diff -r 000000000000 -r 6474c204b198 js/src/gdb/tests/test-jsval.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/gdb/tests/test-jsval.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,36 @@
+#include "gdb-tests.h"
+#include "jsapi.h"
+
+FRAGMENT(jsval, simple) {
+  JS::Rooted<jsval> fortytwo(cx, INT_TO_JSVAL(42));
+  JS::Rooted<jsval> negone(cx, INT_TO_JSVAL(-1));
+  JS::Rooted<jsval> undefined(cx, JSVAL_VOID);
+  JS::Rooted<jsval> null(cx, JSVAL_NULL);
+  JS::Rooted<jsval> js_true(cx, JSVAL_TRUE);
+  JS::Rooted<jsval> js_false(cx, JSVAL_FALSE);
+  JS::Rooted<jsval> elements_hole(cx, js::MagicValue(JS_ELEMENTS_HOLE));
+
+  JS::Rooted<jsval> empty_string(cx);
+  empty_string.setString(JS_NewStringCopyZ(cx, ""));
+  JS::Rooted<jsval> friendly_string(cx);
+  friendly_string.setString(JS_NewStringCopyZ(cx, "Hello!"));
+
+  JS::Rooted<jsval> global(cx);
+  global.setObject(*JS::CurrentGlobalOrNull(cx));
+
+  // Some interesting value that floating-point won't munge.
+  JS::Rooted<jsval> onehundredthirtysevenonehundredtwentyeighths(cx, DOUBLE_TO_JSVAL(137.0 / 128.0));
+
+  breakpoint();
+
+  (void) fortytwo;
+  (void) negone;
+  (void) undefined;
+  (void) js_true;
+  (void) js_false;
+  (void) null;
+  (void) elements_hole;
+  (void) empty_string;
+  (void) friendly_string;
+  (void) global;
+}