diff --git a/llvm/include/llvm/Support/MemoryBuffer.h b/llvm/include/llvm/Support/MemoryBuffer.h
index 38f61935d97a717f7481f839971fa0baededc687..0b6cd12f6f4bb0e818aa1583a446eae6719a1f54 100644
--- a/llvm/include/llvm/Support/MemoryBuffer.h
+++ b/llvm/include/llvm/Support/MemoryBuffer.h
@@ -26,6 +26,11 @@ namespace llvm {
 /// into a memory buffer.  In addition to basic access to the characters in the
 /// file, this interface guarantees you can read one character past the end of
 /// the file, and that this character will read as '\0'.
+///
+/// The '\0' guarantee is needed to support an optimization -- it's intended to
+/// be more efficient for clients which are reading all the data to stop
+/// reading when they encounter a '\0' than to continually check the file
+/// position to see if it has reached the end of the file.
 class MemoryBuffer {
   const char *BufferStart; // Start of the buffer.
   const char *BufferEnd;   // End of the buffer.