Bug 1348772 - Optimize Array.prototype.shift to have O(1) perf instead of O(n). r=jonco

JS code often uses arrays as queues, with a loop to shift() all items, and this resulted in quadratic behavior for us. That kind of code is much faster now.
2017-05-11 17:12:39 +02:00 · 2017-05-11 17:12:39 +02:00 · 7effa6b8fe
--- a/js/src/gc/Barrier.cpp
+++ b/js/src/gc/Barrier.cpp
@ -47,19 +47,26 @@ IsMarkedBlack(JSObject* obj)
 bool
 HeapSlot::preconditionForSet(NativeObject* owner, Kind kind, uint32_t slot) const
 {
-    return kind == Slot
-         ? &owner->getSlotRef(slot) == this
-         : &owner->getDenseElement(slot) == (const Value*)this;
+    if (kind == Slot)
+        return &owner->getSlotRef(slot) == this;
+
+    uint32_t numShifted = owner->getElementsHeader()->numShiftedElements();
+    MOZ_ASSERT(slot >= numShifted);
+    return &owner->getDenseElement(slot - numShifted) == (const Value*)this;
 }

 void
 HeapSlot::assertPreconditionForWriteBarrierPost(NativeObject* obj, Kind kind, uint32_t slot,
                                                const Value& target) const
 {
-    if (kind == Slot)
+    if (kind == Slot) {
        MOZ_ASSERT(obj->getSlotAddressUnchecked(slot)->get() == target);
-    else
-        MOZ_ASSERT(static_cast<HeapSlot*>(obj->getDenseElements() + slot)->get() == target);
+    } else {
+        uint32_t numShifted = obj->getElementsHeader()->numShiftedElements();
+        MOZ_ASSERT(slot >= numShifted);
+        MOZ_ASSERT(static_cast<HeapSlot*>(obj->getDenseElements() + (slot - numShifted))->get() ==
+                   target);
+    }

    CheckEdgeIsNotBlackToGray(obj, target);
 }
--- a/js/src/gc/Marking.cpp
+++ b/js/src/gc/Marking.cpp
@ -1823,7 +1823,10 @@ GCMarker::saveValueRanges()
            HeapSlot* vp = obj->getDenseElementsAllowCopyOnWrite();
            if (array.end == vp + obj->getDenseInitializedLength()) {
                MOZ_ASSERT(array.start >= vp);
-                index = array.start - vp;
+                // Add the number of shifted elements here (and subtract in
+                // restoreValueArray) to ensure shift() calls on the array
+                // are handled correctly.
+                index = obj->unshiftedIndex(array.start - vp);
                kind = HeapSlot::Element;
            } else {
                HeapSlot* vp = obj->fixedSlots();
@ -1865,6 +1868,11 @@ GCMarker::restoreValueArray(const MarkStack::SavedValueArray& array,
            return false;

        uint32_t initlen = obj->getDenseInitializedLength();
+
+        // Account for shifted elements.
+        uint32_t numShifted = obj->getElementsHeader()->numShiftedElements();
+        start = (numShifted < start) ? start - numShifted : 0;
+
        HeapSlot* vp = obj->getDenseElementsAllowCopyOnWrite();
        if (start < initlen) {
            *vpp = vp + start;
@ -2689,8 +2697,11 @@ js::gc::StoreBuffer::SlotsEdge::trace(TenuringTracer& mover) const

    if (kind() == ElementKind) {
        int32_t initLen = obj->getDenseInitializedLength();
-        int32_t clampedStart = Min(start_, initLen);
-        int32_t clampedEnd = Min(start_ + count_, initLen);
+        int32_t numShifted = obj->getElementsHeader()->numShiftedElements();
+        int32_t clampedStart = Min(Max(0, start_ - numShifted), initLen);
+        int32_t clampedEnd = Min(Max(0, start_ + count_ - numShifted), initLen);
+        MOZ_ASSERT(clampedStart >= 0);
+        MOZ_ASSERT(clampedStart <= clampedEnd);
        mover.traceSlots(static_cast<HeapSlot*>(obj->getDenseElements() + clampedStart)
                            ->unsafeUnbarrieredForTracing(), clampedEnd - clampedStart);
    } else {
@ -3014,30 +3025,35 @@ js::TenuringTracer::moveElementsToTenured(NativeObject* dst, NativeObject* src,
    if (src->hasEmptyElements() || src->denseElementsAreCopyOnWrite())
        return 0;

-    Zone* zone = src->zone();
-    ObjectElements* srcHeader = src->getElementsHeader();
-    ObjectElements* dstHeader;
+    void* srcAllocatedHeader = src->getUnshiftedElementsHeader();

    /* TODO Bug 874151: Prefer to put element data inline if we have space. */
-    if (!nursery().isInside(srcHeader)) {
+    if (!nursery().isInside(srcAllocatedHeader)) {
        MOZ_ASSERT(src->elements_ == dst->elements_);
-        nursery().removeMallocedBuffer(srcHeader);
+        nursery().removeMallocedBuffer(srcAllocatedHeader);
        return 0;
    }

-    size_t nslots = ObjectElements::VALUES_PER_HEADER + srcHeader->capacity;
+    ObjectElements* srcHeader = src->getElementsHeader();
+
+    // Shifted elements are copied too.
+    uint32_t numShifted = srcHeader->numShiftedElements();
+    size_t nslots = srcHeader->numAllocatedElements();

    /* Unlike other objects, Arrays can have fixed elements. */
    if (src->is<ArrayObject>() && nslots <= GetGCKindSlots(dstKind)) {
        dst->as<ArrayObject>().setFixedElements();
-        dstHeader = dst->as<ArrayObject>().getElementsHeader();
-        js_memcpy(dstHeader, srcHeader, nslots * sizeof(HeapSlot));
-        nursery().setElementsForwardingPointer(srcHeader, dstHeader, nslots);
+        js_memcpy(dst->getElementsHeader(), srcAllocatedHeader, nslots * sizeof(HeapSlot));
+        dst->elements_ += numShifted;
+        nursery().setElementsForwardingPointer(srcHeader, dst->getElementsHeader(),
+                                               srcHeader->capacity);
        return nslots * sizeof(HeapSlot);
    }

    MOZ_ASSERT(nslots >= 2);

+    Zone* zone = src->zone();
+    ObjectElements* dstHeader;
    {
        AutoEnterOOMUnsafeRegion oomUnsafe;
        dstHeader = reinterpret_cast<ObjectElements*>(zone->pod_malloc<HeapSlot>(nslots));
@ -3047,9 +3063,10 @@ js::TenuringTracer::moveElementsToTenured(NativeObject* dst, NativeObject* src,
        }
    }

-    js_memcpy(dstHeader, srcHeader, nslots * sizeof(HeapSlot));
-    nursery().setElementsForwardingPointer(srcHeader, dstHeader, nslots);
-    dst->elements_ = dstHeader->elements();
+    js_memcpy(dstHeader, srcAllocatedHeader, nslots * sizeof(HeapSlot));
+    dst->elements_ = dstHeader->elements() + numShifted;
+    nursery().setElementsForwardingPointer(srcHeader, dst->getElementsHeader(),
+                                           srcHeader->capacity);
    return nslots * sizeof(HeapSlot);
 }

--- a/js/src/gc/Nursery.cpp
+++ b/js/src/gc/Nursery.cpp
@ -440,11 +440,11 @@ Nursery::setSlotsForwardingPointer(HeapSlot* oldSlots, HeapSlot* newSlots, uint3

 void
 Nursery::setElementsForwardingPointer(ObjectElements* oldHeader, ObjectElements* newHeader,
-                                      uint32_t nelems)
+                                      uint32_t capacity)
 {
    // Only use a direct forwarding pointer if there is enough space for one.
    setForwardingPointer(oldHeader->elements(), newHeader->elements(),
-                         nelems > ObjectElements::VALUES_PER_HEADER);
+                         capacity > 0);
 }

 #ifdef DEBUG
--- a/js/src/gc/Nursery.h
+++ b/js/src/gc/Nursery.h
@ -459,7 +459,7 @@ class Nursery

    void setSlotsForwardingPointer(HeapSlot* oldSlots, HeapSlot* newSlots, uint32_t nslots);
    void setElementsForwardingPointer(ObjectElements* oldHeader, ObjectElements* newHeader,
-                                      uint32_t nelems);
+                                      uint32_t capacity);

    /* Free malloced pointers owned by freed things in the nursery. */
    void freeMallocedBuffers();
--- a/js/src/jit-test/tests/basic/shifted-elements1.js
+++ b/js/src/jit-test/tests/basic/shifted-elements1.js
@ -0,0 +1,14 @@
+function f() {
+    var arr = [];
+    var iters = 1500;
+    for (var i = 0; i < iters; i++) {
+        arr.push(i);
+        if (i % 2 === 0)
+            assertEq(arr.shift(), i / 2);
+    }
+    assertEq(arr.length, iters / 2);
+    for (var i = iters / 2; i < iters; i++)
+        assertEq(arr.shift(), i);
+    assertEq(arr.length, 0);
+}
+f();
--- a/js/src/jit-test/tests/basic/shifted-elements2.js
+++ b/js/src/jit-test/tests/basic/shifted-elements2.js
@ -0,0 +1,22 @@
+// Always use the per-element barrier.
+gczeal(12);
+
+function f() {
+    var arr = [];
+    for (var i = 0; i < 1000; i++)
+        arr.push(i);
+    gc(); // Ensure arr is tenured.
+
+    // Now store a nursery object somewhere in the array, shift elements,
+    // trigger a GC, and check the post barrier kept the object alive.
+    for (var i = 0; i < 20; i++)
+        arr.shift();
+    for (var i = 0; i < 40; i++)
+        arr[900] = {x: i};
+    for (var i = 0; i < 10; i++)
+        arr.shift();
+    gc();
+
+    assertEq(arr[890].x, 39);
+}
+f();
--- a/js/src/jit-test/tests/basic/shifted-elements3.js
+++ b/js/src/jit-test/tests/basic/shifted-elements3.js
@ -0,0 +1,23 @@
+// Always use the per-element barrier.
+gczeal(12);
+
+function f() {
+    var arr = [];
+    for (var i = 0; i < 1000; i++)
+        arr.push(i);
+    gc(); // Ensure arr is tenured.
+
+    for (var i = 0; i < 10; i++)
+        arr.shift();
+
+    // Add a nursery object, shift all elements, and trigger a GC to ensure
+    // the post barrier doesn't misbehave.
+    for (var j = 0; j < 40; j++)
+        arr[500] = {x: j};
+    while (arr.length > 0)
+        arr.shift();
+
+    gc();
+    return arr;
+}
+f();
--- a/js/src/jit-test/tests/basic/shifted-elements4.js
+++ b/js/src/jit-test/tests/basic/shifted-elements4.js
@ -0,0 +1,11 @@
+function f() {
+    var arr = [];
+    for (var i = 0; i < 2; i++) {
+	for (var j = 0; j < 90000; j++)
+	    arr.push(j);
+	for (var j = 0; j < 90000; j++)
+	    assertEq(arr.shift(), j);
+	assertEq(arr.length, 0);
+    }
+}
+f();
--- a/js/src/jit-test/tests/basic/shifted-elements5.js
+++ b/js/src/jit-test/tests/basic/shifted-elements5.js
@ -0,0 +1,39 @@
+function testFreeze() {
+    var arr = [];
+    for (var i = 0; i < 20; i++)
+        arr.push(i);
+    for (var i = 0; i < 10; i++)
+        arr.shift();
+    Object.freeze(arr);
+    assertEq(arr.length, 10);
+    arr[0] = -1;
+    assertEq(arr[0], 10);
+}
+testFreeze();
+testFreeze();
+
+function testCopyOnWrite() {
+    var arr = [1, 2, 3, 4, 5, 6, 7, 8, 9];
+    for (var i = 0; i < 5; i++)
+        assertEq(arr.shift(), i + 1);
+    assertEq(arr.toString(), "6,7,8,9");
+}
+testCopyOnWrite();
+testCopyOnWrite();
+
+function testNonWritableLength() {
+    var arr = [];
+    for (var i = 0; i < 20; i++)
+        arr.push(i);
+    Object.defineProperty(arr, "length", {writable: false, value: arr.length});
+    var ex;
+    try {
+        arr.shift();
+    } catch(e) {
+        ex = e;
+    }
+    assertEq(ex instanceof TypeError, true);
+    assertEq(arr.length, 20);
+}
+testNonWritableLength();
+testNonWritableLength();
--- a/js/src/jit-test/tests/basic/shifted-elements6.js
+++ b/js/src/jit-test/tests/basic/shifted-elements6.js
@ -0,0 +1,17 @@
+// Test incremental GC slices and shifted elements.
+function f() {
+    var arr = [];
+    for (var i = 0; i < 1000; i++)
+        arr.push({x: i});
+    var arr2 = [];
+    for (var i = 0; i < 1000; i++) {
+        gcslice(900);
+        var o = arr.shift();
+        assertEq(o.x, i);
+        arr2.push(o);
+    }
+    gc();
+    for (var i = 0; i < 1000; i++)
+        assertEq(arr2[i].x, i);
+}
+f();
--- a/js/src/jit/CodeGenerator.cpp
+++ b/js/src/jit/CodeGenerator.cpp
@ -8954,7 +8954,26 @@ CodeGenerator::emitArrayPopShift(LInstruction* lir, const MArrayPopShift* mir, R
        Address elementFlags(elementsTemp, ObjectElements::offsetOfFlags());
        Imm32 bit(ObjectElements::NONWRITABLE_ARRAY_LENGTH);
        masm.branchTest32(Assembler::NonZero, elementFlags, bit, ool->entry());
+    }

+    if (mir->mode() == MArrayPopShift::Shift) {
+        // Don't save the elementsTemp register.
+        LiveRegisterSet temps;
+        temps.add(elementsTemp);
+
+        saveVolatile(temps);
+        masm.setupUnalignedABICall(elementsTemp);
+        masm.passABIArg(obj);
+        masm.callWithABI(JS_FUNC_TO_DATA_PTR(void*, js::ArrayShiftMoveElements));
+        restoreVolatile(temps);
+
+        if (mir->unboxedType() == JSVAL_TYPE_MAGIC) {
+            // Reload elementsTemp as ArrayShiftMoveElements may have moved it.
+            masm.loadPtr(Address(obj, NativeObject::offsetOfElements()), elementsTemp);
+        }
+    }
+
+    if (mir->unboxedType() == JSVAL_TYPE_MAGIC) {
        // Now adjust length and initializedLength.
        masm.store32(lengthTemp, Address(elementsTemp, ObjectElements::offsetOfLength()));
        masm.store32(lengthTemp, Address(elementsTemp, ObjectElements::offsetOfInitializedLength()));
@ -8965,19 +8984,6 @@ CodeGenerator::emitArrayPopShift(LInstruction* lir, const MArrayPopShift* mir, R
        masm.add32(Imm32(-1), Address(obj, UnboxedArrayObject::offsetOfCapacityIndexAndInitializedLength()));
    }

-    if (mir->mode() == MArrayPopShift::Shift) {
-        // Don't save the temp registers.
-        LiveRegisterSet temps;
-        temps.add(elementsTemp);
-        temps.add(lengthTemp);
-
-        saveVolatile(temps);
-        masm.setupUnalignedABICall(lengthTemp);
-        masm.passABIArg(obj);
-        masm.callWithABI(JS_FUNC_TO_DATA_PTR(void*, js::ArrayShiftMoveElements));
-        restoreVolatile(temps);
-    }
-
    masm.bind(&done);
    masm.bind(ool->rejoin());
 }
--- a/js/src/jit/VMFunctions.cpp
+++ b/js/src/jit/VMFunctions.cpp
@ -705,7 +705,9 @@ PostWriteElementBarrier(JSRuntime* rt, JSObject* obj, int32_t index)
 #endif
        )
    {
-        rt->gc.storeBuffer().putSlot(nobj, HeapSlot::Element, index, 1);
+        rt->gc.storeBuffer().putSlot(nobj, HeapSlot::Element,
+                                     nobj->unshiftedIndex(index),
+                                     1);
        return;
    }

--- a/js/src/jsarray.cpp
+++ b/js/src/jsarray.cpp
@ -791,6 +791,11 @@ js::ArraySetLength(JSContext* cx, Handle<ArrayObject*> arr, HandleId id,
    header->initializedLength = Min(header->initializedLength, newLen);

    if (attrs & JSPROP_READONLY) {
+        if (header->numShiftedElements() > 0) {
+            arr->unshiftElements();
+            header = arr->getElementsHeader();
+        }
+
        header->setNonwritableArrayLength();

        // When an array's length becomes non-writable, writes to indexes
@ -2229,18 +2234,16 @@ ShiftMoveBoxedOrUnboxedDenseElements(JSObject* obj)
 {
    MOZ_ASSERT(HasBoxedOrUnboxedDenseElements<Type>(obj));

-    /*
-     * At this point the length and initialized length have already been
-     * decremented and the result fetched, so just shift the array elements
-     * themselves.
-     */
    size_t initlen = GetBoxedOrUnboxedInitializedLength<Type>(obj);
+    MOZ_ASSERT(initlen > 0);
+
    if (Type == JSVAL_TYPE_MAGIC) {
-        obj->as<NativeObject>().moveDenseElementsNoPreBarrier(0, 1, initlen);
+        if (!obj->as<NativeObject>().tryShiftDenseElements(1))
+            obj->as<NativeObject>().moveDenseElementsNoPreBarrier(0, 1, initlen - 1);
    } else {
        uint8_t* data = obj->as<UnboxedArrayObject>().elements();
        size_t elementSize = UnboxedTypeSize(Type);
-        memmove(data, data + elementSize, initlen * elementSize);
+        memmove(data, data + elementSize, (initlen - 1) * elementSize);
    }

    return DenseElementResult::Success;
@ -2275,6 +2278,11 @@ ArrayShiftDenseKernel(JSContext* cx, HandleObject obj, MutableHandleValue rval)
    if (rval.isMagic(JS_ELEMENTS_HOLE))
        rval.setUndefined();

+    if (Type == JSVAL_TYPE_MAGIC) {
+        if (obj->as<NativeObject>().tryShiftDenseElements(1))
+            return DenseElementResult::Success;
+    }
+
    DenseElementResult result = MoveBoxedOrUnboxedDenseElements<Type>(cx, obj, 0, 1, initlen - 1);
    if (result != DenseElementResult::Success)
        return result;
--- a/js/src/jsgc.cpp
+++ b/js/src/jsgc.cpp
@ -1971,8 +1971,10 @@ RelocateCell(Zone* zone, TenuredCell* src, AllocKind thingKind, size_t thingSize
            NativeObject* dstNative = &dstObj->as<NativeObject>();

            // Fixup the pointer to inline object elements if necessary.
-            if (srcNative->hasFixedElements())
-                dstNative->setFixedElements();
+            if (srcNative->hasFixedElements()) {
+                uint32_t numShifted = srcNative->getElementsHeader()->numShiftedElements();
+                dstNative->setFixedElements(numShifted);
+            }

            // For copy-on-write objects that own their elements, fix up the
            // owner pointer to point to the relocated object.
--- a/js/src/jsobj.cpp
+++ b/js/src/jsobj.cpp
@ -542,6 +542,8 @@ js::SetIntegrityLevel(JSContext* cx, HandleObject obj, IntegrityLevel level)
        if (level == IntegrityLevel::Frozen && obj->is<ArrayObject>()) {
            if (!obj->as<ArrayObject>().maybeCopyElementsForWrite(cx))
                return false;
+            if (nobj->getElementsHeader()->numShiftedElements() > 0)
+                nobj->unshiftElements();
            obj->as<ArrayObject>().getElementsHeader()->setNonwritableArrayLength();
        }
    } else {
@ -3881,8 +3883,10 @@ JSObject::addSizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf, JS::ClassIn

    if (is<NativeObject>() && as<NativeObject>().hasDynamicElements()) {
        js::ObjectElements* elements = as<NativeObject>().getElementsHeader();
-        if (!elements->isCopyOnWrite() || elements->ownerObject() == this)
-            info->objectsMallocHeapElementsNormal += mallocSizeOf(elements);
+        if (!elements->isCopyOnWrite() || elements->ownerObject() == this) {
+            void* allocatedElements = as<NativeObject>().getUnshiftedElementsHeader();
+            info->objectsMallocHeapElementsNormal += mallocSizeOf(allocatedElements);
+        }
    }

    // Other things may be measured in the future if DMD indicates it is worthwhile.
@ -3947,7 +3951,7 @@ JSObject::sizeOfIncludingThisInNursery() const
        if (native.hasDynamicElements()) {
            js::ObjectElements& elements = *native.getElementsHeader();
            if (!elements.isCopyOnWrite() || elements.ownerObject() == this)
-                size += elements.capacity * sizeof(HeapSlot);
+                size += (elements.capacity + elements.numShiftedElements()) * sizeof(HeapSlot);
        }

        if (is<ArgumentsObject>())
--- a/js/src/jsobjinlines.h
+++ b/js/src/jsobjinlines.h
@ -99,10 +99,11 @@ JSObject::finalize(js::FreeOp* fop)
                // Don't free the elements until object finalization finishes,
                // so that other objects can access these elements while they
                // are themselves finalized.
+                MOZ_ASSERT(elements->numShiftedElements() == 0);
                fop->freeLater(elements);
            }
        } else {
-            fop->free_(elements);
+            fop->free_(nobj->getUnshiftedElementsHeader());
        }
    }

--- a/js/src/vm/NativeObject-inl.h
+++ b/js/src/vm/NativeObject-inl.h
@ -124,7 +124,8 @@ NativeObject::elementsRangeWriteBarrierPost(uint32_t start, uint32_t count)
        const Value& v = elements_[start + i];
        if (v.isObject() && IsInsideNursery(&v.toObject())) {
            zone()->group()->storeBuffer().putSlot(this, HeapSlot::Element,
-                                                   start + i, count - i);
+                                                   unshiftedIndex(start + i),
+                                                   count - i);
            return;
        }
    }
@ -141,8 +142,12 @@ NativeObject::copyDenseElements(uint32_t dstStart, const Value* src, uint32_t co
        checkStoredValue(src[i]);
 #endif
    if (JS::shadow::Zone::asShadowZone(zone())->needsIncrementalBarrier()) {
-        for (uint32_t i = 0; i < count; ++i)
-            elements_[dstStart + i].set(this, HeapSlot::Element, dstStart + i, src[i]);
+        uint32_t numShifted = getElementsHeader()->numShiftedElements();
+        for (uint32_t i = 0; i < count; ++i) {
+            elements_[dstStart + i].set(this, HeapSlot::Element,
+                                        dstStart + i + numShifted,
+                                        src[i]);
+        }
    } else {
        memcpy(&elements_[dstStart], src, count * sizeof(HeapSlot));
        elementsRangeWriteBarrierPost(dstStart, count);
@ -163,6 +168,36 @@ NativeObject::initDenseElements(uint32_t dstStart, const Value* src, uint32_t co
    elementsRangeWriteBarrierPost(dstStart, count);
 }

+inline bool
+NativeObject::tryShiftDenseElements(uint32_t count)
+{
+    ObjectElements* header = getElementsHeader();
+    if (header->isCopyOnWrite() ||
+        header->isFrozen() ||
+        header->hasNonwritableArrayLength() ||
+        header->initializedLength == count)
+    {
+        return false;
+    }
+
+    MOZ_ASSERT(count > 0);
+    MOZ_ASSERT(count < header->initializedLength);
+    MOZ_ASSERT(count <= ObjectElements::MaxShiftedElements);
+
+    if (MOZ_UNLIKELY(header->numShiftedElements() + count > ObjectElements::MaxShiftedElements)) {
+        unshiftElements();
+        header = getElementsHeader();
+    }
+
+    prepareElementRangeForOverwrite(0, count);
+    header->addShiftedElements(count);
+
+    elements_ += count;
+    ObjectElements* newHeader = getElementsHeader();
+    memmove(newHeader, header, sizeof(ObjectElements));
+    return true;
+}
+
 inline void
 NativeObject::moveDenseElements(uint32_t dstStart, uint32_t srcStart, uint32_t count)
 {
@ -184,16 +219,17 @@ NativeObject::moveDenseElements(uint32_t dstStart, uint32_t srcStart, uint32_t c
     * the array before and after the move.
     */
    if (JS::shadow::Zone::asShadowZone(zone())->needsIncrementalBarrier()) {
+        uint32_t numShifted = getElementsHeader()->numShiftedElements();
        if (dstStart < srcStart) {
            HeapSlot* dst = elements_ + dstStart;
            HeapSlot* src = elements_ + srcStart;
            for (uint32_t i = 0; i < count; i++, dst++, src++)
-                dst->set(this, HeapSlot::Element, dst - elements_, *src);
+                dst->set(this, HeapSlot::Element, dst - elements_ + numShifted, *src);
        } else {
            HeapSlot* dst = elements_ + dstStart + count - 1;
            HeapSlot* src = elements_ + srcStart + count - 1;
            for (uint32_t i = 0; i < count; i++, dst--, src--)
-                dst->set(this, HeapSlot::Element, dst - elements_, *src);
+                dst->set(this, HeapSlot::Element, dst - elements_ + numShifted, *src);
        }
    } else {
        memmove(elements_ + dstStart, elements_ + srcStart, count * sizeof(HeapSlot));
@ -231,12 +267,13 @@ NativeObject::ensureDenseInitializedLengthNoPackedCheck(JSContext* cx, uint32_t
    uint32_t& initlen = getElementsHeader()->initializedLength;

    if (initlen < index + extra) {
+        uint32_t numShifted = getElementsHeader()->numShiftedElements();
        size_t offset = initlen;
        for (HeapSlot* sp = elements_ + initlen;
             sp != elements_ + (index + extra);
             sp++, offset++)
        {
-            sp->init(this, HeapSlot::Element, offset, MagicValue(JS_ELEMENTS_HOLE));
+            sp->init(this, HeapSlot::Element, offset + numShifted, MagicValue(JS_ELEMENTS_HOLE));
        }
        initlen = index + extra;
    }
--- a/js/src/vm/NativeObject.cpp
+++ b/js/src/vm/NativeObject.cpp
@ -8,6 +8,7 @@

 #include "mozilla/ArrayUtils.h"
 #include "mozilla/Casting.h"
+#include "mozilla/CheckedInt.h"

 #include "jswatchpoint.h"

@ -28,6 +29,7 @@ using namespace js;
 using JS::AutoCheckCannotGC;
 using JS::GenericNaN;
 using mozilla::ArrayLength;
+using mozilla::CheckedInt;
 using mozilla::DebugOnly;
 using mozilla::PodCopy;
 using mozilla::RoundUpPow2;
@ -112,6 +114,9 @@ ObjectElements::FreezeElements(JSContext* cx, HandleNativeObject obj)
    if (obj->hasEmptyElements())
        return true;

+    if (obj->getElementsHeader()->numShiftedElements() > 0)
+        obj->unshiftElements();
+
    ObjectElements* header = obj->getElementsHeader();

    // Note: this method doesn't update type information to indicate that the
@ -329,7 +334,7 @@ NativeObject::setLastPropertyMakeNonNative(Shape* shape)
    MOZ_ASSERT(shape->numFixedSlots() == 0);

    if (hasDynamicElements())
-        js_free(getElementsHeader());
+        js_free(getUnshiftedElementsHeader());
    if (hasDynamicSlots()) {
        js_free(slots_);
        slots_ = nullptr;
@ -701,6 +706,49 @@ NativeObject::maybeDensifySparseElements(JSContext* cx, HandleNativeObject obj)
    return DenseElementResult::Success;
 }

+void
+NativeObject::unshiftElements()
+{
+    ObjectElements* header = getElementsHeader();
+    uint32_t numShifted = header->numShiftedElements();
+    MOZ_ASSERT(numShifted > 0);
+
+    uint32_t initLength = header->initializedLength;
+
+    ObjectElements* newHeader = static_cast<ObjectElements*>(getUnshiftedElementsHeader());
+    memmove(newHeader, header, sizeof(ObjectElements));
+
+    newHeader->clearShiftedElements();
+    newHeader->capacity += numShifted;
+    elements_ = newHeader->elements();
+
+    // To move the elements, temporarily update initializedLength to include
+    // both shifted and unshifted elements.
+    newHeader->initializedLength += numShifted;
+
+    // Move the elements. Initialize to |undefined| to ensure pre-barriers
+    // don't see garbage.
+    for (size_t i = 0; i < numShifted; i++)
+        initDenseElement(i, UndefinedValue());
+    moveDenseElements(0, numShifted, initLength);
+
+    // Restore the initialized length. We use setDenseInitializedLength to
+    // make sure prepareElementRangeForOverwrite is called on the shifted
+    // elements.
+    setDenseInitializedLength(initLength);
+}
+
+void
+NativeObject::maybeUnshiftElements()
+{
+    ObjectElements* header = getElementsHeader();
+    MOZ_ASSERT(header->numShiftedElements() > 0);
+
+    // Unshift if less than a third of the allocated space is in use.
+    if (header->capacity < header->numAllocatedElements() / 3)
+        unshiftElements();
+}
+
 // Given a requested capacity (in elements) and (potentially) the length of an
 // array for which elements are being allocated, compute an actual allocation
 // amount (in elements).  (Allocation amounts include space for an
@ -799,6 +847,26 @@ NativeObject::growElements(JSContext* cx, uint32_t reqCapacity)
    if (denseElementsAreCopyOnWrite())
        MOZ_CRASH();

+    // If there are shifted elements, consider unshifting them first. If we
+    // don't unshift here, the code below will include the shifted elements in
+    // the resize.
+    uint32_t numShifted = getElementsHeader()->numShiftedElements();
+    if (numShifted > 0) {
+        maybeUnshiftElements();
+        if (getDenseCapacity() >= reqCapacity)
+            return true;
+        numShifted = getElementsHeader()->numShiftedElements();
+
+        // Ensure |reqCapacity + numShifted| below won't overflow by forcing an
+        // unshift in that case.
+        CheckedInt<uint32_t> checkedReqCapacity(reqCapacity);
+        checkedReqCapacity += numShifted;
+        if (MOZ_UNLIKELY(!checkedReqCapacity.isValid())) {
+            unshiftElements();
+            numShifted = 0;
+        }
+    }
+
    uint32_t oldCapacity = getDenseCapacity();
    MOZ_ASSERT(oldCapacity < reqCapacity);

@ -809,13 +877,17 @@ NativeObject::growElements(JSContext* cx, uint32_t reqCapacity)
        // Preserve the |capacity <= length| invariant for arrays with
        // non-writable length.  See also js::ArraySetLength which initially
        // enforces this requirement.
-        newAllocated = reqCapacity + ObjectElements::VALUES_PER_HEADER;
+        newAllocated = reqCapacity + numShifted + ObjectElements::VALUES_PER_HEADER;
    } else {
-        if (!goodElementsAllocationAmount(cx, reqCapacity, getElementsHeader()->length, &newAllocated))
+        if (!goodElementsAllocationAmount(cx, reqCapacity + numShifted,
+                                          getElementsHeader()->length,
+                                          &newAllocated))
+        {
            return false;
+        }
    }

-    uint32_t newCapacity = newAllocated - ObjectElements::VALUES_PER_HEADER;
+    uint32_t newCapacity = newAllocated - ObjectElements::VALUES_PER_HEADER - numShifted;
    MOZ_ASSERT(newCapacity > oldCapacity && newCapacity >= reqCapacity);

    // If newCapacity exceeds MAX_DENSE_ELEMENTS_COUNT, the array should become
@ -824,11 +896,11 @@ NativeObject::growElements(JSContext* cx, uint32_t reqCapacity)

    uint32_t initlen = getDenseInitializedLength();

-    HeapSlot* oldHeaderSlots = reinterpret_cast<HeapSlot*>(getElementsHeader());
+    HeapSlot* oldHeaderSlots = reinterpret_cast<HeapSlot*>(getUnshiftedElementsHeader());
    HeapSlot* newHeaderSlots;
    if (hasDynamicElements()) {
        MOZ_ASSERT(oldCapacity <= MAX_DENSE_ELEMENTS_COUNT);
-        uint32_t oldAllocated = oldCapacity + ObjectElements::VALUES_PER_HEADER;
+        uint32_t oldAllocated = oldCapacity + ObjectElements::VALUES_PER_HEADER + numShifted;

        newHeaderSlots = ReallocateObjectBuffer<HeapSlot>(cx, this, oldHeaderSlots, oldAllocated, newAllocated);
        if (!newHeaderSlots)
@ -837,12 +909,13 @@ NativeObject::growElements(JSContext* cx, uint32_t reqCapacity)
        newHeaderSlots = AllocateObjectBuffer<HeapSlot>(cx, this, newAllocated);
        if (!newHeaderSlots)
            return false;   // Leave elements at its old size.
-        PodCopy(newHeaderSlots, oldHeaderSlots, ObjectElements::VALUES_PER_HEADER + initlen);
+        PodCopy(newHeaderSlots, oldHeaderSlots,
+                ObjectElements::VALUES_PER_HEADER + initlen + numShifted);
    }

    ObjectElements* newheader = reinterpret_cast<ObjectElements*>(newHeaderSlots);
-    newheader->capacity = newCapacity;
-    elements_ = newheader->elements();
+    elements_ = newheader->elements() + numShifted;
+    getElementsHeader()->capacity = newCapacity;

    Debug_SetSlotRangeToCrashOnTouch(elements_ + initlen, newCapacity - initlen);

@ -852,9 +925,6 @@ NativeObject::growElements(JSContext* cx, uint32_t reqCapacity)
 void
 NativeObject::shrinkElements(JSContext* cx, uint32_t reqCapacity)
 {
-    uint32_t oldCapacity = getDenseCapacity();
-    MOZ_ASSERT(reqCapacity < oldCapacity);
-
    MOZ_ASSERT(canHaveNonEmptyElements());
    if (denseElementsAreCopyOnWrite())
        MOZ_CRASH();
@ -862,18 +932,29 @@ NativeObject::shrinkElements(JSContext* cx, uint32_t reqCapacity)
    if (!hasDynamicElements())
        return;

+    // If we have shifted elements, consider unshifting them.
+    uint32_t numShifted = getElementsHeader()->numShiftedElements();
+    if (numShifted > 0) {
+        maybeUnshiftElements();
+        numShifted = getElementsHeader()->numShiftedElements();
+    }
+
+    uint32_t oldCapacity = getDenseCapacity();
+    MOZ_ASSERT(reqCapacity < oldCapacity);
+
    uint32_t newAllocated = 0;
-    MOZ_ALWAYS_TRUE(goodElementsAllocationAmount(cx, reqCapacity, 0, &newAllocated));
+    MOZ_ALWAYS_TRUE(goodElementsAllocationAmount(cx, reqCapacity + numShifted, 0, &newAllocated));
    MOZ_ASSERT(oldCapacity <= MAX_DENSE_ELEMENTS_COUNT);
-    uint32_t oldAllocated = oldCapacity + ObjectElements::VALUES_PER_HEADER;
+
+    uint32_t oldAllocated = oldCapacity + ObjectElements::VALUES_PER_HEADER + numShifted;
    if (newAllocated == oldAllocated)
        return;  // Leave elements at its old size.

    MOZ_ASSERT(newAllocated > ObjectElements::VALUES_PER_HEADER);
-    uint32_t newCapacity = newAllocated - ObjectElements::VALUES_PER_HEADER;
+    uint32_t newCapacity = newAllocated - ObjectElements::VALUES_PER_HEADER - numShifted;
    MOZ_ASSERT(newCapacity <= MAX_DENSE_ELEMENTS_COUNT);

-    HeapSlot* oldHeaderSlots = reinterpret_cast<HeapSlot*>(getElementsHeader());
+    HeapSlot* oldHeaderSlots = reinterpret_cast<HeapSlot*>(getUnshiftedElementsHeader());
    HeapSlot* newHeaderSlots = ReallocateObjectBuffer<HeapSlot>(cx, this, oldHeaderSlots,
                                                                oldAllocated, newAllocated);
    if (!newHeaderSlots) {
@ -882,8 +963,8 @@ NativeObject::shrinkElements(JSContext* cx, uint32_t reqCapacity)
    }

    ObjectElements* newheader = reinterpret_cast<ObjectElements*>(newHeaderSlots);
-    newheader->capacity = newCapacity;
-    elements_ = newheader->elements();
+    elements_ = newheader->elements() + numShifted;
+    getElementsHeader()->capacity = newCapacity;
 }

 /* static */ bool
--- a/js/src/vm/NativeObject.h
+++ b/js/src/vm/NativeObject.h
@ -157,11 +157,28 @@ ArraySetLength(JSContext* cx, Handle<ArrayObject*> obj, HandleId id,
 * Elements do not track property creation order, so enumerating the elements
 * of an object does not necessarily visit indexes in the order they were
 * created.
+ *
+ * Shifted elements
+ * ----------------
+ * It's pretty common to use an array as a queue, like this:
+ *
+ *    while (arr.length > 0)
+ *        foo(arr.shift());
+ *
+ * To ensure we don't get quadratic behavior on this, elements can be 'shifted'
+ * in memory. tryShiftDenseElements does this by incrementing elements_ to point
+ * to the next element and moving the ObjectElements header in memory (so it's
+ * stored where the shifted Value used to be).
+ *
+ * Shifted elements can be unshifted when we grow the array, when the array is
+ * frozen (for simplicity, shifted elements are not supported on objects that
+ * are frozen, have copy-on-write elements, or on arrays with non-writable
+ * length).
 */
 class ObjectElements
 {
  public:
-    enum Flags: uint32_t {
+    enum Flags: uint16_t {
        // Integers written to these elements must be converted to doubles.
        CONVERT_DOUBLE_ELEMENTS     = 0x1,

@ -187,6 +204,15 @@ class ObjectElements
        FROZEN                      = 0x10,
    };

+    // The flags word stores both the flags and the number of shifted elements.
+    // Allow shifting 2047 elements before unshifting.
+    static const size_t NumShiftedElementsBits = 11;
+    static const size_t MaxShiftedElements = (1 << NumShiftedElementsBits) - 1;
+    static const size_t NumShiftedElementsShift = 32 - NumShiftedElementsBits;
+    static const size_t FlagsMask = (1 << NumShiftedElementsShift) - 1;
+    static_assert(MaxShiftedElements == 2047,
+                  "MaxShiftedElements should match the comment");
+
  private:
    friend class ::JSObject;
    friend class ArrayObject;
@ -199,7 +225,9 @@ class ObjectElements
    ArraySetLength(JSContext* cx, Handle<ArrayObject*> obj, HandleId id,
                   unsigned attrs, HandleValue value, ObjectOpResult& result);

-    /* See Flags enum above. */
+    // The NumShiftedElementsBits high bits of this are used to store the
+    // number of shifted elements, the other bits are available for the flags.
+    // See Flags enum above.
    uint32_t flags;

    /*
@ -242,6 +270,21 @@ class ObjectElements
        flags &= ~COPY_ON_WRITE;
    }

+    void addShiftedElements(uint32_t count) {
+        MOZ_ASSERT(count < capacity);
+        MOZ_ASSERT(count < initializedLength);
+        MOZ_ASSERT(!(flags & (NONWRITABLE_ARRAY_LENGTH | FROZEN | COPY_ON_WRITE)));
+        uint32_t numShifted = numShiftedElements() + count;
+        MOZ_ASSERT(numShifted <= MaxShiftedElements);
+        flags = (numShifted << NumShiftedElementsShift) | (flags & FlagsMask);
+        capacity -= count;
+        initializedLength -= count;
+    }
+    void clearShiftedElements() {
+        flags &= FlagsMask;
+        MOZ_ASSERT(numShiftedElements() == 0);
+    }
+
  public:
    constexpr ObjectElements(uint32_t capacity, uint32_t length)
      : flags(0), initializedLength(0), capacity(capacity), length(length)
@ -261,7 +304,7 @@ class ObjectElements
    const HeapSlot* elements() const {
        return reinterpret_cast<const HeapSlot*>(uintptr_t(this) + sizeof(ObjectElements));
    }
-    static ObjectElements * fromElements(HeapSlot* elems) {
+    static ObjectElements* fromElements(HeapSlot* elems) {
        return reinterpret_cast<ObjectElements*>(uintptr_t(elems) - sizeof(ObjectElements));
    }

@ -311,6 +354,17 @@ class ObjectElements
        return JSPROP_ENUMERATE;
    }

+    uint32_t numShiftedElements() const {
+        uint32_t numShifted = flags >> NumShiftedElementsShift;
+        MOZ_ASSERT_IF(numShifted > 0,
+                      !(flags & (NONWRITABLE_ARRAY_LENGTH | FROZEN | COPY_ON_WRITE)));
+        return numShifted;
+    }
+
+    uint32_t numAllocatedElements() const {
+        return VALUES_PER_HEADER + capacity + numShiftedElements();
+    }
+
    // This is enough slots to store an object of this class. See the static
    // assertion below.
    static const size_t VALUES_PER_HEADER = 2;
@ -985,10 +1039,26 @@ class NativeObject : public ShapedObject
                      "uint32_t (and sometimes int32_t ,too)");
    }

-    ObjectElements * getElementsHeader() const {
+    ObjectElements* getElementsHeader() const {
        return ObjectElements::fromElements(elements_);
    }

+    // Returns a pointer to the first element, including shifted elements.
+    inline HeapSlot* unshiftedElements() const {
+        return elements_ - getElementsHeader()->numShiftedElements();
+    }
+
+    // Like getElementsHeader, but returns a pointer to the unshifted header.
+    // This is mainly useful for free()ing dynamic elements: the pointer
+    // returned here is the one we got from malloc.
+    void* getUnshiftedElementsHeader() const {
+        return ObjectElements::fromElements(unshiftedElements());
+    }
+
+    uint32_t unshiftedIndex(uint32_t index) const {
+        return index + getElementsHeader()->numShiftedElements();
+    }
+
    /* Accessors for elements. */
    bool ensureElements(JSContext* cx, uint32_t capacity) {
        MOZ_ASSERT(!denseElementsAreCopyOnWrite());
@ -998,6 +1068,15 @@ class NativeObject : public ShapedObject
        return true;
    }

+    // Try to shift |count| dense elements, see the "Shifted elements" comment.
+    inline bool tryShiftDenseElements(uint32_t count);
+
+    // Unshift all shifted elements so that numShiftedElements is 0.
+    void unshiftElements();
+
+    // If this object has many shifted elements, unshift them.
+    void maybeUnshiftElements();
+
    static bool goodElementsAllocationAmount(JSContext* cx, uint32_t reqAllocated,
                                             uint32_t length, uint32_t* goodAmount);
    bool growElements(JSContext* cx, uint32_t newcap);
@ -1039,7 +1118,7 @@ class NativeObject : public ShapedObject
        MOZ_ASSERT(index < getDenseInitializedLength());
        MOZ_ASSERT(!denseElementsAreCopyOnWrite());
        checkStoredValue(val);
-        elements_[index].set(this, HeapSlot::Element, index, val);
+        elements_[index].set(this, HeapSlot::Element, unshiftedIndex(index), val);
    }

  public:
@ -1061,7 +1140,7 @@ class NativeObject : public ShapedObject
        MOZ_ASSERT(!denseElementsAreCopyOnWrite());
        MOZ_ASSERT(!denseElementsAreFrozen());
        checkStoredValue(val);
-        elements_[index].init(this, HeapSlot::Element, index, val);
+        elements_[index].init(this, HeapSlot::Element, unshiftedIndex(index), val);
    }

    void setDenseElementMaybeConvertDouble(uint32_t index, const Value& val) {
@ -1156,9 +1235,9 @@ class NativeObject : public ShapedObject
    bool canHaveNonEmptyElements();
 #endif

-    void setFixedElements() {
+    void setFixedElements(uint32_t numShifted = 0) {
        MOZ_ASSERT(canHaveNonEmptyElements());
-        elements_ = fixedElements();
+        elements_ = fixedElements() + numShifted;
    }

    inline bool hasDynamicElements() const {
@ -1169,11 +1248,11 @@ class NativeObject : public ShapedObject
         * immediately afterwards. Such cases cannot occur for dense arrays
         * (which have at least two fixed slots) and can only result in a leak.
         */
-        return !hasEmptyElements() && elements_ != fixedElements();
+        return !hasEmptyElements() && !hasFixedElements();
    }

    inline bool hasFixedElements() const {
-        return elements_ == fixedElements();
+        return unshiftedElements() == fixedElements();
    }

    inline bool hasEmptyElements() const {