diff --git a/js/public/RootingAPI.h b/js/public/RootingAPI.h
index ae9cb0c50396..867876b5eea4 100644
--- a/js/public/RootingAPI.h
+++ b/js/public/RootingAPI.h
@@ -642,7 +642,7 @@ class MOZ_STACK_CLASS Rooted : public js::RootedBase<T>
     template <typename CX>
     void init(CX* cx) {
         js::ThingRootKind kind = js::RootKind<T>::rootKind();
-        this->stack = &cx->thingGCRooters[kind];
+        this->stack = &cx->roots.stackRoots_[kind];
         this->prev = *stack;
         *stack = reinterpret_cast<Rooted<void*>*>(this);
     }
diff --git a/js/src/gc/RootMarking.cpp b/js/src/gc/RootMarking.cpp
index 96bec3a60f44..d6ddb25eaad8 100644
--- a/js/src/gc/RootMarking.cpp
+++ b/js/src/gc/RootMarking.cpp
@@ -81,7 +81,7 @@ template <class T, void MarkFunc(JSTracer* trc, T* ref, const char* name), class
 static inline void
 MarkExactStackRootList(JSTracer* trc, Source* s, const char* name)
 {
-    Rooted<T>* rooter = s->template gcRooters<T>();
+    Rooted<T>* rooter = s->roots.template gcRooters<T>();
     while (rooter) {
         T* addr = rooter->address();
         if (!IgnoreExactRoot(addr))
@@ -282,7 +282,7 @@ AutoGCRooter::traceAll(JSTracer* trc)
 AutoGCRooter::traceAllWrappers(JSTracer* trc)
 {
     for (ContextIter cx(trc->runtime()); !cx.done(); cx.next()) {
-        for (AutoGCRooter* gcr = cx->autoGCRooters; gcr; gcr = gcr->down) {
+        for (AutoGCRooter* gcr = cx->roots.autoGCRooters_; gcr; gcr = gcr->down) {
             if (gcr->tag_ == WRAPVECTOR || gcr->tag_ == WRAPPER)
                 gcr->trace(trc);
         }
diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index 6355ece45cc2..7ff44e961472 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -6126,18 +6126,18 @@ JS_CallOnce(JSCallOnceType* once, JSInitCallback func)
 }
 
 AutoGCRooter::AutoGCRooter(JSContext* cx, ptrdiff_t tag)
-  : down(ContextFriendFields::get(cx)->autoGCRooters),
+  : down(ContextFriendFields::get(cx)->roots.autoGCRooters_),
     tag_(tag),
-    stackTop(&ContextFriendFields::get(cx)->autoGCRooters)
+    stackTop(&ContextFriendFields::get(cx)->roots.autoGCRooters_)
 {
     MOZ_ASSERT(this != *stackTop);
     *stackTop = this;
 }
 
 AutoGCRooter::AutoGCRooter(ContextFriendFields* cx, ptrdiff_t tag)
-  : down(cx->autoGCRooters),
+  : down(cx->roots.autoGCRooters_),
     tag_(tag),
-    stackTop(&cx->autoGCRooters)
+    stackTop(&cx->roots.autoGCRooters_)
 {
     MOZ_ASSERT(this != *stackTop);
     *stackTop = this;
diff --git a/js/src/jscntxt.cpp b/js/src/jscntxt.cpp
index 7279e6a3e809..cd06286eadd1 100644
--- a/js/src/jscntxt.cpp
+++ b/js/src/jscntxt.cpp
@@ -156,7 +156,7 @@ js::DestroyContext(JSContext* cx, DestroyContextMode mode)
     if (cx->outstandingRequests != 0)
         MOZ_CRASH();
 
-    cx->checkNoGCRooters();
+    cx->roots.checkNoGCRooters();
 
     if (mode != DCM_NEW_FAILED) {
         if (JSContextCallback cxCallback = rt->cxCallback) {
@@ -188,10 +188,10 @@ js::DestroyContext(JSContext* cx, DestroyContextMode mode)
 }
 
 void
-ContextFriendFields::checkNoGCRooters() {
+RootLists::checkNoGCRooters() {
 #ifdef DEBUG
     for (int i = 0; i < THING_ROOT_LIMIT; ++i)
-        MOZ_ASSERT(thingGCRooters[i] == nullptr);
+        MOZ_ASSERT(stackRoots_[i] == nullptr);
 #endif
 }
 
diff --git a/js/src/jsfriendapi.cpp b/js/src/jsfriendapi.cpp
index 0464f7288702..ffc636f3f976 100644
--- a/js/src/jsfriendapi.cpp
+++ b/js/src/jsfriendapi.cpp
@@ -50,7 +50,6 @@ PerThreadDataFriendFields::PerThreadDataFriendFields()
     for (int i=0; i<StackKindCount; i++)
         nativeStackLimit[i] = UINTPTR_MAX;
 #endif
-    PodArrayZero(thingGCRooters);
 }
 
 JS_FRIEND_API(void)
diff --git a/js/src/jspubtd.h b/js/src/jspubtd.h
index 39e7bb4dbbce..ffa4d4d4f2e9 100644
--- a/js/src/jspubtd.h
+++ b/js/src/jspubtd.h
@@ -239,7 +239,7 @@ class JS_PUBLIC_API(AutoGCRooter)
     /* T must be a context type */
     template<typename T>
     static void traceAllInContext(T* cx, JSTracer* trc) {
-        for (AutoGCRooter* gcr = cx->autoGCRooters; gcr; gcr = gcr->down)
+        for (AutoGCRooter* gcr = cx->roots.autoGCRooters_; gcr; gcr = gcr->down)
             gcr->trace(trc);
     }
 
@@ -355,6 +355,32 @@ template <> struct RootKind<JSScript*> : SpecificRootKind<JSScript*, THING_ROOT_
 template <> struct RootKind<jsid> : SpecificRootKind<jsid, THING_ROOT_ID> {};
 template <> struct RootKind<JS::Value> : SpecificRootKind<JS::Value, THING_ROOT_VALUE> {};
 
+// Abstracts JS rooting mechanisms so they can be shared between the JSContext
+// and JSRuntime.
+class RootLists
+{
+    // Stack GC roots for stack-allocated GC heap pointers.
+    JS::Rooted<void*>* stackRoots_[THING_ROOT_LIMIT];
+    template <typename T> friend class JS::Rooted;
+
+    // Stack GC roots for stack-allocated AutoFooRooter classes.
+    JS::AutoGCRooter* autoGCRooters_;
+    friend class JS::AutoGCRooter;
+
+  public:
+    RootLists() : autoGCRooters_(nullptr) {
+        mozilla::PodArrayZero(stackRoots_);
+    }
+
+    template <class T>
+    inline JS::Rooted<T>* gcRooters() {
+        js::ThingRootKind kind = RootKind<T>::rootKind();
+        return reinterpret_cast<JS::Rooted<T>*>(stackRoots_[kind]);
+    }
+
+    void checkNoGCRooters();
+};
+
 struct ContextFriendFields
 {
   protected:
@@ -367,11 +393,12 @@ struct ContextFriendFields
     JS::Zone*           zone_;
 
   public:
+    /* Rooting structures. */
+    RootLists           roots;
+
     explicit ContextFriendFields(JSRuntime* rt)
-      : runtime_(rt), compartment_(nullptr), zone_(nullptr), autoGCRooters(nullptr)
-    {
-        mozilla::PodArrayZero(thingGCRooters);
-    }
+      : runtime_(rt), compartment_(nullptr), zone_(nullptr)
+    {}
 
     static const ContextFriendFields* get(const JSContext* cx) {
         return reinterpret_cast<const ContextFriendFields*>(cx);
@@ -381,25 +408,6 @@ struct ContextFriendFields
         return reinterpret_cast<ContextFriendFields*>(cx);
     }
 
-  private:
-    /*
-     * Stack allocated GC roots for stack GC heap pointers, which may be
-     * overwritten if moved during a GC.
-     */
-    JS::Rooted<void*>* thingGCRooters[THING_ROOT_LIMIT];
-
-  public:
-    template <class T>
-    inline JS::Rooted<T>* gcRooters() {
-        js::ThingRootKind kind = RootKind<T>::rootKind();
-        return reinterpret_cast<JS::Rooted<T>*>(thingGCRooters[kind]);
-    }
-
-    void checkNoGCRooters();
-
-    /* Stack of thread-stack-allocated GC roots. */
-    JS::AutoGCRooter*  autoGCRooters;
-
     friend JSRuntime* GetRuntime(const JSContext* cx);
     friend JSCompartment* GetContextCompartment(const JSContext* cx);
     friend JS::Zone* GetContextZone(const JSContext* cx);
@@ -454,23 +462,11 @@ struct PerThreadDataFriendFields
     };
 
   public:
+    /* Rooting structures. */
+    RootLists roots;
 
     PerThreadDataFriendFields();
 
-  private:
-    /*
-     * Stack allocated GC roots for stack GC heap pointers, which may be
-     * overwritten if moved during a GC.
-     */
-    JS::Rooted<void*>* thingGCRooters[THING_ROOT_LIMIT];
-
-  public:
-    template <class T>
-    inline JS::Rooted<T>* gcRooters() {
-        js::ThingRootKind kind = RootKind<T>::rootKind();
-        return reinterpret_cast<JS::Rooted<T>*>(thingGCRooters[kind]);
-    }
-
     /* Limit pointer for checking native stack consumption. */
     uintptr_t nativeStackLimit[js::StackKindCount];