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 +6986,15 @@
         continue;
       DeclsInPrototype.push_back(ND);
     }
+    // Sort DeclsInPrototype based on raw encoding of the source location.
+    // Scope::decls() is iterating over a SmallPtrSet so sort the Decls before
+    // moving to DeclContext. This provides a stable ordering for traversing
+    // Decls 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.
diff --git a/clang/test/Modules/decl-params-determinisim.m b/clang/test/Modules/decl-params-determinisim.m
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/decl-params-determinisim.m
@@ -0,0 +1,70 @@
+/// Test determinisim when serializing anonymous decls. Create enough Decls in
+/// DeclContext that can overflow the small storage of SmallPtrSet to make sure
+/// the serialization does not rely on iteration order of SmallPtrSet.
+// RUN: rm -rf %t.dir
+// RUN: split-file %s %t.dir
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps \
+// RUN:   -fmodules-cache-path=%t.dir/cache -triple x86_64-apple-macosx10.11.0 \
+// RUN:   -I%t.dir/headers %t.dir/main.m -fdisable-module-hash -Wno-visibility
+// RUN: mv %t.dir/cache/A.pcm %t1.pcm
+// RUN: rm -rf %t.dir/cache
+// RUN: %clang_cc1 -fmodules -fimplicit-module-maps \
+// RUN:   -fmodules-cache-path=%t.dir/cache -triple x86_64-apple-macosx10.11.0 \
+// RUN:   -I%t.dir/headers %t.dir/main.m -fdisable-module-hash -Wno-visibility
+// RUN: mv %t.dir/cache/A.pcm %t2.pcm
+// RUN: diff %t1.pcm %t2.pcm
+
+//--- headers/a.h
+void f(struct A0 *a0,
+       struct A1 *a1,
+       struct A2 *a2,
+       struct A3 *a3,
+       struct A4 *a4,
+       struct A5 *a5,
+       struct A6 *a6,
+       struct A7 *a7,
+       struct A8 *a8,
+       struct A9 *a9,
+       struct A10 *a10,
+       struct A11 *a11,
+       struct A12 *a12,
+       struct A13 *a13,
+       struct A14 *a14,
+       struct A15 *a15,
+       struct A16 *a16,
+       struct A17 *a17,
+       struct A18 *a18,
+       struct A19 *a19,
+       struct A20 *a20,
+       struct A21 *a21,
+       struct A22 *a22,
+       struct A23 *a23,
+       struct A24 *a24,
+       struct A25 *a25,
+       struct A26 *a26,
+       struct A27 *a27,
+       struct A28 *a28,
+       struct A29 *a29,
+       struct A30 *a30,
+       struct A31 *a31,
+       struct A32 *a32,
+       struct A33 *a33,
+       struct A34 *a34,
+       struct A35 *a35,
+       struct A36 *a36,
+       struct A37 *a37,
+       struct A38 *a38,
+       struct A39 *a39,
+       struct A40 *a40);
+
+
+//--- headers/module.modulemap
+
+module A {
+  header "a.h"
+}
+
+//--- main.m
+
+#import <a.h>
+