Index: include/llvm/Support/MemoryBuffer.h
===================================================================
--- include/llvm/Support/MemoryBuffer.h
+++ include/llvm/Support/MemoryBuffer.h
@@ -132,6 +132,8 @@
   getFileSlice(const Twine &Filename, uint64_t MapSize, uint64_t Offset,
                bool IsVolatile = false);
 
+  static void enableMemoryMapping(bool enable);
+
   //===--------------------------------------------------------------------===//
   // Provided for performance analysis.
   //===--------------------------------------------------------------------===//
Index: lib/Support/MemoryBuffer.cpp
===================================================================
--- lib/Support/MemoryBuffer.cpp
+++ lib/Support/MemoryBuffer.cpp
@@ -39,6 +39,12 @@
 // MemoryBuffer implementation itself.
 //===----------------------------------------------------------------------===//
 
+static bool MemoryMappingEnabled = true;
+
+void MemoryBuffer::enableMemoryMapping(bool enable) {
+  MemoryMappingEnabled = enable;
+}
+
 MemoryBuffer::~MemoryBuffer() { }
 
 /// init - Initialize this MemoryBuffer as a reference to externally allocated
@@ -312,6 +318,9 @@
                           bool RequiresNullTerminator,
                           int PageSize,
                           bool IsVolatile) {
+  if (!MemoryMappingEnabled)
+    return false;
+
   // mmap may leave the buffer without null terminator if the file size changed
   // by the time the last page is mapped in, so avoid it if the file size is
   // likely to change.