diff --git a/js/src/builtin/TestingFunctions.cpp b/js/src/builtin/TestingFunctions.cpp
index c47341508124..d938280934cc 100644
--- a/js/src/builtin/TestingFunctions.cpp
+++ b/js/src/builtin/TestingFunctions.cpp
@@ -570,6 +570,8 @@ GCState(JSContext *cx, unsigned argc, jsval *vp)
         state = "mark";
     else if (globalState == gc::SWEEP)
         state = "sweep";
+    else if (globalState == gc::COMPACT)
+        state = "compact";
     else
         MOZ_CRASH("Unobserveable global GC state");
 
@@ -597,6 +599,48 @@ DeterministicGC(JSContext *cx, unsigned argc, jsval *vp)
 }
 #endif /* JS_GC_ZEAL */
 
+static bool
+StartGC(JSContext *cx, unsigned argc, Value *vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    if (args.length() > 2) {
+        RootedObject callee(cx, &args.callee());
+        ReportUsageError(cx, callee, "Wrong number of arguments");
+        return false;
+    }
+
+    SliceBudget budget;
+    if (args.length() >= 1) {
+        uint32_t work = 0;
+        if (!ToUint32(cx, args[0], &work))
+            return false;
+        budget = SliceBudget(WorkBudget(work));
+    }
+
+    bool shrinking = false;
+    if (args.length() >= 2) {
+        Value arg = args[1];
+        if (arg.isString()) {
+            if (!JS_StringEqualsAscii(cx, arg.toString(), "shrinking", &shrinking))
+                return false;
+        }
+    }
+
+    JSRuntime *rt = cx->runtime();
+    if (rt->gc.isIncrementalGCInProgress()) {
+        RootedObject callee(cx, &args.callee());
+        ReportUsageError(cx, callee, "Incremental GC already in progress");
+        return false;
+    }
+
+    JSGCInvocationKind gckind = shrinking ? GC_SHRINK : GC_NORMAL;
+    rt->gc.startDebugGC(gckind, budget);
+
+    args.rval().setUndefined();
+    return true;
+}
+
 static bool
 GCSlice(JSContext *cx, unsigned argc, Value *vp)
 {
@@ -616,7 +660,12 @@ GCSlice(JSContext *cx, unsigned argc, Value *vp)
         budget = SliceBudget(WorkBudget(work));
     }
 
-    cx->runtime()->gc.gcDebugSlice(budget);
+    JSRuntime *rt = cx->runtime();
+    if (!rt->gc.isIncrementalGCInProgress())
+        rt->gc.startDebugGC(GC_NORMAL, budget);
+    else
+        rt->gc.debugGCSlice(budget);
+
     args.rval().setUndefined();
     return true;
 }
@@ -2190,7 +2239,7 @@ static const JSFunctionSpecWithHelp TestingFunctions[] = {
 "  Run the garbage collector. When obj is given, GC only its compartment.\n"
 "  If 'compartment' is given, GC any compartments that were scheduled for\n"
 "  GC via schedulegc.\n"
-"  If 'shrinking' is passes as the optional second argument, perform a\n"
+"  If 'shrinking' is passed as the optional second argument, perform a\n"
 "  shrinking GC rather than a normal GC."),
 
     JS_FN_HELP("minorgc", ::MinorGC, 0, 0,
@@ -2305,9 +2354,15 @@ gc::ZealModeHelpText),
 "  If true, only allow determinstic GCs to run."),
 #endif
 
+    JS_FN_HELP("startgc", StartGC, 1, 0,
+"startgc([n [, 'shrinking']])",
+"  Start an incremental GC and run a slice that processes about n objects.\n"
+"  If 'shrinking' is passesd as the optional second argument, perform a\n"
+"  shrinking GC rather than a normal GC."),
+
     JS_FN_HELP("gcslice", GCSlice, 1, 0,
-"gcslice(n)",
-"  Run an incremental GC slice that marks about n objects."),
+"gcslice([n])",
+"  Start or continue an an incremental GC, running a slice that processes about n objects."),
 
     JS_FN_HELP("validategc", ValidateGC, 1, 0,
 "validategc(true|false)",
diff --git a/js/src/gc/GCRuntime.h b/js/src/gc/GCRuntime.h
index a79545fd8440..db6dced40200 100644
--- a/js/src/gc/GCRuntime.h
+++ b/js/src/gc/GCRuntime.h
@@ -320,7 +320,8 @@ class GCRuntime
     void startGC(JSGCInvocationKind gckind, JS::gcreason::Reason reason, int64_t millis = 0);
     void gcSlice(JS::gcreason::Reason reason, int64_t millis = 0);
     void finishGC(JS::gcreason::Reason reason);
-    void gcDebugSlice(SliceBudget &budget);
+    void startDebugGC(JSGCInvocationKind gckind, SliceBudget &budget);
+    void debugGCSlice(SliceBudget &budget);
 
     void runDebugGC();
     inline void poke();
diff --git a/js/src/jit-test/tests/gc/incremental-compacting.js b/js/src/jit-test/tests/gc/incremental-compacting.js
new file mode 100644
index 000000000000..1239065ec735
--- /dev/null
+++ b/js/src/jit-test/tests/gc/incremental-compacting.js
@@ -0,0 +1,40 @@
+// Exercise incremental compacting GC
+// Run with MOZ_GCTIMER to see the timings
+
+if (!("gcstate" in this && "gczeal" in this))
+    quit();
+
+gczeal(0);
+
+function testCompacting(zoneCount, objectCount, sliceCount)
+{
+    // Allocate objectCount objects in zoneCount zones
+    // On linux64 debug builds we will move them all
+    // Run compacting GC with multiple slices
+
+    var zones = [];
+    for (var i = 0; i < zoneCount; i++) {
+        var zone = newGlobal();
+        evaluate("var objects; " +
+                 "function makeObjectGraph(objectCount) { " +
+                 "    objects = []; " +
+                 "    for (var i = 0; i < objectCount; i++) " +
+                 "        objects.push({ serial: i }); " +
+                "}",
+                 { global: zone });
+        zone.makeObjectGraph(objectCount);
+        zones.push(zone);
+    }
+
+    startgc(sliceCount, "shrinking");
+    while (gcstate() !== "none") {
+        gcslice(sliceCount);
+    }
+
+    return zones;
+}
+
+testCompacting(1, 100000, 100000);
+testCompacting(2, 100000, 100000);
+testCompacting(4, 50000,  100000);
+testCompacting(2, 100000, 50000);
diff --git a/js/src/jsapi-tests/testGCFinalizeCallback.cpp b/js/src/jsapi-tests/testGCFinalizeCallback.cpp
index b7c681b9aabb..10f7df6a06ad 100644
--- a/js/src/jsapi-tests/testGCFinalizeCallback.cpp
+++ b/js/src/jsapi-tests/testGCFinalizeCallback.cpp
@@ -88,13 +88,13 @@ BEGIN_TEST(testGCFinalizeCallback)
     JS_SetGCZeal(cx, 9, 1000000);
     JS::PrepareForFullGC(rt);
     js::SliceBudget budget(js::WorkBudget(1));
-    rt->gc.gcDebugSlice(budget);
+    rt->gc.startDebugGC(GC_NORMAL, budget);
     CHECK(rt->gc.state() == js::gc::MARK);
     CHECK(rt->gc.isFullGc());
 
     JS::RootedObject global4(cx, createTestGlobal());
     budget = js::SliceBudget(js::WorkBudget(1));
-    rt->gc.gcDebugSlice(budget);
+    rt->gc.debugGCSlice(budget);
     CHECK(!rt->gc.isIncrementalGCInProgress());
     CHECK(!rt->gc.isFullGc());
     CHECK(checkMultipleGroups());
diff --git a/js/src/jsapi-tests/testWeakMap.cpp b/js/src/jsapi-tests/testWeakMap.cpp
index 632e2ce707ea..9569dd677b74 100644
--- a/js/src/jsapi-tests/testWeakMap.cpp
+++ b/js/src/jsapi-tests/testWeakMap.cpp
@@ -91,7 +91,8 @@ BEGIN_TEST(testWeakMap_keyDelegates)
      */
     CHECK(newCCW(map, delegate));
     js::SliceBudget budget(js::WorkBudget(1000000));
-    rt->gc.gcDebugSlice(budget);
+    rt->gc.startDebugGC(GC_NORMAL, budget);
+    CHECK(!JS::IsIncrementalGCInProgress(rt));
 #ifdef DEBUG
     CHECK(map->zone()->lastZoneGroupIndex() < delegate->zone()->lastZoneGroupIndex());
 #endif
@@ -105,7 +106,8 @@ BEGIN_TEST(testWeakMap_keyDelegates)
     key = nullptr;
     CHECK(newCCW(map, delegate));
     budget = js::SliceBudget(js::WorkBudget(100000));
-    rt->gc.gcDebugSlice(budget);
+    rt->gc.startDebugGC(GC_NORMAL, budget);
+    CHECK(!JS::IsIncrementalGCInProgress(rt));
     CHECK(checkSize(map, 1));
 
     /*
diff --git a/js/src/jsgc.cpp b/js/src/jsgc.cpp
index 107b6d79279e..aaef1c085b0e 100644
--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@@ -6402,16 +6402,21 @@ ZonesSelected(JSRuntime *rt)
 }
 
 void
-GCRuntime::gcDebugSlice(SliceBudget &budget)
+GCRuntime::startDebugGC(JSGCInvocationKind gckind, SliceBudget &budget)
 {
-    if (!ZonesSelected(rt)) {
-        if (isIncrementalGCInProgress())
-            JS::PrepareForIncrementalGC(rt);
-        else
-            JS::PrepareForFullGC(rt);
-    }
-    if (!isIncrementalGCInProgress())
-        invocationKind = GC_NORMAL;
+    MOZ_ASSERT(!isIncrementalGCInProgress());
+    if (!ZonesSelected(rt))
+        JS::PrepareForFullGC(rt);
+    invocationKind = gckind;
+    collect(true, budget, JS::gcreason::DEBUG_GC);
+}
+
+void
+GCRuntime::debugGCSlice(SliceBudget &budget)
+{
+    MOZ_ASSERT(isIncrementalGCInProgress());
+    if (!ZonesSelected(rt))
+        JS::PrepareForIncrementalGC(rt);
     collect(true, budget, JS::gcreason::DEBUG_GC);
 }