Index: include/llvm/Support/StreamingMemoryObject.h
===================================================================
--- include/llvm/Support/StreamingMemoryObject.h
+++ include/llvm/Support/StreamingMemoryObject.h
@@ -55,25 +55,32 @@
   std::unique_ptr<DataStreamer> Streamer;
   mutable size_t BytesRead;   // Bytes read from stream
   size_t BytesSkipped;// Bytes skipped at start of stream (e.g. wrapper/header)
-  mutable size_t ObjectSize; // 0 if unknown, set if wrapper seen or EOF reached
-  mutable bool EOFReached;
+  mutable size_t ObjectSize; // 0 if unknown, set if wrapper seen or end of
+                             // object reached.
+  mutable bool EOOReached;  // end of object reached.
 
-  // 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 or end of object is
+  // reached (i.e. BytesRead > Pos). Note: EOF sets end of object if
+  // not already defined.  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)
+    if (EOOReached)
       return Pos < ObjectSize;
     while (Pos >= BytesRead) {
       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;
-        EOFReached = true;
+      if (ObjectSize && BytesRead > ObjectSize) {
+        BytesRead = ObjectSize;
+        EOOReached = true;
+      } else if (bytes == 0) { // reached EOF/ran out of bytes
+        if (ObjectSize == 0)
+          ObjectSize = BytesRead;
+        EOOReached = true;
         break;
       }
     }
Index: lib/Support/StreamingMemoryObject.cpp
===================================================================
--- lib/Support/StreamingMemoryObject.cpp
+++ lib/Support/StreamingMemoryObject.cpp
@@ -93,7 +93,7 @@
   uint64_t End = Address + Size;
   if (End > BytesRead)
     End = BytesRead;
-  assert(static_cast<int64_t>(End - Address) >= 0);
+  assert(End >= Address);
   Size = End - Address;
   memcpy(Buf, &Bytes[Address + BytesSkipped], Size);
   return Size;
@@ -109,6 +109,10 @@
 void StreamingMemoryObject::setKnownObjectSize(size_t size) {
   ObjectSize = size;
   Bytes.reserve(size);
+  if (BytesRead >= ObjectSize) {
+    BytesRead = ObjectSize;
+    EOOReached = true;
+  }
 }
 
 MemoryObject *getNonStreamedMemoryObject(const unsigned char *Start,
@@ -118,7 +122,7 @@
 
 StreamingMemoryObject::StreamingMemoryObject(DataStreamer *streamer) :
   Bytes(kChunkSize), Streamer(streamer), BytesRead(0), BytesSkipped(0),
-  ObjectSize(0), EOFReached(false) {
+  ObjectSize(0), EOOReached(false) {
   BytesRead = streamer->GetBytes(&Bytes[0], kChunkSize);
 }
 }
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));
+}