diff --git a/llvm/lib/DebugInfo/MSF/MappedBlockStream.cpp b/llvm/lib/DebugInfo/MSF/MappedBlockStream.cpp
new file mode 100644
index 0000000..0a3b2b9
--- /dev/null
+++ b/llvm/lib/DebugInfo/MSF/MappedBlockStream.cpp
@@ -0,0 +1,380 @@
+//===- MappedBlockStream.cpp - Reads stream data from an MSF file ---------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/DebugInfo/MSF/MappedBlockStream.h"
+
+#include "llvm/DebugInfo/MSF/IMSFFile.h"
+#include "llvm/DebugInfo/MSF/MSFCommon.h"
+#include "llvm/DebugInfo/MSF/MSFError.h"
+#include "llvm/DebugInfo/MSF/MSFStreamLayout.h"
+
+using namespace llvm;
+using namespace llvm::msf;
+
+namespace {
+template <typename Base> class MappedBlockStreamImpl : public Base {
+public:
+  template <typename... Args>
+  MappedBlockStreamImpl(Args &&... Params)
+      : Base(std::forward<Args>(Params)...) {}
+};
+}
+
+typedef std::pair<uint32_t, uint32_t> Interval;
+static Interval intersect(const Interval &I1, const Interval &I2) {
+  return std::make_pair(std::max(I1.first, I2.first),
+                        std::min(I1.second, I2.second));
+}
+
+MappedBlockStream::MappedBlockStream(uint32_t BlockSize, uint32_t NumBlocks,
+                                     const MSFStreamLayout &Layout,
+                                     const ReadableStream &MsfData)
+    : BlockSize(BlockSize), NumBlocks(NumBlocks), StreamLayout(Layout),
+      MsfData(MsfData) {}
+
+std::unique_ptr<MappedBlockStream>
+MappedBlockStream::createStream(uint32_t BlockSize, uint32_t NumBlocks,
+                                const MSFStreamLayout &Layout,
+                                const ReadableStream &MsfData) {
+  return llvm::make_unique<MappedBlockStreamImpl<MappedBlockStream>>(
+      BlockSize, NumBlocks, Layout, MsfData);
+}
+
+std::unique_ptr<MappedBlockStream>
+MappedBlockStream::createIndexedStream(const MSFLayout &Layout,
+                                       const ReadableStream &MsfData,
+                                       uint32_t StreamIndex) {
+  MSFStreamLayout SL;
+  SL.Blocks = Layout.StreamMap[StreamIndex];
+  SL.Length = Layout.StreamSizes[StreamIndex];
+  return llvm::make_unique<MappedBlockStreamImpl<MappedBlockStream>>(
+      Layout.SB->BlockSize, Layout.SB->NumBlocks, SL, MsfData);
+}
+
+std::unique_ptr<MappedBlockStream>
+MappedBlockStream::createDirectoryStream(const MSFLayout &Layout,
+                                         const ReadableStream &MsfData) {
+  MSFStreamLayout SL;
+  SL.Blocks = Layout.DirectoryBlocks;
+  SL.Length = Layout.SB->NumDirectoryBytes;
+  return createStream(Layout.SB->BlockSize, Layout.SB->NumBlocks, SL, MsfData);
+}
+
+Error MappedBlockStream::readBytes(uint32_t Offset, uint32_t Size,
+                                   ArrayRef<uint8_t> &Buffer) const {
+  // Make sure we aren't trying to read beyond the end of the stream.
+  if (Size > StreamLayout.Length)
+    return make_error<MSFError>(msf_error_code::insufficient_buffer);
+  if (Offset > StreamLayout.Length - Size)
+    return make_error<MSFError>(msf_error_code::insufficient_buffer);
+
+  if (tryReadContiguously(Offset, Size, Buffer))
+    return Error::success();
+
+  auto CacheIter = CacheMap.find(Offset);
+  if (CacheIter != CacheMap.end()) {
+    // Try to find an alloc that was large enough for this request.
+    for (auto &Entry : CacheIter->second) {
+      if (Entry.size() >= Size) {
+        Buffer = Entry.slice(0, Size);
+        return Error::success();
+      }
+    }
+  }
+
+  // We couldn't find a buffer that started at the correct offset (the most
+  // common scenario).  Try to see if there is a buffer that starts at some
+  // other offset but overlaps the desired range.
+  for (auto &CacheItem : CacheMap) {
+    Interval RequestExtent = std::make_pair(Offset, Offset + Size);
+
+    // We already checked this one on the fast path above.
+    if (CacheItem.first == Offset)
+      continue;
+    // If the initial extent of the cached item is beyond the ending extent
+    // of the request, there is no overlap.
+    if (CacheItem.first >= Offset + Size)
+      continue;
+
+    // We really only have to check the last item in the list, since we append
+    // in order of increasing length.
+    if (CacheItem.second.empty())
+      continue;
+
+    auto CachedAlloc = CacheItem.second.back();
+    // If the initial extent of the request is beyond the ending extent of
+    // the cached item, there is no overlap.
+    Interval CachedExtent =
+        std::make_pair(CacheItem.first, CacheItem.first + CachedAlloc.size());
+    if (RequestExtent.first >= CachedExtent.first + CachedExtent.second)
+      continue;
+
+    Interval Intersection = intersect(CachedExtent, RequestExtent);
+    // Only use this if the entire request extent is contained in the cached
+    // extent.
+    if (Intersection != RequestExtent)
+      continue;
+
+    uint32_t CacheRangeOffset =
+        AbsoluteDifference(CachedExtent.first, Intersection.first);
+    Buffer = CachedAlloc.slice(CacheRangeOffset, Size);
+    return Error::success();
+  }
+
+  // Otherwise allocate a large enough buffer in the pool, memcpy the data
+  // into it, and return an ArrayRef to that.  Do not touch existing pool
+  // allocations, as existing clients may be holding a pointer which must
+  // not be invalidated.
+  uint8_t *WriteBuffer = static_cast<uint8_t *>(Pool.Allocate(Size, 8));
+  if (auto EC = readBytes(Offset, MutableArrayRef<uint8_t>(WriteBuffer, Size)))
+    return EC;
+
+  if (CacheIter != CacheMap.end()) {
+    CacheIter->second.emplace_back(WriteBuffer, Size);
+  } else {
+    std::vector<CacheEntry> List;
+    List.emplace_back(WriteBuffer, Size);
+    CacheMap.insert(std::make_pair(Offset, List));
+  }
+  Buffer = ArrayRef<uint8_t>(WriteBuffer, Size);
+  return Error::success();
+}
+
+Error MappedBlockStream::readLongestContiguousChunk(
+    uint32_t Offset, ArrayRef<uint8_t> &Buffer) const {
+  // Make sure we aren't trying to read beyond the end of the stream.
+  if (Offset >= StreamLayout.Length)
+    return make_error<MSFError>(msf_error_code::insufficient_buffer);
+  uint32_t First = Offset / BlockSize;
+  uint32_t Last = First;
+
+  while (Last < NumBlocks - 1) {
+    if (StreamLayout.Blocks[Last] != StreamLayout.Blocks[Last + 1] - 1)
+      break;
+    ++Last;
+  }
+
+  uint32_t OffsetInFirstBlock = Offset % BlockSize;
+  uint32_t BytesFromFirstBlock = BlockSize - OffsetInFirstBlock;
+  uint32_t BlockSpan = Last - First + 1;
+  uint32_t ByteSpan = BytesFromFirstBlock + (BlockSpan - 1) * BlockSize;
+
+  ArrayRef<uint8_t> BlockData;
+  uint32_t MsfOffset = blockToOffset(StreamLayout.Blocks[First], BlockSize);
+  if (auto EC = MsfData.readBytes(MsfOffset, BlockSize, BlockData))
+    return EC;
+
+  BlockData = BlockData.drop_front(OffsetInFirstBlock);
+  Buffer = ArrayRef<uint8_t>(BlockData.data(), ByteSpan);
+  return Error::success();
+}
+
+uint32_t MappedBlockStream::getLength() const { return StreamLayout.Length; }
+
+bool MappedBlockStream::tryReadContiguously(uint32_t Offset, uint32_t Size,
+                                            ArrayRef<uint8_t> &Buffer) const {
+  // Attempt to fulfill the request with a reference directly into the stream.
+  // This can work even if the request crosses a block boundary, provided that
+  // all subsequent blocks are contiguous.  For example, a 10k read with a 4k
+  // block size can be filled with a reference if, from the starting offset,
+  // 3 blocks in a row are contiguous.
+  uint32_t BlockNum = Offset / BlockSize;
+  uint32_t OffsetInBlock = Offset % BlockSize;
+  uint32_t BytesFromFirstBlock = std::min(Size, BlockSize - OffsetInBlock);
+  uint32_t NumAdditionalBlocks =
+      llvm::alignTo(Size - BytesFromFirstBlock, BlockSize) / BlockSize;
+
+  uint32_t RequiredContiguousBlocks = NumAdditionalBlocks + 1;
+  uint32_t E = StreamLayout.Blocks[BlockNum];
+  for (uint32_t I = 0; I < RequiredContiguousBlocks; ++I, ++E) {
+    if (StreamLayout.Blocks[I + BlockNum] != E)
+      return false;
+  }
+
+  // Read out the entire block where the requested offset starts.  Then drop
+  // bytes from the beginning so that the actual starting byte lines up with
+  // the requested starting byte.  Then, since we know this is a contiguous
+  // cross-block span, explicitly resize the ArrayRef to cover the entire
+  // request length.
+  ArrayRef<uint8_t> BlockData;
+  uint32_t FirstBlockAddr = StreamLayout.Blocks[BlockNum];
+  uint32_t MsfOffset = blockToOffset(FirstBlockAddr, BlockSize);
+  if (auto EC = MsfData.readBytes(MsfOffset, BlockSize, BlockData)) {
+    consumeError(std::move(EC));
+    return false;
+  }
+  BlockData = BlockData.drop_front(OffsetInBlock);
+  Buffer = ArrayRef<uint8_t>(BlockData.data(), Size);
+  return true;
+}
+
+Error MappedBlockStream::readBytes(uint32_t Offset,
+                                   MutableArrayRef<uint8_t> Buffer) const {
+  uint32_t BlockNum = Offset / BlockSize;
+  uint32_t OffsetInBlock = Offset % BlockSize;
+
+  // Make sure we aren't trying to read beyond the end of the stream.
+  if (Buffer.size() > StreamLayout.Length)
+    return make_error<MSFError>(msf_error_code::insufficient_buffer);
+  if (Offset > StreamLayout.Length - Buffer.size())
+    return make_error<MSFError>(msf_error_code::insufficient_buffer);
+
+  uint32_t BytesLeft = Buffer.size();
+  uint32_t BytesWritten = 0;
+  uint8_t *WriteBuffer = Buffer.data();
+  while (BytesLeft > 0) {
+    uint32_t StreamBlockAddr = StreamLayout.Blocks[BlockNum];
+
+    ArrayRef<uint8_t> BlockData;
+    uint32_t Offset = blockToOffset(StreamBlockAddr, BlockSize);
+    if (auto EC = MsfData.readBytes(Offset, BlockSize, BlockData))
+      return EC;
+
+    const uint8_t *ChunkStart = BlockData.data() + OffsetInBlock;
+    uint32_t BytesInChunk = std::min(BytesLeft, BlockSize - OffsetInBlock);
+    ::memcpy(WriteBuffer + BytesWritten, ChunkStart, BytesInChunk);
+
+    BytesWritten += BytesInChunk;
+    BytesLeft -= BytesInChunk;
+    ++BlockNum;
+    OffsetInBlock = 0;
+  }
+
+  return Error::success();
+}
+
+uint32_t MappedBlockStream::getNumBytesCopied() const {
+  return static_cast<uint32_t>(Pool.getBytesAllocated());
+}
+
+void MappedBlockStream::invalidateCache() { CacheMap.shrink_and_clear(); }
+
+void MappedBlockStream::fixCacheAfterWrite(uint32_t Offset,
+                                           ArrayRef<uint8_t> Data) const {
+  // If this write overlapped a read which previously came from the pool,
+  // someone may still be holding a pointer to that alloc which is now invalid.
+  // Compute the overlapping range and update the cache entry, so any
+  // outstanding buffers are automatically updated.
+  for (const auto &MapEntry : CacheMap) {
+    // If the end of the written extent precedes the beginning of the cached
+    // extent, ignore this map entry.
+    if (Offset + Data.size() < MapEntry.first)
+      continue;
+    for (const auto &Alloc : MapEntry.second) {
+      // If the end of the cached extent precedes the beginning of the written
+      // extent, ignore this alloc.
+      if (MapEntry.first + Alloc.size() < Offset)
+        continue;
+
+      // If we get here, they are guaranteed to overlap.
+      Interval WriteInterval = std::make_pair(Offset, Offset + Data.size());
+      Interval CachedInterval =
+          std::make_pair(MapEntry.first, MapEntry.first + Alloc.size());
+      // If they overlap, we need to write the new data into the overlapping
+      // range.
+      auto Intersection = intersect(WriteInterval, CachedInterval);
+      assert(Intersection.first <= Intersection.second);
+
+      uint32_t Length = Intersection.second - Intersection.first;
+      uint32_t SrcOffset =
+          AbsoluteDifference(WriteInterval.first, Intersection.first);
+      uint32_t DestOffset =
+          AbsoluteDifference(CachedInterval.first, Intersection.first);
+      ::memcpy(Alloc.data() + DestOffset, Data.data() + SrcOffset, Length);
+    }
+  }
+}
+
+WritableMappedBlockStream::WritableMappedBlockStream(
+    uint32_t BlockSize, uint32_t NumBlocks, const MSFStreamLayout &Layout,
+    const WritableStream &MsfData)
+    : ReadInterface(BlockSize, NumBlocks, Layout, MsfData),
+      WriteInterface(MsfData) {}
+
+std::unique_ptr<WritableMappedBlockStream>
+WritableMappedBlockStream::createStream(uint32_t BlockSize, uint32_t NumBlocks,
+                                        const MSFStreamLayout &Layout,
+                                        const WritableStream &MsfData) {
+  return llvm::make_unique<MappedBlockStreamImpl<WritableMappedBlockStream>>(
+      BlockSize, NumBlocks, Layout, MsfData);
+}
+
+std::unique_ptr<WritableMappedBlockStream>
+WritableMappedBlockStream::createIndexedStream(const MSFLayout &Layout,
+                                               const WritableStream &MsfData,
+                                               uint32_t StreamIndex) {
+  MSFStreamLayout SL;
+  SL.Blocks = Layout.StreamMap[StreamIndex];
+  SL.Length = Layout.StreamSizes[StreamIndex];
+  return createStream(Layout.SB->BlockSize, Layout.SB->NumBlocks, SL, MsfData);
+}
+
+std::unique_ptr<WritableMappedBlockStream>
+WritableMappedBlockStream::createDirectoryStream(
+    const MSFLayout &Layout, const WritableStream &MsfData) {
+  MSFStreamLayout SL;
+  SL.Blocks = Layout.DirectoryBlocks;
+  SL.Length = Layout.SB->NumDirectoryBytes;
+  return createStream(Layout.SB->BlockSize, Layout.SB->NumBlocks, SL, MsfData);
+}
+
+Error WritableMappedBlockStream::readBytes(uint32_t Offset, uint32_t Size,
+                                           ArrayRef<uint8_t> &Buffer) const {
+  return ReadInterface.readBytes(Offset, Size, Buffer);
+}
+
+Error WritableMappedBlockStream::readLongestContiguousChunk(
+    uint32_t Offset, ArrayRef<uint8_t> &Buffer) const {
+  return ReadInterface.readLongestContiguousChunk(Offset, Buffer);
+}
+
+uint32_t WritableMappedBlockStream::getLength() const {
+  return ReadInterface.getLength();
+}
+
+Error WritableMappedBlockStream::writeBytes(uint32_t Offset,
+                                            ArrayRef<uint8_t> Buffer) const {
+  // Make sure we aren't trying to write beyond the end of the stream.
+  if (Buffer.size() > getStreamLength())
+    return make_error<MSFError>(msf_error_code::insufficient_buffer);
+
+  if (Offset > getStreamLayout().Length - Buffer.size())
+    return make_error<MSFError>(msf_error_code::insufficient_buffer);
+
+  uint32_t BlockNum = Offset / getBlockSize();
+  uint32_t OffsetInBlock = Offset % getBlockSize();
+
+  uint32_t BytesLeft = Buffer.size();
+  uint32_t BytesWritten = 0;
+  while (BytesLeft > 0) {
+    uint32_t StreamBlockAddr = getStreamLayout().Blocks[BlockNum];
+    uint32_t BytesToWriteInChunk =
+        std::min(BytesLeft, getBlockSize() - OffsetInBlock);
+
+    const uint8_t *Chunk = Buffer.data() + BytesWritten;
+    ArrayRef<uint8_t> ChunkData(Chunk, BytesToWriteInChunk);
+    uint32_t MsfOffset = blockToOffset(StreamBlockAddr, getBlockSize());
+    MsfOffset += OffsetInBlock;
+    if (auto EC = WriteInterface.writeBytes(MsfOffset, ChunkData))
+      return EC;
+
+    BytesLeft -= BytesToWriteInChunk;
+    BytesWritten += BytesToWriteInChunk;
+    ++BlockNum;
+    OffsetInBlock = 0;
+  }
+
+  ReadInterface.fixCacheAfterWrite(Offset, Buffer);
+
+  return Error::success();
+}
+
+Error WritableMappedBlockStream::commit() const {
+  return WriteInterface.commit();
+}