diff --git a/include/llvm/Bitcode/BitstreamReader.h b/include/llvm/Bitcode/BitstreamReader.h
index 6258cecf7a8..05623fe8e71 100644
--- a/include/llvm/Bitcode/BitstreamReader.h
+++ b/include/llvm/Bitcode/BitstreamReader.h
@@ -341,7 +341,7 @@ public:
     // Read the next word from the stream.
     uint8_t Array[sizeof(word_t)] = {0};
 
-    BitStream->getBitcodeBytes().readBytes(NextChar, sizeof(Array), Array);
+    BitStream->getBitcodeBytes().readBytes(Array, sizeof(Array), NextChar);
 
     // Handle big-endian byte-swapping if necessary.
     support::detail::packed_endian_specific_integral
diff --git a/include/llvm/Support/MemoryObject.h b/include/llvm/Support/MemoryObject.h
index 584a2c55c04..f031a127ce6 100644
--- a/include/llvm/Support/MemoryObject.h
+++ b/include/llvm/Support/MemoryObject.h
@@ -36,18 +36,16 @@ public:
   virtual uint64_t getExtent() const = 0;
 
   /// Tries to read a contiguous range of bytes from the region, up to the end
-  /// of the region. You should override this function if there is a quicker way
-  /// than going back and forth with individual bytes.
+  /// of the region.
   ///
   /// @param address  - The address of the first byte, in the same space as
   ///                   getBase().
   /// @param size     - The number of bytes to copy.
   /// @param buf      - A pointer to a buffer to be filled in.  Must be non-NULL
   ///                   and large enough to hold size bytes.
-  /// @result         - 0 if successful; -1 if not.  Failure may be due to a
-  ///                   bounds violation or an implementation-specific error.
-  virtual int readBytes(uint64_t address, uint64_t size,
-                        uint8_t *buf) const = 0;
+  /// @result         - The number of bytes read.
+  virtual uint64_t readBytes(uint8_t *Buf, uint64_t Size,
+                             uint64_t Address) const = 0;
 
   /// Ensures that the requested data is in memory, and returns a pointer to it.
   /// More efficient than using readBytes if the data is already in memory. May
diff --git a/include/llvm/Support/StreamingMemoryObject.h b/include/llvm/Support/StreamingMemoryObject.h
index da1b88738d0..39d1f4e9dd9 100644
--- a/include/llvm/Support/StreamingMemoryObject.h
+++ b/include/llvm/Support/StreamingMemoryObject.h
@@ -27,8 +27,8 @@ class StreamingMemoryObject : public MemoryObject {
 public:
   StreamingMemoryObject(DataStreamer *streamer);
   uint64_t getExtent() const override;
-  int readBytes(uint64_t address, uint64_t size,
-                uint8_t *buf) const override;
+  uint64_t readBytes(uint8_t *Buf, uint64_t Size,
+                     uint64_t Address) const override;
   const uint8_t *getPointer(uint64_t address, uint64_t size) const override {
     // This could be fixed by ensuring the bytes are fetched and making a copy,
     // requiring that the bitcode size be known, or otherwise ensuring that
diff --git a/lib/Bitcode/Reader/BitcodeReader.cpp b/lib/Bitcode/Reader/BitcodeReader.cpp
index 713b6a3a4e5..b2ca22c74ba 100644
--- a/lib/Bitcode/Reader/BitcodeReader.cpp
+++ b/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -3430,7 +3430,7 @@ std::error_code BitcodeReader::InitLazyStream() {
   Stream.init(&*StreamFile);
 
   unsigned char buf[16];
-  if (Bytes->readBytes(0, 16, buf) == -1)
+  if (Bytes->readBytes(buf, 16, 0) != 16)
     return Error(BitcodeError::InvalidBitcodeSignature);
 
   if (!isBitcode(buf, buf + 16))
diff --git a/lib/Support/StreamingMemoryObject.cpp b/lib/Support/StreamingMemoryObject.cpp
index 7187ce013fa..8884a94181b 100644
--- a/lib/Support/StreamingMemoryObject.cpp
+++ b/lib/Support/StreamingMemoryObject.cpp
@@ -28,8 +28,8 @@ public:
   uint64_t getExtent() const override {
     return LastChar - FirstChar;
   }
-  int readBytes(uint64_t address, uint64_t size,
-                uint8_t *buf) const override;
+  uint64_t readBytes(uint8_t *Buf, uint64_t Size,
+                     uint64_t Address) const override;
   const uint8_t *getPointer(uint64_t address, uint64_t size) const override;
   bool isValidAddress(uint64_t address) const override {
     return validAddress(address);
@@ -55,12 +55,20 @@ private:
   void operator=(const RawMemoryObject&) LLVM_DELETED_FUNCTION;
 };
 
-int RawMemoryObject::readBytes(uint64_t address,
-                               uint64_t size,
-                               uint8_t *buf) const {
-  if (!validAddress(address) || !validAddress(address + size - 1)) return -1;
-  memcpy(buf, (uint8_t *)(uintptr_t)(address + FirstChar), size);
-  return size;
+uint64_t RawMemoryObject::readBytes(uint8_t *Buf, uint64_t Size,
+                                    uint64_t Address) const {
+  uint64_t BufferSize = LastChar - FirstChar;
+  if (Address >= BufferSize)
+    return 0;
+
+  uint64_t End = Address + Size;
+  if (End > BufferSize)
+    End = BufferSize;
+
+  Size = End - Address;
+  assert(Size >= 0);
+  memcpy(Buf, (uint8_t *)(Address + FirstChar), Size);
+  return Size;
 }
 
 const uint8_t *RawMemoryObject::getPointer(uint64_t address,
@@ -91,12 +99,20 @@ uint64_t StreamingMemoryObject::getExtent() const {
   return ObjectSize;
 }
 
-int StreamingMemoryObject::readBytes(uint64_t address,
-                                     uint64_t size,
-                                     uint8_t *buf) const {
-  if (!fetchToPos(address + size - 1)) return -1;
-  memcpy(buf, &Bytes[address + BytesSkipped], size);
-  return 0;
+uint64_t StreamingMemoryObject::readBytes(uint8_t *Buf, uint64_t Size,
+                                          uint64_t Address) const {
+  fetchToPos(Address + Size - 1);
+  uint64_t BufferSize = Bytes.size() - BytesSkipped;
+  if (Address >= BufferSize)
+    return 0;
+
+  uint64_t End = Address + Size;
+  if (End > BufferSize)
+    End = BufferSize;
+  Size = End - Address;
+  assert(Size >= 0);
+  memcpy(Buf, &Bytes[Address + BytesSkipped], Size);
+  return Size;
 }
 
 bool StreamingMemoryObject::dropLeadingBytes(size_t s) {