Subzero: Add necessary PNaCl files for standalone build.

pnacl-llvm/include/llvm/Bitcode/NaCl/NaClBitstreamReader.h

+//===- NaClBitstreamReader.h -----------------------------------*- C++ -*-===//
+//     Low-level bitstream reader interface
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This header defines the BitstreamReader class.  This class can be used to
+// read an arbitrary bitstream, regardless of its contents.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_BITCODE_NACL_NACLBITSTREAMREADER_H
+#define LLVM_BITCODE_NACL_NACLBITSTREAMREADER_H
+
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/Bitcode/NaCl/NaClBitcodeHeader.h"
+#include "llvm/Bitcode/NaCl/NaClLLVMBitCodes.h"
+#include "llvm/Support/Endian.h"
+#include "llvm/Support/StreamingMemoryObject.h"
+#include <atomic>
+#include <climits>
+#include <mutex>
+#include <unordered_map>
+#include <vector>
+
+namespace llvm {
+
+class Deserializer;
+class NaClBitstreamCursor;
+
+namespace naclbitc {
+
+/// Returns the Bit as a Byte:BitInByte string.
+std::string getBitAddress(uint64_t Bit);
+
+/// Severity levels for reporting errors.
+enum ErrorLevel {
+  Warning,
+  Error,
+  Fatal
+};
+
+// Basic printing routine to generate the beginning of an error
+// message. BitPosition is the bit position the error was found.
+// Level is the severity of the error.
+raw_ostream &ErrorAt(raw_ostream &Out, ErrorLevel Level,
+                     uint64_t BitPosition);
+
+} // End namespace naclbitc.
+
+/// This class is used to read from a NaCl bitcode wire format stream,
+/// maintaining information that is global to decoding the entire file.
+/// While a file is being read, multiple cursors can be independently
+/// advanced or skipped around within the file.  These are represented by
+/// the NaClBitstreamCursor class.
+class NaClBitstreamReader {
+public:
+  // Models a raw list of abbreviations.
+  static const size_t DefaultAbbrevListSize = 12;
+  using AbbrevListVector = SmallVector<NaClBitCodeAbbrev *,
+                                       DefaultAbbrevListSize>;
+
+  // Models and maintains a list of abbreviations. In particular, it maintains
+  // updating reference counts of abbreviation operators within the abbreviation
+  // list.
+  class AbbrevList {
+  public:
+    AbbrevList() = default;
+    explicit AbbrevList(const AbbrevList &NewAbbrevs) {
+      appendList(NewAbbrevs);
+    }
+    AbbrevList &operator=(const AbbrevList &Rhs) {
+      clear();
+      appendList(Rhs);
+      return *this;
+    }
+    // Creates a new (empty) abbreviation, appends it to this, and then returns
+    // the new abbreviation.
+    NaClBitCodeAbbrev *appendCreate() {
+      NaClBitCodeAbbrev *Abbv = new NaClBitCodeAbbrev();
+      Abbrevs.push_back(Abbv);
+      return Abbv;
+    }
+    // Appends the given abbreviation to this.
+    void append(NaClBitCodeAbbrev *Abbrv) {
+      Abbrv->addRef();
+      Abbrevs.push_back(Abbrv);
+    }
+    // Appends the contents of NewAbbrevs to this.
+    void appendList(const AbbrevList &NewAbbrevs) {
+      for (NaClBitCodeAbbrev *Abbrv : NewAbbrevs.Abbrevs)
+        append(Abbrv);
+    }
+    // Returns last abbreviation on list.
+    NaClBitCodeAbbrev *last() { return Abbrevs.back(); }
+    // Removes the last element of the list.
+    void popLast() {
+      Abbrevs.back()->dropRef();
+      Abbrevs.pop_back();
+    }
+    // Empties abbreviation list.
+    void clear() {
+      while(!Abbrevs.empty())
+        popLast();
+    }
+    // Allow read access to vector defining list.
+    const AbbrevListVector &getVector() const { return Abbrevs; }
+    ~AbbrevList() { clear(); }
+  private:
+    AbbrevListVector Abbrevs;
+  };
+
+  /// This contains information about abbreviations in blocks defined in the
+  /// BLOCKINFO_BLOCK block.  These describe global abbreviations that apply to
+  /// all succeeding blocks of the specified ID.
+  class BlockInfo {
+    BlockInfo &operator=(const BlockInfo&) = delete;
+  public:
+    BlockInfo() = default;
+    explicit BlockInfo(unsigned BlockID)
+        : BlockID(BlockID), Abbrevs() {}
+    BlockInfo(const BlockInfo&) = default;
+    unsigned getBlockID() const { return BlockID; }
+    void setBlockID(unsigned ID) { BlockID = ID; }
+    AbbrevList &getAbbrevs() { return Abbrevs; }
+    ~BlockInfo() {}
+  private:
+    unsigned BlockID;
+    AbbrevList Abbrevs;
+  };
+
+  class BlockInfoRecordsMap;
+  using SharedBlockInfoMap = std::shared_ptr<BlockInfoRecordsMap>;
+
+  // Holds the global abbreviations in the BlockInfo block of the bitcode file.
+  // Sharing is used to allow parallel parses. Share by using std::share_ptr's
+  // and std::shared_from_this().
+  //
+  // Note: The BlockInfo block must be parsed before sharing of the
+  // BlockInfoRecordsMap.  Therefore, before changing to a parallel parse, the
+  // BlockInfoRecordsMap must be frozen.  Failure to do so, can lead to
+  // unexpected behaviour.
+  //
+  // In practice, this means that only function blocks can be parsed in
+  // parallel.
+  class BlockInfoRecordsMap :
+      public std::enable_shared_from_this<BlockInfoRecordsMap> {
+    friend class NaClBitstreamReader;
+    BlockInfoRecordsMap(const BlockInfoRecordsMap&) = delete;
+    BlockInfoRecordsMap &operator=(const BlockInfoRecordsMap&) = delete;
+  public:
+    using InfosMap = std::unordered_map<unsigned, std::unique_ptr<BlockInfo>>;
+
+    static SharedBlockInfoMap create() {
+      return SharedBlockInfoMap(new BlockInfoRecordsMap());
+    }
+    ~BlockInfoRecordsMap() = default;
+
+    bool isFrozen() const {
+      return IsFrozen.load();
+    }
+
+    // Returns true if already frozen.
+    bool freeze() {
+      return IsFrozen.exchange(true);
+    }
+
+    BlockInfo *getBlockInfo(unsigned BlockID) {
+      auto Pos = KnownInfos.find(BlockID);
+      if (Pos != KnownInfos.end())
+        return Pos->second.get();
+      return getOrCreateUnknownBlockInfo(BlockID);
+    }
+
+    // Locks the BlockInfoRecordsMap for the lifetime of the UpdateLock.  Used
+    // to allow the parsing of a BlockInfo block, and install global
+    // abbreviations.
+    //
+    // Verifies that the BlockInfoRecordsMap didn't get frozen during the
+    // instance's lifetime as a safety precaution. That is, it checks that no
+    // bitstream reader was created to share the global abbreviations before the
+    // global abbreviations are defined.
+    class UpdateLock {
+      UpdateLock() = delete;
+      UpdateLock(const UpdateLock&) = delete;
+      UpdateLock &operator=(const UpdateLock&) = delete;
+    public:
+      explicit UpdateLock(BlockInfoRecordsMap &BlockInfoRecords);
+      ~UpdateLock();
+    private:
+      // The BlockInfoRecordsMap to update.
+      BlockInfoRecordsMap &BlockInfoRecords;
+      // The locked mutex from BlockInfoRecordsMap;
+      std::unique_lock<std::mutex> Lock;
+    };
+
+  private:
+    // The set of known BlockInfo's. This map is prepopulated so that fast
+    // lookup can be performed thread safe (i.e. without using a lock).
+    InfosMap KnownInfos;
+    // The set of unknown BlockInfo's. This map is to handle unknown (and hence,
+    // invalid) PNaCl bitcode files. This map is updated incrementally, and uses
+    // UnknownBlockInfoLock to make it thread safe.
+    InfosMap UnknownInfos;
+    // True if the known BlockInfo blocks are frozen (i.e. the bitstream reader
+    // will ignore the BlockInfo block).
+    std::atomic_bool IsFrozen;
+    // Lock to use to update this data structure.
+    std::mutex UpdateRecordsLock;
+    // Lock to get/create an unknonw block info.
+    std::mutex UnknownBlockInfoLock;
+
+    BlockInfoRecordsMap();
+
+    BlockInfo *getOrCreateUnknownBlockInfo(unsigned BlockID);
+  };
+
+private:
+  friend class NaClBitstreamCursor;
+
+  std::unique_ptr<MemoryObject> BitcodeBytes;
+
+  SharedBlockInfoMap BlockInfoRecords;
+
+  /// \brief Holds the offset of the first byte after the header.
+  size_t InitialAddress;
+
+  // Holds the number of bytes to add to the bitcode position, when reporting
+  // errors. Useful when using parallel parses of function blocks.
+  size_t ErrorOffset = 0;
+
+  // True if filler should be added to byte align records.
+  bool AlignBitcodeRecords = false;
+  NaClBitstreamReader(const NaClBitstreamReader&) = delete;
+  void operator=(const NaClBitstreamReader&) = delete;
+
+
+  void initFromHeader(NaClBitcodeHeader &Header) {
+    InitialAddress = Header.getHeaderSize();
+    AlignBitcodeRecords = Header.getAlignBitcodeRecords();
+  }
+
+public:
+  /// Read stream from sequence of bytes [Start .. End) after parsing
+  /// the given bitcode header.
+  NaClBitstreamReader(const unsigned char *Start, const unsigned char *End,
+                      NaClBitcodeHeader &Header)
+      : BitcodeBytes(getNonStreamedMemoryObject(Start, End)),
+        BlockInfoRecords(BlockInfoRecordsMap::create()) {
+    initFromHeader(Header);
+  }
+
+  /// Read stream from Bytes, after parsing the given bitcode header.
+  NaClBitstreamReader(MemoryObject *Bytes, NaClBitcodeHeader &Header)
+      : BitcodeBytes(Bytes), BlockInfoRecords(BlockInfoRecordsMap::create())
+  { initFromHeader(Header); }
+
+  /// Read stream from bytes, starting at the given initial address.
+  /// Provides simple API for unit testing.
+  NaClBitstreamReader(MemoryObject *Bytes, size_t InitialAddress)
+      : BitcodeBytes(Bytes), BlockInfoRecords(BlockInfoRecordsMap::create()),
+        InitialAddress(InitialAddress) {}
+
+  /// Read stream from sequence of bytes [Start .. End), using the global
+  /// abbreviations of the given bitstream reader. Assumes that [Start .. End)
+  /// is copied from Reader's memory object.
+  NaClBitstreamReader(size_t StartAddress, const unsigned char *Start,
+                      const unsigned char *End, NaClBitstreamReader *Reader)
+      : BitcodeBytes(getNonStreamedMemoryObject(Start, End)),
+        BlockInfoRecords(Reader->BlockInfoRecords), InitialAddress(0),
+        ErrorOffset(StartAddress) { BlockInfoRecords->freeze(); }
+
+  // Returns the memory object that is being read.
+  MemoryObject &getBitcodeBytes() { return *BitcodeBytes; }
+
+  ~NaClBitstreamReader() {}
+
+  /// \brief Returns the initial address (after the header) of the input stream.
+  size_t getInitialAddress() const {
+    return InitialAddress;
+  }
+
+  /// Returns the byte address of the first byte in the bitstream. Used
+  /// for error reporting.
+  size_t getErrorOffset() const { return ErrorOffset; }
+
+  //===--------------------------------------------------------------------===//
+  // Block Manipulation
+  //===--------------------------------------------------------------------===//
+
+  BlockInfo *getBlockInfo(unsigned BlockID) {
+    return BlockInfoRecords->getBlockInfo(BlockID);
+  }
+};
+
+/// When advancing through a bitstream cursor, each advance can discover a few
+/// different kinds of entries:
+struct NaClBitstreamEntry {
+  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 NaClBitstreamEntry getError() {
+    NaClBitstreamEntry E; E.Kind = Error; return E;
+  }
+  static NaClBitstreamEntry getEndBlock() {
+    NaClBitstreamEntry E; E.Kind = EndBlock; return E;
+  }
+  static NaClBitstreamEntry getSubBlock(unsigned ID) {
+    NaClBitstreamEntry E; E.Kind = SubBlock; E.ID = ID; return E;
+  }
+  static NaClBitstreamEntry getRecord(unsigned AbbrevID) {
+    NaClBitstreamEntry E; E.Kind = Record; E.ID = AbbrevID; return E;
+  }
+};
+
+/// Models default view of a bitcode record.
+typedef SmallVector<uint64_t, 8> NaClBitcodeRecordVector;
+
+/// Class NaClAbbrevListener is used to allow instances of class
+/// NaClBitcodeParser to listen to record details when processing
+/// abbreviations. The major reason for using a listener is that the
+/// NaCl bitcode reader would require a major rewrite (including the
+/// introduction of more overhead) if we were to lift abbreviations up
+/// to the bitcode reader. That is, not only would we have to lift the
+/// block processing up into the readers (i.e. many blocks in
+/// NaClBitcodeReader and NaClBitcodeParser), but add many new API's
+/// to allow the readers to update internals of the bit stream reader
+/// appropriately.
+class NaClAbbrevListener {
+  NaClAbbrevListener(const NaClAbbrevListener&) = delete;
+  void operator=(const NaClAbbrevListener&) = delete;
+public:
+  NaClAbbrevListener() {}
+  virtual ~NaClAbbrevListener() {}
+
+  /// Called to process the read abbreviation.
+  virtual void ProcessAbbreviation(NaClBitCodeAbbrev *Abbrv,
+                                   bool IsLocal) = 0;
+
+  /// Called after entering block. NumWords is the number of words
+  /// in the block.
+  virtual void BeginBlockInfoBlock(unsigned NumWords) = 0;
+
+  /// Called if a naclbitc::BLOCKINFO_CODE_SETBID record is found in
+  /// NaClBitstreamCursor::ReadBlockInfoBlock.
+  virtual void SetBID() = 0;
+
+  /// Called just before an EndBlock record is processed by
+  /// NaClBitstreamCursor::ReadBlockInfoBlock
+  virtual void EndBlockInfoBlock() = 0;
+
+  /// The values of the bitcode record associated with the called
+  /// virtual function.
+  NaClBitcodeRecordVector Values;
+
+  /// Start bit for current record being processed in
+  /// NaClBitstreamCursor::ReadBlockInfoBlock.
+  uint64_t StartBit;
+};
+
+/// This represents a position within a bitcode file. There may be multiple
+/// independent cursors reading within one bitstream, each maintaining their
+/// own local state.
+///
+/// Unlike iterators, NaClBitstreamCursors are heavy-weight objects
+/// that should not be passed by value.
+class NaClBitstreamCursor {
+public:
+  /// This class handles errors in the bitstream reader. Redirects
+  /// fatal error messages to virtual method Fatal.
+  class ErrorHandler {
+    ErrorHandler(const ErrorHandler &) = delete;
+    ErrorHandler &operator=(const ErrorHandler &) = delete;
+  public:
+    explicit ErrorHandler(NaClBitstreamCursor &Cursor) : Cursor(Cursor) {}
+    LLVM_ATTRIBUTE_NORETURN
+    virtual void Fatal(const std::string &ErrorMessage) const;
+    virtual ~ErrorHandler() {}
+    uint64_t getCurrentBitNo() const {
+      return Cursor.GetCurrentBitNo();
+    }
+  private:
+    NaClBitstreamCursor &Cursor;
+  };
+
+private:
+  friend class Deserializer;
+  NaClBitstreamReader *BitStream;
+  size_t NextChar;
+  // The current error handler for the bitstream reader.
+  std::unique_ptr<ErrorHandler> ErrHandler;
+
+  // The size of the bitcode. 0 if we don't know it yet.
+  size_t Size;
+
+  /// 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.
+  typedef size_t word_t;
+  word_t CurWord;
+
+  /// This is the number of bits in CurWord that are valid. This
+  /// is always from [0...bits_of(word_t)-1] inclusive.
+  unsigned BitsInCurWord;
+
+  // Data specific to a block being scanned.
+  class Block {
+  public:
+    Block() = delete;
+    Block &operator=(const Block &Rhs) {
+      GlobalAbbrevs = Rhs.GlobalAbbrevs;
+      NumGlobalAbbrevs = Rhs.NumGlobalAbbrevs;
+      LocalAbbrevs = Rhs.LocalAbbrevs;
+      CodeAbbrev = Rhs.CodeAbbrev;
+      return *this;
+    }
+    Block(NaClBitstreamReader::BlockInfo *GlobalAbbrevs,
+          NaClBitcodeSelectorAbbrev& CodeAbbrev)
+        : GlobalAbbrevs(GlobalAbbrevs),
+          NumGlobalAbbrevs(GlobalAbbrevs->getAbbrevs().getVector().size()),
+          LocalAbbrevs(), CodeAbbrev(CodeAbbrev) {}
+    Block(NaClBitstreamReader::BlockInfo *GlobalAbbrevs)
+        : GlobalAbbrevs(GlobalAbbrevs),
+          NumGlobalAbbrevs(GlobalAbbrevs->getAbbrevs().getVector().size()),
+          LocalAbbrevs(), CodeAbbrev() {}
+    ~Block() = default;
+    const NaClBitstreamReader::AbbrevList &getGlobalAbbrevs() const {
+      return GlobalAbbrevs->getAbbrevs();
+    }
+    unsigned getNumGlobalAbbrevs() const { return NumGlobalAbbrevs; }
+    const NaClBitstreamReader::AbbrevList &getLocalAbbrevs() const {
+      return LocalAbbrevs;
+    }
+    const NaClBitcodeSelectorAbbrev &getCodeAbbrev() const {
+      return CodeAbbrev;
+    }
+    void setCodeAbbrev(NaClBitcodeSelectorAbbrev &Abbrev) {
+      CodeAbbrev = Abbrev;
+    }
+    NaClBitCodeAbbrev *appendLocalCreate() {
+      return LocalAbbrevs.appendCreate();
+    }
+    void moveLocalAbbrevToAbbrevList(NaClBitstreamReader::AbbrevList *List) {
+      if (List != &LocalAbbrevs) {
+        NaClBitCodeAbbrev *Abbv = LocalAbbrevs.last();
+        List->append(Abbv);
+        LocalAbbrevs.popLast();
+      }
+    }
+  private:
+    friend class NaClBitstreamCursor;
+    // The global abbreviations associated with this scope.
+    NaClBitstreamReader::BlockInfo *GlobalAbbrevs;
+    // Number of abbreviations when block was entered. Used to limit scope of
+    // CurBlockInfo, since any abbreviation added inside a BlockInfo block
+    // (within this block) must not effect global abbreviations.
+    unsigned NumGlobalAbbrevs;
+    NaClBitstreamReader::AbbrevList LocalAbbrevs;
+    // This is the declared size of code values used for the current block, in
+    // bits.
+    NaClBitcodeSelectorAbbrev CodeAbbrev;
+  };
+
+  /// This tracks the Block-specific information for each nested block.
+  SmallVector<Block, 8> BlockScope;
+
+  NaClBitstreamCursor(const NaClBitstreamCursor &) = delete;
+  NaClBitstreamCursor &operator=(const NaClBitstreamCursor &) = delete;
+
+public:
+  NaClBitstreamCursor() : ErrHandler(new ErrorHandler(*this)) {
+    init(nullptr);
+  }
+
+  explicit NaClBitstreamCursor(NaClBitstreamReader &R)
+      : ErrHandler(new ErrorHandler(*this)) { init(&R); }
+
+  void init(NaClBitstreamReader *R) {
+    freeState();
+    BitStream = R;
+    NextChar = (BitStream == nullptr) ? 0 : BitStream->getInitialAddress();
+    Size = 0;
+    BitsInCurWord = 0;
+    if (BitStream) {
+      BlockScope.push_back(
+          Block(BitStream->getBlockInfo(naclbitc::TOP_LEVEL_BLOCKID)));
+    }
+  }
+
+  ~NaClBitstreamCursor() {
+    freeState();
+  }
+
+  void freeState() {
+    while (!BlockScope.empty())
+      BlockScope.pop_back();
+  }
+
+  // Replaces the current bitstream error handler with the new
+  // handler. Takes ownership of the new handler and deletes it when
+  // it is no longer needed.
+  void setErrorHandler(std::unique_ptr<ErrorHandler> &NewHandler) {
+    ErrHandler = std::move(NewHandler);
+  }
+
+  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 == 0 || BitStream->getBitcodeBytes().isValidAddress(
+        static_cast<uint64_t>(pos - 1));
+  }
+
+  bool AtEndOfStream() {
+    if (BitsInCurWord != 0)
+      return false;
+    if (Size != 0)
+      return Size == NextChar;
+    fillCurWord();
+    return BitsInCurWord == 0;
+  }
+
+  /// Return the number of bits used to encode an abbrev #.
+  unsigned getAbbrevIDWidth() const {
+    return BlockScope.back().getCodeAbbrev().NumBits;
+  }
+
+  /// Return the bit # of the bit we are reading.
+  uint64_t GetCurrentBitNo() const {
+    return NextChar*CHAR_BIT - BitsInCurWord;
+  }
+
+  /// Converts the given position into the corresponding Error position.
+  uint64_t getErrorBitNo(uint64_t Position) const {
+    return BitStream->getErrorOffset() * CHAR_BIT + Position;
+  }
+
+  /// Returns the current bit address for reporting errors.
+  uint64_t getErrorBitNo() const {
+    return getErrorBitNo(GetCurrentBitNo());
+  }
+
+  NaClBitstreamReader *getBitStreamReader() {
+    return BitStream;
+  }
+  const NaClBitstreamReader *getBitStreamReader() const {
+    return BitStream;
+  }
+
+  /// Returns the current bit address (string) of the bit cursor.
+  std::string getCurrentBitAddress() const {
+    return naclbitc::getBitAddress(GetCurrentBitNo());
+  }
+
+  /// 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.
+  /// Use the given abbreviation listener (if provided).
+  NaClBitstreamEntry advance(unsigned Flags, NaClAbbrevListener *Listener) {
+    while (1) {
+      unsigned Code = ReadCode();
+      if (Code == naclbitc::END_BLOCK) {
+        // Pop the end of the block unless Flags tells us not to.
+        if (!(Flags & AF_DontPopBlockAtEnd) && ReadBlockEnd())
+          return NaClBitstreamEntry::getError();
+        return NaClBitstreamEntry::getEndBlock();
+      }
+
+      if (Code == naclbitc::ENTER_SUBBLOCK)
+        return NaClBitstreamEntry::getSubBlock(ReadSubBlockID());
+
+      if (Code == naclbitc::DEFINE_ABBREV &&
+          !(Flags & AF_DontAutoprocessAbbrevs)) {
+        // We read and accumulate abbrev's, the client can't do anything with
+        // them anyway.
+        ReadAbbrevRecord(true, Listener);
+        continue;
+      }
+
+      return NaClBitstreamEntry::getRecord(Code);
+    }
+  }
+
+  /// This is a convenience function for clients that don't expect any
+  /// subblocks. This just skips over them automatically.
+  NaClBitstreamEntry advanceSkippingSubblocks(unsigned Flags = 0) {
+    while (1) {
+      // If we found a normal entry, return it.
+      NaClBitstreamEntry Entry = advance(Flags, 0);
+      if (Entry.Kind != NaClBitstreamEntry::SubBlock)
+        return Entry;
+
+      // If we found a sub-block, just skip over it and check the next entry.
+      if (SkipBlock())
+        return NaClBitstreamEntry::getError();
+    }
+  }
+
+  /// Returns the starting byte of the word containing BitNo.
+  uintptr_t getStartWordByteForBit(uint64_t BitNo) const {
+    return uintptr_t(BitNo/CHAR_BIT) & ~(sizeof(word_t)-1);
+  }
+
+  /// Returns the index of BitNo within the word it appears in.
+  unsigned getWordBitNo(uint64_t BitNo) const {
+    return unsigned(BitNo & (sizeof(word_t)*CHAR_BIT-1));
+  }
+
+  /// Returns the ending byte of the word containing BitNo.
+  uintptr_t getEndWordByteForBit(uint64_t BitNo) const {
+    return getStartWordByteForBit(BitNo) +
+        (getWordBitNo(BitNo)
+         ? sizeof(word_t)
+         : 0);
+  }
+
+  /// Fills Buffer[Size] using bytes at Address (in the memory object being
+  /// read). Returns number of bytes filled (less than Size if at end of memory
+  /// object).
+  uint64_t fillBuffer(uint8_t *Buffer, size_t Size, size_t Address) const {
+    return BitStream->getBitcodeBytes().readBytes(Buffer, Size, Address);
+  }
+
+  /// Reset the stream to the specified bit number.
+  void JumpToBit(uint64_t BitNo) {
+    const uintptr_t ByteNo = getStartWordByteForBit(BitNo);
+    const unsigned WordBitNo = getWordBitNo(BitNo);
+    if (!canSkipToPos(ByteNo))
+      reportInvalidJumpToBit(BitNo);
+
+    // Move the cursor to the right word.
+    NextChar = ByteNo;
+    BitsInCurWord = 0;
+
+    // Skip over any bits that are already consumed.
+    if (WordBitNo)
+      Read(WordBitNo);
+  }
+
+  void fillCurWord() {
+    assert(Size == 0 || NextChar < (unsigned)Size);
+
+    // Read the next word from the stream.
+    uint8_t Array[sizeof(word_t)] = {0};
+
+    uint64_t BytesRead = fillBuffer(Array, sizeof(Array), NextChar);
+
+    // If we run out of data, stop at the end of the stream.
+    if (BytesRead == 0) {
+      Size = NextChar;
+      return;
+    }
+
+    CurWord =
+        support::endian::read<word_t, support::little, support::unaligned>(
+            Array);
+    NextChar += BytesRead;
+    BitsInCurWord = BytesRead * CHAR_BIT;
+  }
+
+  word_t Read(unsigned NumBits) {
+    static const unsigned BitsInWord = sizeof(word_t) * CHAR_BIT;
+
+    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;
+
+    fillCurWord();
+
+    // If we run out of data, stop at the end of the stream.
+    if (BitsLeft > BitsInCurWord)
+      return 0;
+
+    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;
+  }
+
+  uint32_t ReadVBR(unsigned NumBits) {
+    uint32_t Piece = Read(NumBits);
+    if ((Piece & (1U << (NumBits-1))) == 0)
+      return Piece;
+
+    uint32_t Result = 0;
+    unsigned NextBit = 0;
+    while (1) {
+      Result |= (Piece & ((1U << (NumBits-1))-1)) << NextBit;
+
+      if ((Piece & (1U << (NumBits-1))) == 0)
+        return Result;
+
+      NextBit += NumBits-1;
+      Piece = Read(NumBits);
+    }
+  }
+
+  // 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.
+  uint64_t ReadVBR64(unsigned NumBits) {
+    uint32_t Piece = Read(NumBits);
+    if ((Piece & (1U << (NumBits-1))) == 0)
+      return uint64_t(Piece);
+
+    uint64_t Result = 0;
+    unsigned NextBit = 0;
+    while (1) {
+      Result |= uint64_t(Piece & ((1U << (NumBits-1))-1)) << NextBit;
+
+      if ((Piece & (1U << (NumBits-1))) == 0)
+        return Result;
+
+      NextBit += NumBits-1;
+      Piece = Read(NumBits);
+    }
+  }
+
+private:
+  void SkipToByteBoundary() {
+    unsigned BitsToSkip = BitsInCurWord % CHAR_BIT;
+    if (BitsToSkip) {
+      CurWord >>= BitsToSkip;
+      BitsInCurWord -= BitsToSkip;
+    }
+  }
+
+  void SkipToByteBoundaryIfAligned() {
+    if (BitStream->AlignBitcodeRecords)
+      SkipToByteBoundary();
+  }
+
+  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;
+  }
+public:
+
+  unsigned ReadCode() {
+    const NaClBitcodeSelectorAbbrev &CodeAbbrev =
+        BlockScope.back().getCodeAbbrev();
+    return CodeAbbrev.IsFixed
+        ? Read(CodeAbbrev.NumBits)
+        : ReadVBR(CodeAbbrev.NumBits);
+  }
+
+  // Block header:
+  //    [ENTER_SUBBLOCK, blockid, newcodelen, <align4bytes>, blocklen]
+
+  /// Having read the ENTER_SUBBLOCK code, read the BlockID for the block.
+  unsigned ReadSubBlockID() {
+    return ReadVBR(naclbitc::BlockIDWidth);
+  }
+
+  /// Having read the ENTER_SUBBLOCK abbrevid and a BlockID, skip over the body
+  /// of this block. If the block record is malformed, return true.
+  bool SkipBlock() {
+    // Read and ignore the codelen value.  Since we are skipping this block, we
+    // don't care what code widths are used inside of it.
+    ReadVBR(naclbitc::CodeLenWidth);
+    SkipToFourByteBoundary();
+    unsigned NumFourBytes = Read(naclbitc::BlockSizeWidth);
+
+    // Check that the block wasn't partially defined, and that the offset isn't
+    // bogus.
+    size_t SkipTo = GetCurrentBitNo() + NumFourBytes*4*CHAR_BIT;
+    if (AtEndOfStream() || !canSkipToPos(SkipTo/CHAR_BIT))
+      return true;
+
+    JumpToBit(SkipTo);
+    return false;
+  }
+
+  /// Having read the ENTER_SUBBLOCK abbrevid, enter the block, and return true
+  /// if the block has an error.
+  bool EnterSubBlock(unsigned BlockID, unsigned *NumWordsP = nullptr);
+
+  bool ReadBlockEnd() {
+    if (BlockScope.empty()) return true;
+
+    // Block tail:
+    //    [END_BLOCK, <align4bytes>]
+    SkipToFourByteBoundary();
+
+    BlockScope.pop_back();
+    return false;
+  }
+
+private:
+
+  //===--------------------------------------------------------------------===//
+  // Record Processing
+  //===--------------------------------------------------------------------===//
+
+private:
+  // Returns abbreviation encoding associated with Value.
+  NaClBitCodeAbbrevOp::Encoding getEncoding(uint64_t Value);
+
+  void skipAbbreviatedField(const NaClBitCodeAbbrevOp &Op);
+
+  // Reads the next Value using the abbreviation Op. Returns true only
+  // if Op is an array (and sets Value to the number of elements in the
+  // array).
+  inline bool readRecordAbbrevField(const NaClBitCodeAbbrevOp &Op,
+                                    uint64_t &Value);
+
+  // Reads and returns the next value using the abbreviation Op,
+  // assuming Op appears after an array abbreviation.
+  inline uint64_t readArrayAbbreviatedField(const NaClBitCodeAbbrevOp &Op);
+
+  // Reads the array abbreviation Op, NumArrayElements times, putting
+  // the read values in Vals.
+  inline void readArrayAbbrev(const NaClBitCodeAbbrevOp &Op,
+                              unsigned NumArrayElements,
+                              SmallVectorImpl<uint64_t> &Vals);
+
+  // Reports that that abbreviation Index is not valid.
+  void reportInvalidAbbrevNumber(unsigned Index) const;
+
+  // Reports that jumping to Bit is not valid.
+  void reportInvalidJumpToBit(uint64_t Bit) const;
+
+public:
+
+  /// Return the abbreviation for the specified AbbrevId.
+  const NaClBitCodeAbbrev *getAbbrev(unsigned AbbrevID) const {
+    unsigned AbbrevNo = AbbrevID-naclbitc::FIRST_APPLICATION_ABBREV;
+    const Block &CurBlock = BlockScope.back();
+    const unsigned NumGlobalAbbrevs = CurBlock.getNumGlobalAbbrevs();
+    if (AbbrevNo < NumGlobalAbbrevs)
+      return CurBlock.getGlobalAbbrevs().getVector()[AbbrevNo];
+    unsigned LocalAbbrevNo = AbbrevNo - NumGlobalAbbrevs;
+    NaClBitstreamReader::AbbrevListVector
+        LocalAbbrevs = CurBlock.getLocalAbbrevs().getVector();
+    if (LocalAbbrevNo >= LocalAbbrevs.size())
+      reportInvalidAbbrevNumber(AbbrevID);
+    return LocalAbbrevs[LocalAbbrevNo];
+  }
+
+  /// Read the current record and discard it.
+  void skipRecord(unsigned AbbrevID);
+  
+  unsigned readRecord(unsigned AbbrevID, SmallVectorImpl<uint64_t> &Vals);
+
+  //===--------------------------------------------------------------------===//
+  // Abbrev Processing
+  //===--------------------------------------------------------------------===//
+  // IsLocal indicates where the abbreviation occurs. If it is in the
+  // BlockInfo block, IsLocal is false. In all other cases, IsLocal is
+  // true.
+  void ReadAbbrevRecord(bool IsLocal,
+                        NaClAbbrevListener *Listener);
+
+  // Skips over an abbreviation record. Duplicates code of ReadAbbrevRecord,
+  // except that no abbreviation is built.
+  void SkipAbbrevRecord();
+  
+  bool ReadBlockInfoBlock(NaClAbbrevListener *Listener);
+};
+
+} // End llvm namespace
+
+#endif