Index: include/llvm/Support/StreamingMemoryObject.h
===================================================================
--- include/llvm/Support/StreamingMemoryObject.h
+++ include/llvm/Support/StreamingMemoryObject.h
@@ -58,26 +58,27 @@
   mutable size_t ObjectSize; // 0 if unknown, set if wrapper seen or EOF reached
   mutable bool EOFReached;
 
-  // Fetch enough bytes such that Pos can be read or EOF is reached
-  // (i.e. BytesRead > Pos). Return true if Pos can be read.
-  // Unlike most of the functions in BitcodeReader, returns true on success.
-  // Most of the requests will be small, but we fetch at kChunkSize bytes
-  // at a time to avoid making too many potentially expensive GetBytes calls
+  // Fetch enough bytes such that Pos can be read (i.e. BytesRead >
+  // Pos). Returns true if Pos can be read.  Unlike most of the
+  // functions in BitcodeReader, returns true on success.  Most of the
+  // requests will be small, but we fetch at kChunkSize bytes at a
+  // time to avoid making too many potentially expensive GetBytes
+  // calls.
   bool fetchToPos(size_t Pos) const {
-    if (EOFReached)
-      return Pos < ObjectSize;
     while (Pos >= BytesRead) {
+      if (EOFReached)
+        return false;
       Bytes.resize(BytesRead + BytesSkipped + kChunkSize);
       size_t bytes = Streamer->GetBytes(&Bytes[BytesRead + BytesSkipped],
                                         kChunkSize);
       BytesRead += bytes;
-      if (bytes != kChunkSize) { // reached EOF/ran out of bytes
-        ObjectSize = BytesRead;
+      if (bytes == 0) { // reached EOF/ran out of bytes
+        if (ObjectSize == 0)
+          ObjectSize = BytesRead;
         EOFReached = true;
-        break;
       }
     }
-    return Pos < BytesRead;
+    return !ObjectSize || Pos < ObjectSize;
   }
 
   StreamingMemoryObject(const StreamingMemoryObject&) = delete;
Index: lib/Support/StreamingMemoryObject.cpp
===================================================================
--- lib/Support/StreamingMemoryObject.cpp
+++ lib/Support/StreamingMemoryObject.cpp
@@ -73,7 +73,7 @@
 // block until we actually want to read it.
 bool StreamingMemoryObject::isValidAddress(uint64_t address) const {
   if (ObjectSize && address < ObjectSize) return true;
-    return fetchToPos(address);
+  return fetchToPos(address);
 }
 
 uint64_t StreamingMemoryObject::getExtent() const {
@@ -87,18 +87,30 @@
 uint64_t StreamingMemoryObject::readBytes(uint8_t *Buf, uint64_t Size,
                                           uint64_t Address) const {
   fetchToPos(Address + Size - 1);
-  if (Address >= BytesRead)
+  // Note: For wrapped bitcode files will set ObjectSize after the
+  // first call to fetchToPos. In such cases, ObjectSize can be
+  // smaller than BytesRead.
+  size_t MaxAddress =
+      (ObjectSize && ObjectSize < BytesRead) ? ObjectSize : BytesRead;
+  if (Address >= MaxAddress)
     return 0;
 
   uint64_t End = Address + Size;
-  if (End > BytesRead)
-    End = BytesRead;
-  assert(static_cast<int64_t>(End - Address) >= 0);
+  if (End > MaxAddress)
+    End = MaxAddress;
+  assert(End >= Address);
   Size = End - Address;
   memcpy(Buf, &Bytes[Address + BytesSkipped], Size);
   return Size;
 }
 
+void StreamingMemoryObject::setKnownObjectSize(size_t size) {
+  ObjectSize = size;
+  Bytes.reserve(size);
+  if (ObjectSize <= BytesRead)
+    EOFReached = true;
+}
+
 bool StreamingMemoryObject::dropLeadingBytes(size_t s) {
   if (BytesRead < s) return true;
   BytesSkipped = s;
@@ -106,11 +118,6 @@
   return false;
 }
 
-void StreamingMemoryObject::setKnownObjectSize(size_t size) {
-  ObjectSize = size;
-  Bytes.reserve(size);
-}
-
 MemoryObject *getNonStreamedMemoryObject(const unsigned char *Start,
                                          const unsigned char *End) {
   return new RawMemoryObject(Start, End);
Index: unittests/Support/StreamingMemoryObject.cpp
===================================================================
--- unittests/Support/StreamingMemoryObject.cpp
+++ unittests/Support/StreamingMemoryObject.cpp
@@ -27,3 +27,12 @@
   StreamingMemoryObject O(DS);
   EXPECT_TRUE(O.isValidAddress(32 * 1024));
 }
+
+TEST(StreamingMemoryObject, TestSetKnownObjectSize) {
+  auto *DS = new NullDataStreamer();
+  StreamingMemoryObject O(DS);
+  uint8_t Buf[32];
+  EXPECT_EQ((uint64_t) 16, O.readBytes(Buf, 16, 0));
+  O.setKnownObjectSize(24);
+  EXPECT_EQ((uint64_t) 8, O.readBytes(Buf, 16, 16));
+}