diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -6986,6 +6986,13 @@
         continue;
       DeclsInPrototype.push_back(ND);
     }
+    // Sort DeclsInPrototype based on raw encoding of the source location. This
+    // provides a stable iterating order in DeclContext, which is important for
+    // tasks like ASTWriter for deterministic output.
+    llvm::sort(DeclsInPrototype, [](Decl *D1, Decl *D2) {
+      return D1->getLocation().getRawEncoding() <
+             D2->getLocation().getRawEncoding();
+    });
   }
 
   // Remember that we parsed a function type, and remember the attributes.