Index: lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -3826,6 +3826,32 @@
   return nullptr;
 }
 
+// Try to convert VarArg calls to regular calls if the callee is a regular
+// function, but is casted to a VarArg function, in case the actual arguments
+// match the callee's signature.
+static bool transformVarArgCast(CallSite CS) {
+  auto *Callee = dyn_cast<Function>(CS.getCalledValue()->stripPointerCasts());
+  if (CS.getCalledValue() == Callee || !Callee)
+    return false;
+
+  FunctionType *CalleeTy = Callee->getFunctionType();
+  if (CalleeTy->isVarArg() ||
+      CalleeTy->getNumParams() != CS.getNumArgOperands() ||
+      CalleeTy->getReturnType() != CS.getFunctionType()->getReturnType())
+    return false;
+
+  // Check if operand types at the call site match the parameter types
+  // of the original function.
+  for (unsigned i = 0; i < CS.getNumArgOperands(); i++)
+    if (CS.getArgOperand(i)->getType() !=
+        Callee->getFunctionType()->getParamType(i))
+      return false;
+
+  CS.mutateFunctionType(Callee->getFunctionType());
+  CS.setCalledFunction(Callee);
+  return true;
+}
+
 // Given a call to llvm.adjust.trampoline, find and return the corresponding
 // call to llvm.init.trampoline if the call to the trampoline can be optimized
 // to a direct call to a function.  Otherwise return NULL.
@@ -3976,6 +4002,7 @@
     if (I) return eraseInstFromFunction(*I);
   }
 
+  transformVarArgCast(CS);
   return Changed ? CS.getInstruction() : nullptr;
 }
 
Index: test/Transforms/InstCombine/call.ll
===================================================================
--- test/Transforms/InstCombine/call.ll
+++ test/Transforms/InstCombine/call.ll
@@ -287,3 +287,12 @@
 ; CHECK-LABEL: @test17(
 ; CHECK: call i32 @pr28655(i32 0)
 ; CHECK: ret i32 0
+
+declare void @ext_method(i8*, i32) local_unnamed_addr
+
+define void @test_cast_to_vararg(i8* %this) {
+; CHECK-LABEL: test_cast_to_vararg
+; CHECK:  call void @ext_method(i8* %this, i32 42)
+  call void (i8*, ...) bitcast (void (i8*, i32)* @ext_method to void (i8*, ...)*)(i8* %this, i32 42)
+  ret void
+}