diff --git a/clang/include/clang/Interpreter/Interpreter.h b/clang/include/clang/Interpreter/Interpreter.h
index d972d960dcb7c..469ce1fd75bf8 100644
--- a/clang/include/clang/Interpreter/Interpreter.h
+++ b/clang/include/clang/Interpreter/Interpreter.h
@@ -169,6 +169,7 @@ class Interpreter {
 
 private:
   size_t getEffectivePTUSize() const;
+  void markUserCodeStart();
 
   llvm::DenseMap<CXXRecordDecl *, llvm::orc::ExecutorAddr> Dtors;
 
diff --git a/clang/lib/Interpreter/Interpreter.cpp b/clang/lib/Interpreter/Interpreter.cpp
index 3485da8196683..e293fefb52496 100644
--- a/clang/lib/Interpreter/Interpreter.cpp
+++ b/clang/lib/Interpreter/Interpreter.cpp
@@ -280,15 +280,14 @@ Interpreter::create(std::unique_ptr<CompilerInstance> CI) {
   if (Err)
     return std::move(Err);
 
+  // Add runtime code and set a marker to hide it from user code. Undo will not
+  // go through that.
   auto PTU = Interp->Parse(Runtimes);
   if (!PTU)
     return PTU.takeError();
+  Interp->markUserCodeStart();
 
   Interp->ValuePrintingInfo.resize(4);
-  // FIXME: This is a ugly hack. Undo command checks its availability by looking
-  // at the size of the PTU list. However we have parsed something in the
-  // beginning of the REPL so we have to mark them as 'Irrevocable'.
-  Interp->InitPTUSize = Interp->IncrParser->getPTUs().size();
   return std::move(Interp);
 }
 
@@ -345,6 +344,11 @@ const ASTContext &Interpreter::getASTContext() const {
   return getCompilerInstance()->getASTContext();
 }
 
+void Interpreter::markUserCodeStart() {
+  assert(!InitPTUSize && "We only do this once");
+  InitPTUSize = IncrParser->getPTUs().size();
+}
+
 size_t Interpreter::getEffectivePTUSize() const {
   std::list<PartialTranslationUnit> &PTUs = IncrParser->getPTUs();
   assert(PTUs.size() >= InitPTUSize && "empty PTU list?");