GCIntegration - SpiderMonkey Redirect 1
if a pointer is not traced via js_call_tracer or any similar mechanism, then there's no need for a write barrier (although see below about read barriers).
...for pointers that are traced using js_call_tracer we will most likely change the api so that the address of the pointer being traced is passed in, rather than the pointer itself.
... if a pointer is not traced, as by js_call_tracer, then there is no need for a write barrier.

JSVAL_TO_GCTHING
another possible alternative is to avoid casting altogether by using an api that operates on jsvals rather than raw pointers (for example, js_call_value_tracer rather than js_call_tracer).

JSAPI reference
it added in spidermonkey 1.8 obsolete since jsapi 12 js_tracerinit added in spidermonkey 12 obsolete since jsapi 31 js_calltracer added in spidermonkey 1.8 obsolete since jsapi 22 js_set_tracing_details added in spidermonkey 1.8 obsolete since jsapi 26 js_set_tracing_index added in spidermonkey 1.8 obsolete since jsapi 26 js_set_tracing_name added in spidermonkey 1.8 obsolete since jsapi 26 js_call_tracer added in spidermonkey 1.8 obsolete since jsapi 23 js_call_value_tracer added in spidermonkey 1.8 obsolete since jsapi 23 js_call_object_tracer added in spidermonkey 1.8 obsolete since jsapi 23 js_call_string_tracer added in spidermonkey 1.8 obsolete since jsapi 23 js_call_double_tracer added in spidermonkey 1.8 obsolete since javascript 1.8.5 js_printtracethinginfo (debug-only) added in spid...