Index: lib/Transforms/IPO/MergeSimilarFunctions.cpp
===================================================================
--- lib/Transforms/IPO/MergeSimilarFunctions.cpp
+++ lib/Transforms/IPO/MergeSimilarFunctions.cpp
@@ -1830,7 +1830,10 @@
   if (F1->hasSection())
     NewF->setSection(F1->getSection());
 
-  NewF->setCallingConv(CallingConv::Fast);
+  if (F1->getFunctionType()->isVarArg())
+    NewF->setCallingConv(CallingConv::C);
+  else
+    NewF->setCallingConv(CallingConv::Fast);
 
   Function::arg_iterator DestI = NewF->arg_begin();
   for (auto &Arg : F1->args()) {