diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index 1039a996eb8..a5d08f1bdd2 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -734,6 +734,7 @@ JSRuntime::JSRuntime()
     gcStats(thisFromCtor()),
     gcNumber(0),
     gcStartNumber(0),
+    gcIsFull(false),
     gcTriggerReason(gcreason::NO_REASON),
     gcStrictCompartmentChecking(false),
     gcIncrementalState(gc::NO_INCREMENTAL),
diff --git a/js/src/jscntxt.h b/js/src/jscntxt.h
index 23b9543e5f0..f4aef744eaa 100644
--- a/js/src/jscntxt.h
+++ b/js/src/jscntxt.h
@@ -362,6 +362,9 @@ struct JSRuntime : js::RuntimeFriendFields
     /* The gcNumber at the time of the most recent GC's first slice. */
     uint64_t            gcStartNumber;
 
+    /* Whether all compartments are being collected in first GC slice. */
+    bool                gcIsFull;
+
     /* The reason that an interrupt-triggered GC should be called. */
     js::gcreason::Reason gcTriggerReason;
 
diff --git a/js/src/jsgc.cpp b/js/src/jsgc.cpp
index 09212e872c7..10a99d169f1 100644
--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@@ -2280,14 +2280,7 @@ MarkRuntime(JSTracer *trc, bool useSavedRoots = false)
      * atoms. Otherwise, the non-collected compartments could contain pointers
      * to atoms that we would miss.
      */
-    bool isFullGC = true;
-    if (IS_GC_MARKING_TRACER(trc)) {
-        for (CompartmentsIter c(rt); !c.done(); c.next()) {
-            if (!c->isCollecting())
-                isFullGC = false;
-        }
-    }
-    MarkAtomState(trc, rt->gcKeepAtoms || !isFullGC);
+    MarkAtomState(trc, rt->gcKeepAtoms || (IS_GC_MARKING_TRACER(trc) && !rt->gcIsFull));
     rt->staticStrings.trace(trc);
 
     for (ContextIter acx(rt); !acx.done(); acx.next())
@@ -2903,6 +2896,12 @@ PurgeRuntime(JSTracer *trc)
 static void
 BeginMarkPhase(JSRuntime *rt)
 {
+    rt->gcIsFull = true;
+    for (CompartmentsIter c(rt); !c.done(); c.next()) {
+        if (!c->isCollecting())
+            rt->gcIsFull = false;
+    }
+
     rt->gcMarker.start(rt);
     JS_ASSERT(!rt->gcMarker.callback);
     JS_ASSERT(IS_GC_MARKING_TRACER(&rt->gcMarker));