diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -822,7 +822,7 @@
   // Gather Info for preprocessor construction later on.
 
   HeaderSearch &HeaderInfo = *AST->HeaderInfo;
-  unsigned Counter;
+  unsigned Counter = 0;
 
   AST->PP = std::make_shared<Preprocessor>(
       AST->PPOpts, AST->getDiagnostics(), *AST->LangOpts,