Index: lib/CodeGen/TargetInfo.cpp
===================================================================
--- lib/CodeGen/TargetInfo.cpp	(revision 126172)
+++ lib/CodeGen/TargetInfo.cpp	(working copy)
@@ -2105,8 +2105,9 @@
         RT->getDecl()->hasFlexibleArrayMember())
       return ABIArgInfo::getIndirect(0, /*ByVal=*/false);
 
-    // FIXME: mingw64-gcc emits 128-bit struct as i128
-    if (Size <= 128 &&
+    // MS x64 ABI requirement: "Any argument that doesn’t fit in 8 bytes, or is 
+    // not 1, 2, 4, or 8 bytes, must be passed by reference."
+    if (Size <= 64 &&
         (Size & (Size - 1)) == 0)
       return ABIArgInfo::getDirect(llvm::IntegerType::get(getVMContext(),
                                                           Size));