_t) * 8;
+
+  SimpleBitstreamCursor() = default;
+  explicit SimpleBitstreamCursor(ArrayRef<uint8_t> BitcodeBytes)
+      : BitcodeBytes(BitcodeBytes) {}
+  explicit SimpleBitstreamCursor(StringRef BitcodeBytes)
+      : BitcodeBytes(arrayRefFromStringRef(BitcodeBytes)) {}
+  explicit SimpleBitstreamCursor(MemoryBufferRef BitcodeBytes)
+      : SimpleBitstreamCursor(BitcodeBytes.getBuffer()) {}
+
+  bool canSkipToPos(size_t pos) const {
+    // pos can be skipped to if it is a valid address or one byte past the end.
+    return pos <= BitcodeBytes.size();
+  }
+
+  bool AtEndOfStream() {
+    return BitsInCurWord == 0 && BitcodeBytes.size() <= NextChar;
+  }
+
+  /// Return the bit # of the bit we are reading.
+  uint64_t GetCurrentBitNo() const {
+    return NextChar*CHAR_BIT - BitsInCurWord;
+  }
+
+  // Return the byte # of the current bit.
+  uint64_t getCurrentByteNo() const { return GetCurrentBitNo() / 8; }
+
+  ArrayRef<uint8_t> getBitcodeBytes() const { return BitcodeBytes; }
+
+  /// Reset the stream to the specified bit number.
+  Error JumpToBit(uint64_t BitNo) {
+    size_t ByteNo = size_t(BitNo/8) & ~(sizeof(word_t)-1);
+    unsigned WordBitNo = unsigned(BitNo & (sizeof(word_t)*8-1));
+    assert(canSkipToPos(ByteNo) && "Invalid location");
+
+    // Move the cursor to the right word.
+    NextChar = ByteNo;
+    BitsInCurWord = 0;
+
+    // Skip over any bits that are already consumed.
+    if (WordBitNo) {
+      if (Expected<word_t> Res = Read(WordBitNo))
+        return Error::success();
+      else
+        return Res.takeError();
+    }
+
+    return Error::success();
+  }
+
+  /// Get a pointer into the bitstream at the specified byte offset.
+  const uint8_t *getPointerToByte(uint64_t ByteNo, uint64_t NumBytes) {
+    return BitcodeBytes.data() + ByteNo;
+  }
+
+  /// Get a pointer into the bitstream at the specified bit offset.
+  ///
+  /// The bit offset must be on a byte boundary.
+  const uint8_t *getPointerToBit(uint64_t BitNo, uint64_t NumBytes) {
+    assert(!(BitNo % 8) && "Expected bit on byte boundary");
+    return getPointerToByte(BitNo / 8, NumBytes);
+  }
+
+  Error fillCurWord() {
+    if (NextChar >= BitcodeBytes.size())
+      return createStringError(std::errc::io_error,
+                               "Unexpected end of file reading %u of %u bytes",
+                               NextChar, BitcodeBytes.size());
+
+    // Read the next word from the stream.
+    const uint8_t *NextCharPtr = BitcodeBytes.data() + NextChar;
+    unsigned BytesRead;
+    if (BitcodeBytes.size() >= NextChar + sizeof(word_t)) {
+      BytesRead = sizeof(word_t);
+      CurWord =
+          support::endian::read<word_t, support::little, support::unaligned>(
+              NextCharPtr);
+    } else {
+      // Short read.
+      BytesRead = BitcodeBytes.size() - NextChar;
+      CurWord = 0;
+      for (unsigned B = 0; B != BytesRead; ++B)
+        CurWord |= uint64_t(NextCharPtr[B]) << (B * 8);
+    }
+    NextChar += BytesRead;
+    BitsInCurWord = BytesRead * 8;
+    return Error::success();
+  }
+
+  Expected<word_t> Read(unsigned NumBits) {
+    static const unsigned BitsInWord = MaxChunkSize;
+
+    assert(NumBits && NumBits <= BitsInWord &&
+           "Cannot return zero or more than BitsInWord bits!");
+
+    static const unsigned Mask = sizeof(word_t) > 4 ? 0x3f : 0x1f;
+
+    // If the field is fully contained by CurWord, return it quickly.
+    if (BitsInCurWord >= NumBits) {
+      word_t R = CurWord & (~word_t(0) >> (BitsInWord - NumBits));
+
+      // Use a mask to avoid undefined behavior.
+      CurWord >>= (NumBits & Mask);
+
+      BitsInCurWord -= NumBits;
+      return R;
+    }
+
+    word_t R = BitsInCurWord ? CurWord : 0;
+    unsigned BitsLeft = NumBits - BitsInCurWord;
+
+    if (Error fillResult = fillCurWord())
+      return std::move(fillResult);
+
+    // If we run out of data, abort.
+    if (BitsLeft > BitsInCurWord)
+      return createStringError(std::errc::io_error,
+                               "Unexpected end of file reading %u of %u bits",
+                               BitsInCurWord, BitsLeft);
+
+    word_t R2 = CurWord & (~word_t(0) >> (BitsInWord - BitsLeft));
+
+    // Use a mask to avoid undefined behavior.
+    CurWord >>= (BitsLeft & Mask);
+
+    BitsInCurWord -= BitsLeft;
+
+    R |= R2 << (NumBits - BitsLeft);
+
+    return R;
+  }
+
+  Expected<uint32_t> ReadVBR(unsigned NumBits) {
+    Expected<unsigned> MaybeRead = Read(NumBits);
+    if (!MaybeRead)
+      return MaybeRead;
+    uint32_t Piece = MaybeRead.get();
+
+    if ((Piece & (1U << (NumBits-1))) == 0)
+      return Piece;
+
+    uint32_t Result = 0;
+    unsigned NextBit = 0;
+    while (true) {
+      Result |= (Piece & ((1U << (NumBits-1))-1)) << NextBit;
+
+      if ((Piece & (1U << (NumBits-1))) == 0)
+        return Result;
+
+      NextBit += NumBits-1;
+      MaybeRead = Read(NumBits);
+      if (!MaybeRead)
+        return MaybeRead;
+      Piece = MaybeRead.get();
+    }
+  }
+
+  // Read a VBR that may have a value up to 64-bits in size. The chunk size of
+  // the VBR must still be <= 32 bits though.
+  Expected<uint64_t> ReadVBR64(unsigned NumBits) {
+    Expected<uint64_t> MaybeRead = Read(NumBits);
+    if (!MaybeRead)
+      return MaybeRead;
+    uint32_t Piece = MaybeRead.get();
+
+    if ((Piece & (1U << (NumBits-1))) == 0)
+      return uint64_t(Piece);
+
+    uint64_t Result = 0;
+    unsigned NextBit = 0;
+    while (true) {
+      Result |= uint64_t(Piece & ((1U << (NumBits-1))-1)) << NextBit;
+
+      if ((Piece & (1U << (NumBits-1))) == 0)
+        return Result;
+
+      NextBit += NumBits-1;
+      MaybeRead = Read(NumBits);
+      if (!MaybeRead)
+        return MaybeRead;
+      Piece = MaybeRead.get();
+    }
+  }
+
+  void SkipToFourByteBoundary() {
+    // If word_t is 64-bits and if we've read less than 32 bits, just dump
+    // the bits we have up to the next 32-bit boundary.
+    if (sizeof(word_t) > 4 &&
+        BitsInCurWord >= 32) {
+      CurWord >>= BitsInCurWord-32;
+      BitsInCurWord = 32;
+      return;
+    }
+
+    BitsInCurWord = 0;
+  }
+
+  /// Return the size of the stream in bytes.
+  size_t SizeInBytes() const { return BitcodeBytes.size(); }
+
+  /// Skip to the end of the file.
+  void skipToEnd() { NextChar = BitcodeBytes.size(); }
+};
+
+/// When advancing through a bitstream cursor, each advance can discover a few
+/// different kinds of entries:
+struct BitstreamEntry {
+  enum {
+    Error,    // Malformed bitcode was found.
+    EndBlock, // We've reached the end of the current block, (or the end of the
+              // file, which is treated like a series of EndBlock records.
+    SubBlock, // This is the start of a new subblock of a specific ID.
+    Record    // This is a record with a specific AbbrevID.
+  } Kind;
+
+  unsigned ID;
+
+  static BitstreamEntry getError() {
+    BitstreamEntry E; E.Kind = Error; return E;
+  }
+
+  static BitstreamEntry getEndBlock() {
+    BitstreamEntry E; E.Kind = EndBlock; return E;
+  }
+
+  static BitstreamEntry getSubBlock(unsigned ID) {
+    BitstreamEntry E; E.Kind = SubBlock; E.ID = ID; return E;
+  }
+
+  static BitstreamEntry getRecord(unsigned AbbrevID) {
+    BitstreamEntry E; E.Kind = Record; E.ID = AbbrevID; return E;
+  }
+};
+
+/// This represents a position within a bitcode file, implemented on top of a
+/// SimpleBitstreamCursor.
+///
+/// Unlike iterators, BitstreamCursors are heavy-weight objects that should not
+/// be passed by value.
+class BitstreamCursor : SimpleBitstreamCursor {
+  // This is the declared size of code values used for the current block, in
+  // bits.
+  unsigned CurCodeSize = 2;
+
+  /// Abbrevs installed at in this block.
+  std::vector<std::shared_ptr<BitCodeAbbrev>> CurAbbrevs;
+
+  struct Block {
+    unsigned PrevCodeSize;
+    std::vector<std::shared_ptr<BitCodeAbbrev>> PrevAbbrevs;
+
+    explicit Block(unsigned PCS) : PrevCodeSize(PCS) {}
+  };
+
+  /// This tracks the codesize of parent blocks.
+  SmallVector<Block, 8> BlockScope;
+
+  BitstreamBlockInfo *BlockInfo = nullptr;
+
+public:
+  static const size_t MaxChunkSize = sizeof(word_t) * 8;
+
+  BitstreamCursor() = default;
+  explicit BitstreamCursor(ArrayRef<uint8_t> BitcodeBytes)
+      : SimpleBitstreamCursor(BitcodeBytes) {}
+  explicit BitstreamCursor(StringRef BitcodeBytes)
+      : SimpleBitstreamCursor(BitcodeBytes) {}
+  explicit BitstreamCursor(MemoryBufferRef BitcodeBytes)
+      : SimpleBitstreamCursor(BitcodeBytes) {}
+
+  using SimpleBitstreamCursor::AtEndOfStream;
+  using SimpleBitstreamCursor::canSkipToPos;
+  using SimpleBitstreamCursor::fillCurWord;
+  using SimpleBitstreamCursor::getBitcodeBytes;
+  using SimpleBitstreamCursor::GetCurrentBitNo;
+  using SimpleBitstreamCursor::getCurrentByteNo;
+  using SimpleBitstreamCursor::getPointerToByte;
+  using SimpleBitstreamCursor::JumpToBit;
+  using SimpleBitstreamCursor::Read;
+  using SimpleBitstreamCursor::ReadVBR;
+  using SimpleBitstreamCursor::ReadVBR64;
+  using SimpleBitstreamCursor::SizeInBytes;
+
+  /// Return the number of bits used to encode an abbrev #.
+  unsigned getAbbrevIDWidth() const { return CurCodeSize; }
+
+  /// Flags that modify the behavior of advance().
+  enum {
+    /// If this flag is used, the advance() method does not automatically pop
+    /// the block scope when the end of a block is reached.
+    AF_DontPopBlockAtEnd = 1,
+
+    /// If this flag is used, abbrev entries are returned just like normal
+    /// records.
+    AF_DontAutoprocessAbbrevs = 2
+  };
+
+  /// Advance the current bitstream, returning the next entry in the stream.
+  Expected<BitstreamEntry> advance(unsigned Flags = 0) {
+    while (true) {
+      if (AtEndOfStream())
+        return BitstreamEntry::getError();
+
+      Expected<unsigned> MaybeCode = ReadCode();
+      if (!MaybeCode)
+        return MaybeCode.takeError();
+      unsigned Code = MaybeCode.get();
+
+      if (Code == bitc::END_BLOCK) {
+        // Pop the end of the block unless Flags tells us not to.
+        if (!(Flags & AF_DontPopBlockAtEnd) && ReadBlockEnd())
+          return BitstreamEntry::getError();
+        return BitstreamEntry::getEndBlock();
+      }
+
+      if (Code == bitc::ENTER_SUBBLOCK) {
+        if (Expected<unsigned> MaybeSubBlock = ReadSubBlockID())
+          return BitstreamEntry::getSubBlock(MaybeSubBlock.get());
+        else
+          return MaybeSubBlock.takeError();
+      }
+
+      if (Code == bitc::DEFINE_ABBREV &&
+          !(Flags & AF_DontAutoprocessAbbrevs)) {
+        // We read and accumulate abbrev's, the client can't do anything with
+        // them anyway.
+        if (Error Err = ReadAbbrevRecord())
+          return std::move(Err);
+        continue;
+      }
+
+      return BitstreamEntry::getRecord(Code);
+    }
+  }
+
+  /// This is a convenience function for clients that don't expect any
+  /// subblocks. This just skips over them automatically.
+  Expected<BitstreamEntry> advanceSkippingSubblocks(unsigned Flags = 0) {
+    while (true) {
+      // If we found a normal entry, return it.
+      Expected<BitstreamEntry> MaybeEntry = advance(Flags);
+      if (!MaybeEntry)
+        return MaybeEntry;
+      BitstreamEntry Entry = MaybeEntry.get();
+
+      if (Entry.Kind != BitstreamEntry::SubBlock)
+        return Entry;
+
+      // If we found a sub-block, just skip over it and check the next entry.
+      if (Error Err = SkipBlock())
+        return std::move(Err);
+    }
+  }
+
+  Expected<unsigned> ReadCode() { return Read(CurCodeSize); }
+
+  // Block header:
+  //    [ENTER_SUBBLOCK, blockid, newcodelen, <align4bytes>, blocklen]
+
+  /// Having read the ENTER_SUBBLOCK code, read the BlockID for the block.
+  Expected<unsigned> ReadSubBlockID() { return ReadVBR(bitc::BlockIDWidth); }
+
+  /// Having read the ENTER_SUBBLOCK abbrevid and a BlockID, skip over the body
+  /// of this block.
+  Error SkipBlock() {
+    // Read and ignore the codelen value.
+    if (Expected<uint32_t> Res = ReadVBR(bitc::CodeLenWidth))
+      ; // Since we are skipping this block, we don't care what code widths are
+        // used inside of it.
+    else
+      return Res.takeError();
+
+    SkipToFourByteBoundary();
+    Expected<unsigned> MaybeNum = Read(bitc::BlockSizeWidth);
+    if (!MaybeNum)
+      return MaybeNum.takeError();
+    size_t NumFourBytes = MaybeNum.get();
+
+    // Check that the block wasn't partially defined, and that the offset isn't
+    // bogus.
+    size_t SkipTo = GetCurrentBitNo() + NumFourBytes * 4 * 8;
+    if (AtEndOfStream())
+      return createStringError(std::errc::illegal_byte_sequence,
+                               "can't skip block: already at end of stream");
+    if (!canSkipToPos(SkipTo / 8))
+      return createStringError(std::errc::illegal_byte_sequence,
+                               "can't skip to bit %zu from %" PRIu64, SkipTo,
+                               GetCurrentBitNo());
+
+    if (Error Res = JumpToBit(SkipTo))
+      return Res;
+
+    return Error::success();
+  }
+
+  /// Having read the ENTER_SUBBLOCK abbrevid, and enter the block.
+  Error EnterSubBlock(unsigned BlockID, unsigned *NumWordsP = nullptr);
+
+  bool ReadBlockEnd() {
+    if (BlockScope.empty()) return true;
+
+    // Block tail:
+    //    [END_BLOCK, <align4bytes>]
+    SkipToFourByteBoundary();
+
+    popBlockScope();
+    return false;
+  }
+
+private:
+  void popBlockScope() {
+    CurCodeSize = BlockScope.back().PrevCodeSize;
+
+    CurAbbrevs = std::move(BlockScope.back().PrevAbbrevs);
+    BlockScope.pop_back();
+  }
+
+  //===--------------------------------------------------------------------===//
+  // Record Processing
+  //===--------------------------------------------------------------------===//
+
+public:
+  /// Return the abbreviation for the specified AbbrevId.
+  const BitCodeAbbrev *getAbbrev(unsigned AbbrevID) {
+    unsigned AbbrevNo = AbbrevID - bitc::FIRST_APPLICATION_ABBREV;
+    if (AbbrevNo >= CurAbbrevs.size())
+      report_fatal_error("Invalid abbrev number");
+    return CurAbbrevs[AbbrevNo].get();
+  }
+
+  /// Read the current record and discard it, returning the code for the record.
+  Expected<unsigned> skipRecord(unsigned AbbrevID);
+
+  Expected<unsigned> readRecord(unsigned AbbrevID,
+                                SmallVectorImpl<uint64_t> &Vals,
+                                StringRef *Blob = nullptr);
+
+  //===--------------------------------------------------------------------===//
+  // Abbrev Processing
+  //===--------------------------------------------------------------------===//
+  Error ReadAbbrevRecord();
+
+  /// Read and return a block info block from the bitstream. If an error was
+  /// encountered, return None.
+  ///
+  /// \param ReadBlockInfoNames Whether to read block/record name information in
+  /// the BlockInfo block. Only llvm-bcanalyzer uses this.
+  Expected<Optional<BitstreamBlockInfo>>
+  ReadBlockInfoBlock(bool ReadBlockInfoNames = false);
+
+  /// Set the block info to be used by this BitstreamCursor to interpret
+  /// abbreviated records.
+  void setBlockInfo(BitstreamBlockInfo *BI) { BlockInfo = BI; }
+};
+
+} // end llvm namespace
+
+#endif // LLVM_BITSTREAM_BITSTREAMREADER_H