diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -2173,10 +2173,10 @@
   NegFlag<SetFalse>>;
 
 defm xray_function_index : BoolFOption<"xray-function-index",
-  CodeGenOpts<"XRayOmitFunctionIndex">, DefaultTrue,
-  NegFlag<SetFalse, [CC1Option], "Omit function index section at the"
+  CodeGenOpts<"XRayOmitFunctionIndex">, DefaultFalse,
+  NegFlag<SetTrue, [CC1Option], "Omit function index section at the"
           " expense of single-function patching performance">,
-  PosFlag<SetTrue>>;
+  PosFlag<SetFalse>>;
 
 def fxray_link_deps : Flag<["-"], "fxray-link-deps">, Group<f_Group>,
   Flags<[CC1Option]>,
diff --git a/clang/test/CodeGen/xray-function-index.cpp b/clang/test/CodeGen/xray-function-index.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/CodeGen/xray-function-index.cpp
@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -fxray-instrument                          -x c++ -std=c++11 -triple x86_64-unknown-linux-gnu -S -o - %s | FileCheck %s --check-prefixes=CHECK,CHECK-ENABLED
+// RUN: %clang_cc1 -fxray-instrument -fno-xray-function-index -x c++ -std=c++11 -triple x86_64-unknown-linux-gnu -S -o - %s | FileCheck %s --check-prefixes=CHECK,CHECK-DISABLED
+
+[[clang::xray_always_instrument]] void foo() {}
+
+// CHECK-LABEL: .section xray_instr_map,"ao",@progbits,_Z3foov
+// CHECK-ENABLED: .section xray_fn_idx,"awo",@progbits,_Z3foov
+// CHECK-DISABLED-NOT: xray_fn_idx