diff --git a/dom/base/nsJSEnvironment.cpp b/dom/base/nsJSEnvironment.cpp
index fdd5a37ba253..b8d256a5d983 100644
--- a/dom/base/nsJSEnvironment.cpp
+++ b/dom/base/nsJSEnvironment.cpp
@@ -1459,7 +1459,7 @@ nsJSContext::GarbageCollectNow(JS::gcreason::Reason aReason,
   if (sCCLockedOut && aIncremental == IncrementalGC) {
     // We're in the middle of incremental GC. Do another slice.
     JS::PrepareForIncrementalGC(sRuntime);
-    JS::IncrementalGC(sRuntime, aReason, aSliceMillis);
+    JS::IncrementalGCSlice(sRuntime, aReason, aSliceMillis);
     return;
   }
 
@@ -1472,7 +1472,7 @@ nsJSContext::GarbageCollectNow(JS::gcreason::Reason aReason,
 
   if (aIncremental == IncrementalGC) {
     MOZ_ASSERT(aShrinking == NonShrinkingGC);
-    JS::IncrementalGC(sRuntime, aReason, aSliceMillis);
+    JS::StartIncrementalGC(sRuntime, aReason, aSliceMillis);
   } else if (aShrinking == ShrinkingGC) {
     JS::ShrinkingGC(sRuntime, aReason);
   } else {
diff --git a/js/public/GCAPI.h b/js/public/GCAPI.h
index 7f4b58a44101..9da731c5901b 100644
--- a/js/public/GCAPI.h
+++ b/js/public/GCAPI.h
@@ -207,16 +207,27 @@ ShrinkingGC(JSRuntime *rt, gcreason::Reason reason);
  */
 
 /*
- * Begin an incremental collection and perform one slice worth of work or
- * perform a slice of an ongoing incremental collection. When this function
- * returns, the collection is not complete. This function must be called
- * repeatedly until !IsIncrementalGCInProgress(rt).
+ * Begin an incremental collection and perform one slice worth of work. When
+ * this function returns, the collection may not be complete.
+ * IncrementalGCSlice() must be called repeatedly until
+ * !IsIncrementalGCInProgress(rt).
  *
  * Note: SpiderMonkey's GC is not realtime. Slices in practice may be longer or
  *       shorter than the requested interval.
  */
 extern JS_FRIEND_API(void)
-IncrementalGC(JSRuntime *rt, gcreason::Reason reason, int64_t millis = 0);
+StartIncrementalGC(JSRuntime *rt, gcreason::Reason reason, int64_t millis = 0);
+
+/*
+ * Perform a slice of an ongoing incremental collection. When this function
+ * returns, the collection may not be complete. It must be called repeatedly
+ * until !IsIncrementalGCInProgress(rt).
+ *
+ * Note: SpiderMonkey's GC is not realtime. Slices in practice may be longer or
+ *       shorter than the requested interval.
+ */
+extern JS_FRIEND_API(void)
+IncrementalGCSlice(JSRuntime *rt, gcreason::Reason reason, int64_t millis = 0);
 
 /*
  * If IsIncrementalGCInProgress(rt), this call finishes the ongoing collection
diff --git a/js/src/jsapi-tests/testGCFinalizeCallback.cpp b/js/src/jsapi-tests/testGCFinalizeCallback.cpp
index e526ce1c1173..99673d6dd1c7 100644
--- a/js/src/jsapi-tests/testGCFinalizeCallback.cpp
+++ b/js/src/jsapi-tests/testGCFinalizeCallback.cpp
@@ -24,7 +24,7 @@ BEGIN_TEST(testGCFinalizeCallback)
     /* Full GC, incremental. */
     FinalizeCalls = 0;
     JS::PrepareForFullGC(rt);
-    JS::IncrementalGC(rt, JS::gcreason::API, 1000000);
+    JS::StartIncrementalGC(rt, JS::gcreason::API, 1000000);
     CHECK(!rt->gc.isIncrementalGCInProgress());
     CHECK(rt->gc.isFullGc());
     CHECK(checkMultipleGroups());
@@ -61,7 +61,7 @@ BEGIN_TEST(testGCFinalizeCallback)
     /* Compartment GC, incremental, single compartment. */
     FinalizeCalls = 0;
     JS::PrepareZoneForGC(global1->zone());
-    JS::IncrementalGC(rt, JS::gcreason::API, 1000000);
+    JS::StartIncrementalGC(rt, JS::gcreason::API, 1000000);
     CHECK(!rt->gc.isIncrementalGCInProgress());
     CHECK(!rt->gc.isFullGc());
     CHECK(checkSingleGroup());
@@ -73,7 +73,7 @@ BEGIN_TEST(testGCFinalizeCallback)
     JS::PrepareZoneForGC(global1->zone());
     JS::PrepareZoneForGC(global2->zone());
     JS::PrepareZoneForGC(global3->zone());
-    JS::IncrementalGC(rt, JS::gcreason::API, 1000000);
+    JS::StartIncrementalGC(rt, JS::gcreason::API, 1000000);
     CHECK(!rt->gc.isIncrementalGCInProgress());
     CHECK(!rt->gc.isFullGc());
     CHECK(checkMultipleGroups());
diff --git a/js/src/jsfriendapi.cpp b/js/src/jsfriendapi.cpp
index 1182ac528eee..41aca0d0ec4f 100644
--- a/js/src/jsfriendapi.cpp
+++ b/js/src/jsfriendapi.cpp
@@ -208,12 +208,15 @@ JS::ShrinkingGC(JSRuntime *rt, gcreason::Reason reason)
 }
 
 JS_FRIEND_API(void)
-JS::IncrementalGC(JSRuntime *rt, gcreason::Reason reason, int64_t millis)
+JS::StartIncrementalGC(JSRuntime *rt, gcreason::Reason reason, int64_t millis)
 {
-    if (!rt->gc.isIncrementalGCInProgress())
-        rt->gc.startGC(GC_NORMAL, reason, millis);
-    else
-        rt->gc.gcSlice(reason, millis);
+    rt->gc.startGC(GC_NORMAL, reason, millis);
+}
+
+JS_FRIEND_API(void)
+JS::IncrementalGCSlice(JSRuntime *rt, gcreason::Reason reason, int64_t millis)
+{
+    rt->gc.gcSlice(reason, millis);
 }
 
 JS_FRIEND_API(void)