gecko-dev/js/src/jsarray.cpp

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set sw=4 ts=8 et tw=78:
 *
 * ***** BEGIN LICENSE BLOCK *****
 * Version: MPL 1.1/GPL 2.0/LGPL 2.1
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 * http://www.mozilla.org/MPL/
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is Mozilla Communicator client code, released
 * March 31, 1998.
 *
 * The Initial Developer of the Original Code is
 * Netscape Communications Corporation.
 * Portions created by the Initial Developer are Copyright (C) 1998
 * the Initial Developer. All Rights Reserved.
 *
 * Contributor(s):
 *
 * Alternatively, the contents of this file may be used under the terms of
 * either of the GNU General Public License Version 2 or later (the "GPL"),
 * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
 * in which case the provisions of the GPL or the LGPL are applicable instead
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 * under the terms of either the GPL or the LGPL, and not to allow others to
 * use your version of this file under the terms of the MPL, indicate your
 * decision by deleting the provisions above and replace them with the notice
 * and other provisions required by the GPL or the LGPL. If you do not delete
 * the provisions above, a recipient may use your version of this file under
 * the terms of any one of the MPL, the GPL or the LGPL.
 *
 * ***** END LICENSE BLOCK ***** */

/*
 * JS array class.
 *
 * Array objects begin as "dense" arrays, optimized for index-only property
 * access over a vector of slots with high load factor.  Array methods
 * optimize for denseness by testing that the object's class is
 * &js_ArrayClass, and can then directly manipulate the slots for efficiency.
 *
 * We track these pieces of metadata for arrays in dense mode:
 *  - The array's length property as a uint32, accessible with
 *    getArrayLength(), setArrayLength().
 *  - The number of element slots (capacity), gettable with
 *    getDenseArrayCapacity().
 *
 * In dense mode, holes in the array are represented by
 * MagicValue(JS_ARRAY_HOLE) invalid values.
 *
 * NB: the capacity and length of a dense array are entirely unrelated!  The
 * length may be greater than, less than, or equal to the capacity. The first
 * case may occur when the user writes "new Array(100), in which case the
 * length is 100 while the capacity remains 0 (indices below length and above
 * capaicty must be treated as holes). See array_length_setter for another
 * explanation of how the first case may occur.
 *
 * Arrays are converted to use js_SlowArrayClass when any of these conditions
 * are met:
 *  - there are more than MIN_SPARSE_INDEX slots total
 *  - the load factor (COUNT / capacity) is less than 0.25
 *  - a property is set that is not indexed (and not "length")
 *  - a property is defined that has non-default property attributes.
 *
 * Dense arrays do not track property creation order, so unlike other native
 * objects and slow arrays, enumerating an array does not necessarily visit the
 * properties in the order they were created.  We could instead maintain the
 * scope to track property enumeration order, but still use the fast slot
 * access.  That would have the same memory cost as just using a
 * js_SlowArrayClass, but have the same performance characteristics as a dense
 * array for slot accesses, at some cost in code complexity.
 */
#include <stdlib.h>
#include <string.h>
#include "jstypes.h"
#include "jsstdint.h"
#include "jsutil.h"
#include "jsapi.h"
#include "jsarray.h"
#include "jsatom.h"
#include "jsbit.h"
#include "jsbool.h"
#include "jstracer.h"
#include "jsbuiltins.h"
#include "jscntxt.h"
#include "jsversion.h"
#include "jsdbgapi.h" /* for js_TraceWatchPoints */
#include "jsfun.h"
#include "jsgc.h"
#include "jsinterp.h"
#include "jsiter.h"
#include "jslock.h"
#include "jsnum.h"
#include "jsobj.h"
#include "jsscope.h"
#include "jsstr.h"
#include "jsstaticcheck.h"
#include "jsvector.h"
#include "jswrapper.h"

#include "jsatominlines.h"
#include "jscntxtinlines.h"
#include "jsinterpinlines.h"
#include "jsobjinlines.h"

using namespace js;
using namespace js::gc;

/* 2^32 - 1 as a number and a string */
#define MAXINDEX 4294967295u
#define MAXSTR   "4294967295"

static inline bool
ENSURE_SLOW_ARRAY(JSContext *cx, JSObject *obj)
{
    return obj->getClass() == &js_SlowArrayClass ||
           obj->makeDenseArraySlow(cx);
}

/*
 * Determine if the id represents an array index or an XML property index.
 *
 * An id is an array index according to ECMA by (15.4):
 *
 * "Array objects give special treatment to a certain class of property names.
 * A property name P (in the form of a string value) is an array index if and
 * only if ToString(ToUint32(P)) is equal to P and ToUint32(P) is not equal
 * to 2^32-1."
 *
 * In our implementation, it would be sufficient to check for JSVAL_IS_INT(id)
 * except that by using signed 31-bit integers we miss the top half of the
 * valid range. This function checks the string representation itself; note
 * that calling a standard conversion routine might allow strings such as
 * "08" or "4.0" as array indices, which they are not.
 *
 * 'id' is passed as a jsboxedword since the given id need not necessarily hold
 * an atomized string.
 */
bool
js_StringIsIndex(JSLinearString *str, jsuint *indexp)
{
    const jschar *cp = str->chars();
    if (JS7_ISDEC(*cp) && str->length() < sizeof(MAXSTR)) {
        jsuint index = JS7_UNDEC(*cp++);
        jsuint oldIndex = 0;
        jsuint c = 0;
        if (index != 0) {
            while (JS7_ISDEC(*cp)) {
                oldIndex = index;
                c = JS7_UNDEC(*cp);
                index = 10*index + c;
                cp++;
            }
        }

        /* Ensure that all characters were consumed and we didn't overflow. */
        if (*cp == 0 &&
             (oldIndex < (MAXINDEX / 10) ||
              (oldIndex == (MAXINDEX / 10) && c < (MAXINDEX % 10))))
        {
            *indexp = index;
            return true;
        }
    }
    return false;
}

static bool 
ValueToLength(JSContext *cx, Value* vp, jsuint* plength)
{
    if (vp->isInt32()) {
        int32_t i = vp->toInt32();
        if (i < 0)
            goto error;

        *plength = (jsuint)(i);
        return true;
    }

    jsdouble d;
    if (!ValueToNumber(cx, *vp, &d))
        goto error;

    if (JSDOUBLE_IS_NaN(d))
        goto error;

    jsuint length;
    length = (jsuint) d;
    if (d != (jsdouble) length)
        goto error;


    *plength = length;
    return true;

error:
    JS_ReportErrorNumber(cx, js_GetErrorMessage, NULL,
                         JSMSG_BAD_ARRAY_LENGTH);
    return false;
}

JSBool
js_GetLengthProperty(JSContext *cx, JSObject *obj, jsuint *lengthp)
{
    if (obj->isArray()) {
        *lengthp = obj->getArrayLength();
        return true;
    }

    if (obj->isArguments() && !obj->isArgsLengthOverridden()) {
        *lengthp = obj->getArgsInitialLength();
        return true;
    }

    AutoValueRooter tvr(cx);
    if (!obj->getProperty(cx, ATOM_TO_JSID(cx->runtime->atomState.lengthAtom), tvr.addr()))
        return false;

    if (tvr.value().isInt32()) {
        *lengthp = jsuint(jsint(tvr.value().toInt32())); /* jsuint cast does ToUint32 */
        return true;
    }

    JS_STATIC_ASSERT(sizeof(jsuint) == sizeof(uint32_t));
    return ValueToECMAUint32(cx, tvr.value(), (uint32_t *)lengthp);
}

JSBool JS_FASTCALL
js_IndexToId(JSContext *cx, jsuint index, jsid *idp)
{
    JSString *str;

    if (index <= JSID_INT_MAX) {
        *idp = INT_TO_JSID(index);
        return JS_TRUE;
    }
    str = js_NumberToString(cx, index);
    if (!str)
        return JS_FALSE;
    return js_ValueToStringId(cx, StringValue(str), idp);
}

static JSBool
BigIndexToId(JSContext *cx, JSObject *obj, jsuint index, JSBool createAtom,
             jsid *idp)
{
    jschar buf[10], *start;
    Class *clasp;
    JSAtom *atom;
    JS_STATIC_ASSERT((jsuint)-1 == 4294967295U);

    JS_ASSERT(index > JSID_INT_MAX);

    start = JS_ARRAY_END(buf);
    do {
        --start;
        *start = (jschar)('0' + index % 10);
        index /= 10;
    } while (index != 0);

    /*
     * Skip the atomization if the class is known to store atoms corresponding
     * to big indexes together with elements. In such case we know that the
     * array does not have an element at the given index if its atom does not
     * exist.  Fast arrays (clasp == &js_ArrayClass) don't use atoms for
     * any indexes, though it would be rare to see them have a big index
     * in any case.
     */
    if (!createAtom &&
        ((clasp = obj->getClass()) == &js_SlowArrayClass ||
         clasp == &js_ArgumentsClass ||
         clasp == &js_ObjectClass)) {
        atom = js_GetExistingStringAtom(cx, start, JS_ARRAY_END(buf) - start);
        if (!atom) {
            *idp = JSID_VOID;
            return JS_TRUE;
        }
    } else {
        atom = js_AtomizeChars(cx, start, JS_ARRAY_END(buf) - start, 0);
        if (!atom)
            return JS_FALSE;
    }

    *idp = ATOM_TO_JSID(atom);
    return JS_TRUE;
}

bool
JSObject::willBeSparseDenseArray(uintN requiredCapacity, uintN newElementsHint)
{
    JS_ASSERT(isDenseArray());
    JS_ASSERT(requiredCapacity > MIN_SPARSE_INDEX);

    uintN cap = numSlots();
    JS_ASSERT(requiredCapacity >= cap);

    if (requiredCapacity >= JSObject::NSLOTS_LIMIT)
        return true;
    
    uintN minimalDenseCount = requiredCapacity / 4;
    if (newElementsHint >= minimalDenseCount)
        return false;
    minimalDenseCount -= newElementsHint;

    if (minimalDenseCount > cap)
        return true;
    
    Value *elems = getDenseArrayElements();
    for (uintN i = 0; i < cap; i++) {
        if (!elems[i].isMagic(JS_ARRAY_HOLE) && !--minimalDenseCount)
            return false;
    }
    return true;
}

static bool
ReallyBigIndexToId(JSContext* cx, jsdouble index, jsid* idp)
{
    return js_ValueToStringId(cx, DoubleValue(index), idp);
}

static bool
IndexToId(JSContext* cx, JSObject* obj, jsdouble index, JSBool* hole, jsid* idp,
          JSBool createAtom = JS_FALSE)
{
    if (index <= JSID_INT_MAX) {
        *idp = INT_TO_JSID(int(index));
        return JS_TRUE;
    }

    if (index <= jsuint(-1)) {
        if (!BigIndexToId(cx, obj, jsuint(index), createAtom, idp))
            return JS_FALSE;
        if (hole && JSID_IS_VOID(*idp))
            *hole = JS_TRUE;
        return JS_TRUE;
    }

    return ReallyBigIndexToId(cx, index, idp);
}

/*
 * If the property at the given index exists, get its value into location
 * pointed by vp and set *hole to false. Otherwise set *hole to true and *vp
 * to JSVAL_VOID. This function assumes that the location pointed by vp is
 * properly rooted and can be used as GC-protected storage for temporaries.
 */
static JSBool
GetElement(JSContext *cx, JSObject *obj, jsdouble index, JSBool *hole, Value *vp)
{
    JS_ASSERT(index >= 0);
    if (obj->isDenseArray() && index < obj->getDenseArrayCapacity() &&
        !(*vp = obj->getDenseArrayElement(uint32(index))).isMagic(JS_ARRAY_HOLE)) {
        *hole = JS_FALSE;
        return JS_TRUE;
    }
    if (obj->isArguments() &&
        index < obj->getArgsInitialLength() &&
        !(*vp = obj->getArgsElement(uint32(index))).isMagic(JS_ARGS_HOLE)) {
        *hole = JS_FALSE;
        JSStackFrame *fp = (JSStackFrame *)obj->getPrivate();
        if (fp != JS_ARGUMENTS_OBJECT_ON_TRACE) {
            if (fp)
                *vp = fp->canonicalActualArg(index);
            return JS_TRUE;
        }
    }

    AutoIdRooter idr(cx);

    *hole = JS_FALSE;
    if (!IndexToId(cx, obj, index, hole, idr.addr()))
        return JS_FALSE;
    if (*hole) {
        vp->setUndefined();
        return JS_TRUE;
    }

    JSObject *obj2;
    JSProperty *prop;
    if (!obj->lookupProperty(cx, idr.id(), &obj2, &prop))
        return JS_FALSE;
    if (!prop) {
        *hole = JS_TRUE;
        vp->setUndefined();
    } else {
        if (!obj->getProperty(cx, idr.id(), vp))
            return JS_FALSE;
        *hole = JS_FALSE;
    }
    return JS_TRUE;
}

namespace js {

bool
GetElements(JSContext *cx, JSObject *aobj, jsuint length, Value *vp)
{
    if (aobj->isDenseArray() && length <= aobj->getDenseArrayCapacity() &&
        !js_PrototypeHasIndexedProperties(cx, aobj)) {
        /* The prototype does not have indexed properties so hole = undefined */
        Value *srcbeg = aobj->getDenseArrayElements();
        Value *srcend = srcbeg + length;
        for (Value *dst = vp, *src = srcbeg; src < srcend; ++dst, ++src)
            *dst = src->isMagic(JS_ARRAY_HOLE) ? UndefinedValue() : *src;
    } else if (aobj->isArguments() && !aobj->isArgsLengthOverridden() &&
               !js_PrototypeHasIndexedProperties(cx, aobj)) {
        /*
         * Two cases, two loops: note how in the case of an active stack frame
         * backing aobj, even though we copy from fp->argv, we still must check
         * aobj->getArgsElement(i) for a hole, to handle a delete on the
         * corresponding arguments element. See args_delProperty.
         */
        if (JSStackFrame *fp = (JSStackFrame *) aobj->getPrivate()) {
            JS_ASSERT(fp->numActualArgs() <= JS_ARGS_LENGTH_MAX);
            fp->forEachCanonicalActualArg(CopyNonHoleArgsTo(aobj, vp));
        } else {
            Value *srcbeg = aobj->getArgsElements();
            Value *srcend = srcbeg + length;
            for (Value *dst = vp, *src = srcbeg; src < srcend; ++dst, ++src)
                *dst = src->isMagic(JS_ARGS_HOLE) ? UndefinedValue() : *src;
        }
    } else {
        for (uintN i = 0; i < length; i++) {
            if (!aobj->getProperty(cx, INT_TO_JSID(jsint(i)), &vp[i]))
                return JS_FALSE;
        }
    }

    return true;
}

}

/*
 * Set the value of the property at the given index to v assuming v is rooted.
 */
static JSBool
SetArrayElement(JSContext *cx, JSObject *obj, jsdouble index, const Value &v)
{
    JS_ASSERT(index >= 0);

    if (obj->isDenseArray()) {
        /* Predicted/prefetched code should favor the remains-dense case. */
        JSObject::EnsureDenseResult result = JSObject::ED_SPARSE;
        do {
            if (index > jsuint(-1))
                break;
            jsuint idx = jsuint(index);
            result = obj->ensureDenseArrayElements(cx, idx, 1);
            if (result != JSObject::ED_OK)
                break;
            if (idx >= obj->getArrayLength())
                obj->setArrayLength(idx + 1);
            obj->setDenseArrayElement(idx, v);
            return true;
        } while (false);

        if (result == JSObject::ED_FAILED)
            return false;
        JS_ASSERT(result == JSObject::ED_SPARSE);
        if (!obj->makeDenseArraySlow(cx))
            return JS_FALSE;
    }

    AutoIdRooter idr(cx);

    if (!IndexToId(cx, obj, index, NULL, idr.addr(), JS_TRUE))
        return JS_FALSE;
    JS_ASSERT(!JSID_IS_VOID(idr.id()));

    Value tmp = v;
    return obj->setProperty(cx, idr.id(), &tmp, true);
}

#ifdef JS_TRACER
JSBool JS_FASTCALL
js_EnsureDenseArrayCapacity(JSContext *cx, JSObject *obj, jsint i)
{
#ifdef DEBUG
    Class *origObjClasp = obj->clasp; 
#endif
    jsuint u = jsuint(i);
    JSBool ret = (obj->ensureDenseArrayElements(cx, u, 1) == JSObject::ED_OK);

    /* Partially check the CallInfo's storeAccSet is correct. */
    JS_ASSERT(obj->clasp == origObjClasp);
    return ret;
}
/* This function and its callees do not touch any object's .clasp field. */
JS_DEFINE_CALLINFO_3(extern, BOOL, js_EnsureDenseArrayCapacity, CONTEXT, OBJECT, INT32,
                     0, nanojit::ACCSET_STORE_ANY & ~tjit::ACCSET_OBJ_CLASP)
#endif

static JSBool
DeleteArrayElement(JSContext *cx, JSObject *obj, jsdouble index, JSBool strict)
{
    JS_ASSERT(index >= 0);
    if (obj->isDenseArray()) {
        if (index <= jsuint(-1)) {
            jsuint idx = jsuint(index);
            if (idx < obj->getDenseArrayCapacity()) {
                obj->setDenseArrayElement(idx, MagicValue(JS_ARRAY_HOLE));
                return js_SuppressDeletedIndexProperties(cx, obj, idx, idx+1);
            }
        }
        return JS_TRUE;
    }

    AutoIdRooter idr(cx);

    if (!IndexToId(cx, obj, index, NULL, idr.addr()))
        return JS_FALSE;
    if (JSID_IS_VOID(idr.id()))
        return JS_TRUE;

    Value junk;
    return obj->deleteProperty(cx, idr.id(), &junk, strict);
}

/*
 * When hole is true, delete the property at the given index. Otherwise set
 * its value to v assuming v is rooted.
 */
static JSBool
SetOrDeleteArrayElement(JSContext *cx, JSObject *obj, jsdouble index,
                        JSBool hole, const Value &v)
{
    if (hole) {
        JS_ASSERT(v.isUndefined());
        return DeleteArrayElement(cx, obj, index, true);
    }
    return SetArrayElement(cx, obj, index, v);
}

JSBool
js_SetLengthProperty(JSContext *cx, JSObject *obj, jsdouble length)
{
    Value v;
    jsid id;

    v.setNumber(length);
    id = ATOM_TO_JSID(cx->runtime->atomState.lengthAtom);
    /* We don't support read-only array length yet. */
    return obj->setProperty(cx, id, &v, false);
}

JSBool
js_HasLengthProperty(JSContext *cx, JSObject *obj, jsuint *lengthp)
{
    JSErrorReporter older = JS_SetErrorReporter(cx, NULL);
    AutoValueRooter tvr(cx);
    jsid id = ATOM_TO_JSID(cx->runtime->atomState.lengthAtom);
    JSBool ok = obj->getProperty(cx, id, tvr.addr());
    JS_SetErrorReporter(cx, older);
    if (!ok)
        return false;

    if (!ValueToLength(cx, tvr.addr(), lengthp))
        return false;

    return true;
}

/*
 * Since SpiderMonkey supports cross-class prototype-based delegation, we have
 * to be careful about the length getter and setter being called on an object
 * not of Array class. For the getter, we search obj's prototype chain for the
 * array that caused this getter to be invoked. In the setter case to overcome
 * the JSPROP_SHARED attribute, we must define a shadowing length property.
 */
static JSBool
array_length_getter(JSContext *cx, JSObject *obj, jsid id, Value *vp)
{
    do {
        if (obj->isArray()) {
            vp->setNumber(obj->getArrayLength());
            return JS_TRUE;
        }
    } while ((obj = obj->getProto()) != NULL);
    return JS_TRUE;
}

static JSBool
array_length_setter(JSContext *cx, JSObject *obj, jsid id, Value *vp)
{
    jsuint newlen, oldlen, gap, index;
    Value junk;

    if (!obj->isArray()) {
        jsid lengthId = ATOM_TO_JSID(cx->runtime->atomState.lengthAtom);

        return obj->defineProperty(cx, lengthId, *vp, NULL, NULL, JSPROP_ENUMERATE);
    }

    if (!ValueToLength(cx, vp, &newlen))
        return false;

    oldlen = obj->getArrayLength();

    if (oldlen == newlen)
        return true;

    vp->setNumber(newlen);
    if (oldlen < newlen) {
        obj->setArrayLength(newlen);
        return true;
    }

    if (obj->isDenseArray()) {
        /*
         * Don't reallocate if we're not actually shrinking our slots. If we do
         * shrink slots here, ensureDenseArrayElements will fill all slots to the
         * right of newlen with JS_ARRAY_HOLE. This permits us to disregard
         * length when reading from arrays as long we are within the capacity.
         */
        jsuint oldcap = obj->getDenseArrayCapacity();
        if (oldcap > newlen)
            obj->shrinkDenseArrayElements(cx, newlen);
        obj->setArrayLength(newlen);
    } else if (oldlen - newlen < (1 << 24)) {
        do {
            --oldlen;
            if (!JS_CHECK_OPERATION_LIMIT(cx)) {
                obj->setArrayLength(oldlen + 1);
                return false;
            }
            if (!DeleteArrayElement(cx, obj, oldlen, true)) {
                obj->setArrayLength(oldlen + 1);
                JS_ClearPendingException(cx);
                return true;
            }
        } while (oldlen != newlen);
        obj->setArrayLength(newlen);
    } else {
        /*
         * We are going to remove a lot of indexes in a presumably sparse
         * array. So instead of looping through indexes between newlen and
         * oldlen, we iterate through all properties and remove those that
         * correspond to indexes in the half-open range [newlen, oldlen).  See
         * bug 322135.
         */
        JSObject *iter = JS_NewPropertyIterator(cx, obj);
        if (!iter)
            return false;

        /* Protect iter against GC under JSObject::deleteProperty. */
        AutoObjectRooter tvr(cx, iter);

        gap = oldlen - newlen;
        for (;;) {
            if (!JS_CHECK_OPERATION_LIMIT(cx) || !JS_NextProperty(cx, iter, &id))
                return false;
            if (JSID_IS_VOID(id))
                break;
            if (js_IdIsIndex(id, &index) && index - newlen < gap &&
                !obj->deleteProperty(cx, id, &junk, false)) {
                return false;
            }
        }
        obj->setArrayLength(newlen);
    }

    return true;
}

/*
 * We have only indexed properties up to capacity (excepting holes), plus the
 * length property. For all else, we delegate to the prototype.
 */
static inline bool
IsDenseArrayId(JSContext *cx, JSObject *obj, jsid id)
{
    JS_ASSERT(obj->isDenseArray());

    uint32 i;
    return JSID_IS_ATOM(id, cx->runtime->atomState.lengthAtom) ||
           (js_IdIsIndex(id, &i) &&
            obj->getArrayLength() != 0 &&
            i < obj->getDenseArrayCapacity() &&
            !obj->getDenseArrayElement(i).isMagic(JS_ARRAY_HOLE));
}

static JSBool
array_lookupProperty(JSContext *cx, JSObject *obj, jsid id, JSObject **objp,
                     JSProperty **propp)
{
    if (!obj->isDenseArray())
        return js_LookupProperty(cx, obj, id, objp, propp);

    if (IsDenseArrayId(cx, obj, id)) {
        *propp = (JSProperty *) 1;  /* non-null to indicate found */
        *objp = obj;
        return JS_TRUE;
    }

    JSObject *proto = obj->getProto();
    if (!proto) {
        *objp = NULL;
        *propp = NULL;
        return JS_TRUE;
    }
    return proto->lookupProperty(cx, id, objp, propp);
}

JSBool
js_GetDenseArrayElementValue(JSContext *cx, JSObject *obj, jsid id, Value *vp)
{
    JS_ASSERT(obj->isDenseArray());

    uint32 i;
    if (!js_IdIsIndex(id, &i)) {
        JS_ASSERT(JSID_IS_ATOM(id, cx->runtime->atomState.lengthAtom));
        vp->setNumber(obj->getArrayLength());
        return JS_TRUE;
    }
    *vp = obj->getDenseArrayElement(i);
    return JS_TRUE;
}

static JSBool
array_getProperty(JSContext *cx, JSObject *obj, JSObject *receiver, jsid id, Value *vp)
{
    uint32 i;

    if (JSID_IS_ATOM(id, cx->runtime->atomState.lengthAtom)) {
        vp->setNumber(obj->getArrayLength());
        return JS_TRUE;
    }

    if (JSID_IS_ATOM(id, cx->runtime->atomState.protoAtom)) {
        vp->setObjectOrNull(obj->getProto());
        return JS_TRUE;
    }

    if (!obj->isDenseArray())
        return js_GetProperty(cx, obj, id, vp);

    if (!js_IdIsIndex(id, &i) || i >= obj->getDenseArrayCapacity() ||
        obj->getDenseArrayElement(i).isMagic(JS_ARRAY_HOLE)) {
        JSObject *obj2;
        JSProperty *prop;
        const Shape *shape;

        JSObject *proto = obj->getProto();
        if (!proto) {
            vp->setUndefined();
            return JS_TRUE;
        }

        vp->setUndefined();
        if (js_LookupPropertyWithFlags(cx, proto, id, cx->resolveFlags,
                                       &obj2, &prop) < 0)
            return JS_FALSE;

        if (prop && obj2->isNative()) {
            shape = (const Shape *) prop;
            if (!js_NativeGet(cx, obj, obj2, shape, JSGET_METHOD_BARRIER, vp))
                return JS_FALSE;
        }
        return JS_TRUE;
    }

    *vp = obj->getDenseArrayElement(i);
    return JS_TRUE;
}

static JSBool
slowarray_addProperty(JSContext *cx, JSObject *obj, jsid id, Value *vp)
{
    jsuint index, length;

    if (!js_IdIsIndex(id, &index))
        return JS_TRUE;
    length = obj->getArrayLength();
    if (index >= length)
        obj->setArrayLength(index + 1);
    return JS_TRUE;
}

static JSType
array_typeOf(JSContext *cx, JSObject *obj)
{
    return JSTYPE_OBJECT;
}

static JSBool
array_setProperty(JSContext *cx, JSObject *obj, jsid id, Value *vp, JSBool strict)
{
    uint32 i;

    if (JSID_IS_ATOM(id, cx->runtime->atomState.lengthAtom))
        return array_length_setter(cx, obj, id, vp);

    if (!obj->isDenseArray())
        return js_SetProperty(cx, obj, id, vp, strict);

    do {
        if (!js_IdIsIndex(id, &i))
            break;
        if (js_PrototypeHasIndexedProperties(cx, obj))
            break;

        JSObject::EnsureDenseResult result = obj->ensureDenseArrayElements(cx, i, 1);
        if (result != JSObject::ED_OK) {
            if (result == JSObject::ED_FAILED)
                return false;
            JS_ASSERT(result == JSObject::ED_SPARSE);
            break;
        }

        if (i >= obj->getArrayLength())
            obj->setArrayLength(i + 1);
        obj->setDenseArrayElement(i, *vp);
        return true;
    } while (false);

    if (!obj->makeDenseArraySlow(cx))
        return false;
    return js_SetProperty(cx, obj, id, vp, strict);
}

JSBool
js_PrototypeHasIndexedProperties(JSContext *cx, JSObject *obj)
{
    /*
     * Walk up the prototype chain and see if this indexed element already
     * exists. If we hit the end of the prototype chain, it's safe to set the
     * element on the original object.
     */
    while ((obj = obj->getProto()) != NULL) {
        /*
         * If the prototype is a non-native object (possibly a dense array), or
         * a native object (possibly a slow array) that has indexed properties,
         * return true.
         */
        if (!obj->isNative())
            return JS_TRUE;
        if (obj->isIndexed())
            return JS_TRUE;
    }
    return JS_FALSE;
}

static JSBool
array_defineProperty(JSContext *cx, JSObject *obj, jsid id, const Value *value,
                     PropertyOp getter, PropertyOp setter, uintN attrs)
{
    if (JSID_IS_ATOM(id, cx->runtime->atomState.lengthAtom))
        return JS_TRUE;

    if (!obj->isDenseArray())
        return js_DefineProperty(cx, obj, id, value, getter, setter, attrs);

    do {
        uint32 i = 0;       // init to shut GCC up
        bool isIndex = js_IdIsIndex(id, &i);
        if (!isIndex || attrs != JSPROP_ENUMERATE)
            break;

        JSObject::EnsureDenseResult result = obj->ensureDenseArrayElements(cx, i, 1);
        if (result != JSObject::ED_OK) {
            if (result == JSObject::ED_FAILED)
                return false;
            JS_ASSERT(result == JSObject::ED_SPARSE);
            break;
        }

        if (i >= obj->getArrayLength())
            obj->setArrayLength(i + 1);
        obj->setDenseArrayElement(i, *value);
        return true;
    } while (false);

    if (!obj->makeDenseArraySlow(cx))
        return false;
    return js_DefineProperty(cx, obj, id, value, getter, setter, attrs);
}

static JSBool
array_getAttributes(JSContext *cx, JSObject *obj, jsid id, uintN *attrsp)
{
    *attrsp = JSID_IS_ATOM(id, cx->runtime->atomState.lengthAtom)
        ? JSPROP_PERMANENT : JSPROP_ENUMERATE;
    return JS_TRUE;
}

static JSBool
array_setAttributes(JSContext *cx, JSObject *obj, jsid id, uintN *attrsp)
{
    JS_ReportErrorNumber(cx, js_GetErrorMessage, NULL,
                         JSMSG_CANT_SET_ARRAY_ATTRS);
    return JS_FALSE;
}

static JSBool
array_deleteProperty(JSContext *cx, JSObject *obj, jsid id, Value *rval, JSBool strict)
{
    uint32 i;

    if (!obj->isDenseArray())
        return js_DeleteProperty(cx, obj, id, rval, strict);

    if (JSID_IS_ATOM(id, cx->runtime->atomState.lengthAtom)) {
        rval->setBoolean(false);
        return JS_TRUE;
    }

    if (js_IdIsIndex(id, &i) && i < obj->getDenseArrayCapacity())
        obj->setDenseArrayElement(i, MagicValue(JS_ARRAY_HOLE));

    if (!js_SuppressDeletedProperty(cx, obj, id))
        return false;

    rval->setBoolean(true);
    return JS_TRUE;
}

static void
array_trace(JSTracer *trc, JSObject *obj)
{
    JS_ASSERT(obj->isDenseArray());

    uint32 capacity = obj->getDenseArrayCapacity();
    for (uint32 i = 0; i < capacity; i++)
        MarkValue(trc, obj->getDenseArrayElement(i), "dense_array_elems");
}

static JSBool
array_fix(JSContext *cx, JSObject *obj, bool *success, AutoIdVector *props)
{
    JS_ASSERT(obj->isDenseArray());

    /*
     * We must slowify dense arrays; otherwise, we'd need to detect assignments to holes,
     * since that is effectively adding a new property to the array.
     */
    if (!obj->makeDenseArraySlow(cx) ||
        !GetPropertyNames(cx, obj, JSITER_HIDDEN | JSITER_OWNONLY, props))
        return false;

    *success = true;
    return true;
}

Class js_ArrayClass = {
    "Array",
    Class::NON_NATIVE |
    JSCLASS_HAS_PRIVATE |
    JSCLASS_HAS_CACHED_PROTO(JSProto_Array),
    PropertyStub,   /* addProperty */
    PropertyStub,   /* delProperty */
    PropertyStub,   /* getProperty */
    PropertyStub,   /* setProperty */
    EnumerateStub,
    ResolveStub,
    js_TryValueOf,
    NULL,
    NULL,           /* reserved0   */
    NULL,           /* checkAccess */
    NULL,           /* call        */
    NULL,           /* construct   */
    NULL,           /* xdrObject   */
    NULL,           /* hasInstance */
    NULL,           /* mark        */
    JS_NULL_CLASS_EXT,
    {
        array_lookupProperty,
        array_defineProperty,
        array_getProperty,
        array_setProperty,
        array_getAttributes,
        array_setAttributes,
        array_deleteProperty,
        NULL,       /* enumerate      */
        array_typeOf,
        array_trace,
        array_fix,
        NULL,       /* thisObject     */
        NULL,       /* clear          */
    }
};

Class js_SlowArrayClass = {
    "Array",
    JSCLASS_HAS_PRIVATE |
    JSCLASS_HAS_CACHED_PROTO(JSProto_Array),
    slowarray_addProperty,
    PropertyStub,   /* delProperty */
    PropertyStub,   /* getProperty */
    PropertyStub,   /* setProperty */
    EnumerateStub,
    ResolveStub,
    js_TryValueOf
};

/*
 * Convert an array object from fast-and-dense to slow-and-flexible.
 */
JSBool
JSObject::makeDenseArraySlow(JSContext *cx)
{
    JS_ASSERT(isDenseArray());

    /*
     * Save old map now, before calling InitScopeForObject. We'll have to undo
     * on error. This is gross, but a better way is not obvious.
     */
    JSObjectMap *oldMap = map;

    /* Create a native scope. */
    JSObject *arrayProto = getProto();
    js::gc::FinalizeKind kind = js::gc::FinalizeKind(arena()->header()->thingKind);
    if (!InitScopeForObject(cx, this, &js_SlowArrayClass, arrayProto, kind))
        return false;

    uint32 capacity = getDenseArrayCapacity();

    /*
     * Begin with the length property to share more of the property tree.
     * The getter/setter here will directly access the object's private value.
     */
    if (!addProperty(cx, ATOM_TO_JSID(cx->runtime->atomState.lengthAtom),
                     array_length_getter, array_length_setter,
                     SHAPE_INVALID_SLOT, JSPROP_PERMANENT | JSPROP_SHARED, 0, 0)) {
        setMap(oldMap);
        return false;
    }

    /*
     * Create new properties pointing to existing elements. Pack the array to
     * remove holes, so that shapes use successive slots (as for other objects).
     */
    uint32 next = 0;
    for (uint32 i = 0; i < capacity; i++) {
        jsid id;
        if (!ValueToId(cx, Int32Value(i), &id)) {
            setMap(oldMap);
            return false;
        }

        if (getDenseArrayElement(i).isMagic(JS_ARRAY_HOLE))
            continue;

        setDenseArrayElement(next, getDenseArrayElement(i));

        if (!addDataProperty(cx, id, next, JSPROP_ENUMERATE)) {
            setMap(oldMap);
            return false;
        }

        next++;
    }

    /*
     * Dense arrays with different numbers of slots but the same number of fixed
     * slots and the same non-hole indexes must use their fixed slots consistently.
     */
    if (hasSlotsArray() && next <= numFixedSlots())
        revertToFixedSlots(cx);

    ClearValueRange(slots + next, this->capacity - next, false);

    /*
     * Finally, update class. If |this| is Array.prototype, then js_InitClass
     * will create an emptyShape whose class is &js_SlowArrayClass, to ensure
     * that delegating instances can share shapes in the tree rooted at the
     * proto's empty shape.
     */
    clasp = &js_SlowArrayClass;
    return true;
}

/* Transfer ownership of buffer to returned string. */
static inline JSBool
BufferToString(JSContext *cx, JSCharBuffer &cb, Value *rval)
{
    JSString *str = js_NewStringFromCharBuffer(cx, cb);
    if (!str)
        return false;
    rval->setString(str);
    return true;
}

#if JS_HAS_TOSOURCE
static JSBool
array_toSource(JSContext *cx, uintN argc, Value *vp)
{
    JS_CHECK_RECURSION(cx, return false);

    JSObject *obj = ComputeThisFromVp(cx, vp);
    if (!obj ||
        (obj->getClass() != &js_SlowArrayClass &&
         !InstanceOf(cx, obj, &js_ArrayClass, vp + 2))) {
        return false;
    }

    /* Find joins or cycles in the reachable object graph. */
    jschar *sharpchars;
    JSHashEntry *he = js_EnterSharpObject(cx, obj, NULL, &sharpchars);
    if (!he)
        return false;
    bool initiallySharp = IS_SHARP(he);

    /* After this point, all paths exit through the 'out' label. */
    MUST_FLOW_THROUGH("out");
    bool ok = false;

    /*
     * This object will take responsibility for the jschar buffer until the
     * buffer is transferred to the returned JSString.
     */
    JSCharBuffer cb(cx);

    /* Cycles/joins are indicated by sharp objects. */
#if JS_HAS_SHARP_VARS
    if (IS_SHARP(he)) {
        JS_ASSERT(sharpchars != 0);
        cb.replaceRawBuffer(sharpchars, js_strlen(sharpchars));
        goto make_string;
    } else if (sharpchars) {
        MAKE_SHARP(he);
        cb.replaceRawBuffer(sharpchars, js_strlen(sharpchars));
    }
#else
    if (IS_SHARP(he)) {
        if (!js_AppendLiteral(cb, "[]"))
            goto out;
        cx->free(sharpchars);
        goto make_string;
    }
#endif

    if (!cb.append('['))
        goto out;

    jsuint length;
    if (!js_GetLengthProperty(cx, obj, &length))
        goto out;

    for (jsuint index = 0; index < length; index++) {
        /* Use vp to locally root each element value. */
        JSBool hole;
        if (!JS_CHECK_OPERATION_LIMIT(cx) ||
            !GetElement(cx, obj, index, &hole, vp)) {
            goto out;
        }

        /* Get element's character string. */
        JSString *str;
        if (hole) {
            str = cx->runtime->emptyString;
        } else {
            str = js_ValueToSource(cx, *vp);
            if (!str)
                goto out;
        }
        vp->setString(str);

        const jschar *chars = str->getChars(cx);
        if (!chars)
            goto out;

        /* Append element to buffer. */
        if (!cb.append(chars, chars + str->length()))
            goto out;
        if (index + 1 != length) {
            if (!js_AppendLiteral(cb, ", "))
                goto out;
        } else if (hole) {
            if (!cb.append(','))
                goto out;
        }
    }

    /* Finalize the buffer. */
    if (!cb.append(']'))
        goto out;

  make_string:
    if (!BufferToString(cx, cb, vp))
        goto out;

    ok = true;

  out:
    if (!initiallySharp)
        js_LeaveSharpObject(cx, NULL);
    return ok;
}
#endif

static JSBool
array_toString_sub(JSContext *cx, JSObject *obj, JSBool locale,
                   JSString *sepstr, Value *rval)
{
    JS_CHECK_RECURSION(cx, return false);

    /* Get characters to use for the separator. */
    static const jschar comma = ',';
    const jschar *sep;
    size_t seplen;
    if (sepstr) {
        seplen = sepstr->length();
        sep = sepstr->getChars(cx);
        if (!sep)
            return false;
    } else {
        sep = &comma;
        seplen = 1;
    }

    /*
     * Use HashTable entry as the cycle indicator. On first visit, create the
     * entry, and, when leaving, remove the entry.
     */
    typedef js::HashSet<JSObject *> ObjSet;
    ObjSet::AddPtr hashp = cx->busyArrays.lookupForAdd(obj);
    uint32 genBefore;
    if (!hashp) {
        /* Not in hash table, so not a cycle. */
        if (!cx->busyArrays.add(hashp, obj))
            return false;
        genBefore = cx->busyArrays.generation();
    } else {
        /* Cycle, so return empty string. */
        rval->setString(ATOM_TO_STRING(cx->runtime->atomState.emptyAtom));
        return true;
    }

    AutoObjectRooter tvr(cx, obj);

    /* After this point, all paths exit through the 'out' label. */
    MUST_FLOW_THROUGH("out");
    bool ok = false;

    /*
     * This object will take responsibility for the jschar buffer until the
     * buffer is transferred to the returned JSString.
     */
    JSCharBuffer cb(cx);

    jsuint length;
    if (!js_GetLengthProperty(cx, obj, &length))
        goto out;

    for (jsuint index = 0; index < length; index++) {
        /* Use rval to locally root each element value. */
        JSBool hole;
        if (!JS_CHECK_OPERATION_LIMIT(cx) ||
            !GetElement(cx, obj, index, &hole, rval)) {
            goto out;
        }

        /* Get element's character string. */
        if (!(hole || rval->isNullOrUndefined())) {
            if (locale) {
                /* Work on obj.toLocalString() instead. */
                JSObject *robj;

                if (!js_ValueToObjectOrNull(cx, *rval, &robj))
                    goto out;
                rval->setObjectOrNull(robj);
                JSAtom *atom = cx->runtime->atomState.toLocaleStringAtom;
                if (!js_TryMethod(cx, robj, atom, 0, NULL, rval))
                    goto out;
            }

            if (!js_ValueToCharBuffer(cx, *rval, cb))
                goto out;
        }

        /* Append the separator. */
        if (index + 1 != length) {
            if (!cb.append(sep, seplen))
                goto out;
        }
    }

    /* Finalize the buffer. */
    if (!BufferToString(cx, cb, rval))
        goto out;

    ok = true;

  out:
    if (genBefore == cx->busyArrays.generation())
        cx->busyArrays.remove(hashp);
    else
        cx->busyArrays.remove(obj);
    return ok;
}

/* ES5 15.4.4.2. NB: The algorithm here differs from the one in ES3. */
static JSBool
array_toString(JSContext *cx, uintN argc, Value *vp)
{
    JSObject *obj = ComputeThisFromVp(cx, vp);
    if (!obj)
        return false;

    Value &join = vp[0];
    if (!obj->getProperty(cx, ATOM_TO_JSID(cx->runtime->atomState.joinAtom), &join))
        return false;

    if (!js_IsCallable(join)) {
        JSString *str = obj_toStringHelper(cx, obj);
        if (!str)
            return false;
        vp->setString(str);
        return true;
    }

    LeaveTrace(cx);
    InvokeArgsGuard args;
    if (!cx->stack().pushInvokeArgs(cx, 0, &args))
        return false;

    args.callee() = join;
    args.thisv().setObject(*obj);

    /* Do the call. */
    if (!Invoke(cx, args, 0))
        return false;
    *vp = args.rval();
    return true;
}

static JSBool
array_toLocaleString(JSContext *cx, uintN argc, Value *vp)
{
    JSObject *obj = ComputeThisFromVp(cx, vp);
    if (!obj)
        return false;

    /*
     *  Passing comma here as the separator. Need a way to get a
     *  locale-specific version.
     */
    return array_toString_sub(cx, obj, JS_TRUE, NULL, vp);
}

static JSBool
InitArrayElements(JSContext *cx, JSObject *obj, jsuint start, jsuint count, Value *vector)
{
    JS_ASSERT(count < MAXINDEX);

    /*
     * Optimize for dense arrays so long as adding the given set of elements
     * wouldn't otherwise make the array slow.
     */
    do {
        if (!obj->isDenseArray())
            break;
        if (js_PrototypeHasIndexedProperties(cx, obj))
            break;

        JSObject::EnsureDenseResult result = obj->ensureDenseArrayElements(cx, start, count);
        if (result != JSObject::ED_OK) {
            if (result == JSObject::ED_FAILED)
                return false;
            JS_ASSERT(result == JSObject::ED_SPARSE);
            break;
        }
        jsuint newlen = start + count;
        if (newlen > obj->getArrayLength())
            obj->setArrayLength(newlen);

        JS_ASSERT(count < uint32(-1) / sizeof(Value));
        memcpy(obj->getDenseArrayElements() + start, vector, sizeof(jsval) * count);
        JS_ASSERT_IF(count != 0, !obj->getDenseArrayElement(newlen - 1).isMagic(JS_ARRAY_HOLE));
        return true;
    } while (false);

    Value* end = vector + count;
    while (vector != end && start < MAXINDEX) {
        if (!JS_CHECK_OPERATION_LIMIT(cx) ||
            !SetArrayElement(cx, obj, start++, *vector++)) {
            return JS_FALSE;
        }
    }

    if (vector == end)
        return JS_TRUE;

    /* Finish out any remaining elements past the max array index. */
    if (obj->isDenseArray() && !ENSURE_SLOW_ARRAY(cx, obj))
        return JS_FALSE;

    JS_ASSERT(start == MAXINDEX);
    AutoValueRooter tvr(cx);
    AutoIdRooter idr(cx);
    Value idval = DoubleValue(MAXINDEX);
    do {
        *tvr.addr() = *vector++;
        if (!js_ValueToStringId(cx, idval, idr.addr()) ||
            !obj->setProperty(cx, idr.id(), tvr.addr(), true)) {
            return JS_FALSE;
        }
        idval.getDoubleRef() += 1;
    } while (vector != end);

    return JS_TRUE;
}

static JSBool
InitArrayObject(JSContext *cx, JSObject *obj, jsuint length, const Value *vector)
{
    JS_ASSERT(obj->isArray());

    JS_ASSERT(obj->isDenseArray());
    obj->setArrayLength(length);
    if (!vector || !length)
        return true;

    /* Avoid ensureDenseArrayElements to skip sparse array checks there. */
    if (!obj->ensureSlots(cx, length))
        return false;
    memcpy(obj->getDenseArrayElements(), vector, length * sizeof(Value));
    return true;
}

/*
 * Perl-inspired join, reverse, and sort.
 */
static JSBool
array_join(JSContext *cx, uintN argc, Value *vp)
{
    JSString *str;
    if (argc == 0 || vp[2].isUndefined()) {
        str = NULL;
    } else {
        str = js_ValueToString(cx, vp[2]);
        if (!str)
            return JS_FALSE;
        vp[2].setString(str);
    }
    JSObject *obj = ComputeThisFromVp(cx, vp);
    return obj && array_toString_sub(cx, obj, JS_FALSE, str, vp);
}

static JSBool
array_reverse(JSContext *cx, uintN argc, Value *vp)
{
    jsuint len;
    JSObject *obj = ComputeThisFromVp(cx, vp);
    if (!obj || !js_GetLengthProperty(cx, obj, &len))
        return JS_FALSE;
    vp->setObject(*obj);

    do {
        if (!obj->isDenseArray())
            break;
        if (js_PrototypeHasIndexedProperties(cx, obj))
            break;
        
        /* An empty array or an array with no elements is already reversed. */
        if (len == 0 || obj->getDenseArrayCapacity() == 0)
            return JS_TRUE;

        /*
         * It's actually surprisingly complicated to reverse an array due to the
         * orthogonality of array length and array capacity while handling
         * leading and trailing holes correctly.  Reversing seems less likely to
         * be a common operation than other array mass-mutation methods, so for
         * now just take a probably-small memory hit (in the absence of too many
         * holes in the array at its start) and ensure that the capacity is
         * sufficient to hold all the elements in the array if it were full.
         */
        JSObject::EnsureDenseResult result = obj->ensureDenseArrayElements(cx, len, 0);
        if (result != JSObject::ED_OK) {
            if (result == JSObject::ED_FAILED)
                return false;
            JS_ASSERT(result == JSObject::ED_SPARSE);
            break;
        }

        uint32 lo = 0, hi = len - 1;
        for (; lo < hi; lo++, hi--) {
            Value origlo = obj->getDenseArrayElement(lo);
            Value orighi = obj->getDenseArrayElement(hi);
            obj->setDenseArrayElement(lo, orighi);
            if (orighi.isMagic(JS_ARRAY_HOLE))
                js_SuppressDeletedProperty(cx, obj, INT_TO_JSID(lo));
            obj->setDenseArrayElement(hi, origlo);
            if (origlo.isMagic(JS_ARRAY_HOLE))
                js_SuppressDeletedProperty(cx, obj, INT_TO_JSID(hi));
            }

        /*
         * Per ECMA-262, don't update the length of the array, even if the new
         * array has trailing holes (and thus the original array began with
         * holes).
         */
        return JS_TRUE;
    } while (false);

    AutoValueRooter tvr(cx);
    for (jsuint i = 0, half = len / 2; i < half; i++) {
        JSBool hole, hole2;
        if (!JS_CHECK_OPERATION_LIMIT(cx) ||
            !GetElement(cx, obj, i, &hole, tvr.addr()) ||
            !GetElement(cx, obj, len - i - 1, &hole2, vp) ||
            !SetOrDeleteArrayElement(cx, obj, len - i - 1, hole, tvr.value()) ||
            !SetOrDeleteArrayElement(cx, obj, i, hole2, *vp)) {
            return false;
        }
    }
    vp->setObject(*obj);
    return true;
}

typedef struct MSortArgs {
    size_t       elsize;
    JSComparator cmp;
    void         *arg;
    JSBool       isValue;
} MSortArgs;

/* Helper function for js_MergeSort. */
static JSBool
MergeArrays(MSortArgs *msa, void *src, void *dest, size_t run1, size_t run2)
{
    void *arg, *a, *b, *c;
    size_t elsize, runtotal;
    int cmp_result;
    JSComparator cmp;
    JSBool isValue;

    runtotal = run1 + run2;

    elsize = msa->elsize;
    cmp = msa->cmp;
    arg = msa->arg;
    isValue = msa->isValue;

#define CALL_CMP(a, b) \
    if (!cmp(arg, (a), (b), &cmp_result)) return JS_FALSE;

    /* Copy runs already in sorted order. */
    b = (char *)src + run1 * elsize;
    a = (char *)b - elsize;
    CALL_CMP(a, b);
    if (cmp_result <= 0) {
        memcpy(dest, src, runtotal * elsize);
        return JS_TRUE;
    }

#define COPY_ONE(p,q,n) \
    (isValue ? (void)(*(Value*)p = *(Value*)q) : (void)memcpy(p, q, n))

    a = src;
    c = dest;
    for (; runtotal != 0; runtotal--) {
        JSBool from_a = run2 == 0;
        if (!from_a && run1 != 0) {
            CALL_CMP(a,b);
            from_a = cmp_result <= 0;
        }

        if (from_a) {
            COPY_ONE(c, a, elsize);
            run1--;
            a = (char *)a + elsize;
        } else {
            COPY_ONE(c, b, elsize);
            run2--;
            b = (char *)b + elsize;
        }
        c = (char *)c + elsize;
    }
#undef COPY_ONE
#undef CALL_CMP

    return JS_TRUE;
}

/*
 * This sort is stable, i.e. sequence of equal elements is preserved.
 * See also bug #224128.
 */
bool
js_MergeSort(void *src, size_t nel, size_t elsize,
             JSComparator cmp, void *arg, void *tmp,
             JSMergeSortElemType elemType)
{
    void *swap, *vec1, *vec2;
    MSortArgs msa;
    size_t i, j, lo, hi, run;
    int cmp_result;

    JS_ASSERT_IF(JS_SORTING_VALUES, elsize == sizeof(Value));
    bool isValue = elemType == JS_SORTING_VALUES;

    /* Avoid memcpy overhead for word-sized and word-aligned elements. */
#define COPY_ONE(p,q,n) \
    (isValue ? (void)(*(Value*)p = *(Value*)q) : (void)memcpy(p, q, n))
#define CALL_CMP(a, b) \
    if (!cmp(arg, (a), (b), &cmp_result)) return JS_FALSE;
#define INS_SORT_INT 4

    /*
     * Apply insertion sort to small chunks to reduce the number of merge
     * passes needed.
     */
    for (lo = 0; lo < nel; lo += INS_SORT_INT) {
        hi = lo + INS_SORT_INT;
        if (hi >= nel)
            hi = nel;
        for (i = lo + 1; i < hi; i++) {
            vec1 = (char *)src + i * elsize;
            vec2 = (char *)vec1 - elsize;
            for (j = i; j > lo; j--) {
                CALL_CMP(vec2, vec1);
                /* "<=" instead of "<" insures the sort is stable */
                if (cmp_result <= 0) {
                    break;
                }

                /* Swap elements, using "tmp" as tmp storage */
                COPY_ONE(tmp, vec2, elsize);
                COPY_ONE(vec2, vec1, elsize);
                COPY_ONE(vec1, tmp, elsize);
                vec1 = vec2;
                vec2 = (char *)vec1 - elsize;
            }
        }
    }
#undef CALL_CMP
#undef COPY_ONE

    msa.elsize = elsize;
    msa.cmp = cmp;
    msa.arg = arg;
    msa.isValue = isValue;

    vec1 = src;
    vec2 = tmp;
    for (run = INS_SORT_INT; run < nel; run *= 2) {
        for (lo = 0; lo < nel; lo += 2 * run) {
            hi = lo + run;
            if (hi >= nel) {
                memcpy((char *)vec2 + lo * elsize, (char *)vec1 + lo * elsize,
                       (nel - lo) * elsize);
                break;
            }
            if (!MergeArrays(&msa, (char *)vec1 + lo * elsize,
                             (char *)vec2 + lo * elsize, run,
                             hi + run > nel ? nel - hi : run)) {
                return JS_FALSE;
            }
        }
        swap = vec1;
        vec1 = vec2;
        vec2 = swap;
    }
    if (src != vec1)
        memcpy(src, tmp, nel * elsize);

    return JS_TRUE;
}

struct CompareArgs
{
    JSContext          *context;
    InvokeSessionGuard session;

    CompareArgs(JSContext *cx)
      : context(cx)
    {}
};

static JS_REQUIRES_STACK JSBool
sort_compare(void *arg, const void *a, const void *b, int *result)
{
    const Value *av = (const Value *)a, *bv = (const Value *)b;
    CompareArgs *ca = (CompareArgs *) arg;
    JSContext *cx = ca->context;

    /*
     * array_sort deals with holes and undefs on its own and they should not
     * come here.
     */
    JS_ASSERT(!av->isMagic() && !av->isUndefined());
    JS_ASSERT(!av->isMagic() && !bv->isUndefined());

    if (!JS_CHECK_OPERATION_LIMIT(cx))
        return JS_FALSE;

    InvokeSessionGuard &session = ca->session;
    session[0] = *av;
    session[1] = *bv;

    if (!session.invoke(cx))
        return JS_FALSE;

    jsdouble cmp;
    if (!ValueToNumber(cx, session.rval(), &cmp))
        return JS_FALSE;

    /* Clamp cmp to -1, 0, 1. */
    *result = 0;
    if (!JSDOUBLE_IS_NaN(cmp) && cmp != 0)
        *result = cmp > 0 ? 1 : -1;

    /*
     * XXX else report some kind of error here?  ECMA talks about 'consistent
     * compare functions' that don't return NaN, but is silent about what the
     * result should be.  So we currently ignore it.
     */

    return JS_TRUE;
}

typedef JSBool (JS_REQUIRES_STACK *JSRedComparator)(void*, const void*,
                                                    const void*, int *);

static inline JS_IGNORE_STACK JSComparator
comparator_stack_cast(JSRedComparator func)
{
    return func;
}

static int
sort_compare_strings(void *arg, const void *a, const void *b, int *result)
{
    JSContext *cx = (JSContext *)arg;
    JSString *astr = ((const Value *)a)->toString();
    JSString *bstr = ((const Value *)b)->toString();
    return JS_CHECK_OPERATION_LIMIT(cx) && CompareStrings(cx, astr, bstr, result);
}

JSBool
js::array_sort(JSContext *cx, uintN argc, Value *vp)
{
    jsuint len, newlen, i, undefs;
    size_t elemsize;
    JSString *str;

    Value *argv = JS_ARGV(cx, vp);
    Value fval;
    if (argc > 0 && !argv[0].isUndefined()) {
        if (argv[0].isPrimitive()) {
            JS_ReportErrorNumber(cx, js_GetErrorMessage, NULL, JSMSG_BAD_SORT_ARG);
            return false;
        }
        fval = argv[0];     /* non-default compare function */
    } else {
        fval.setNull();
    }

    JSObject *obj = ComputeThisFromVp(cx, vp);
    if (!obj || !js_GetLengthProperty(cx, obj, &len))
        return false;
    if (len == 0) {
        vp->setObject(*obj);
        return true;
    }

    /*
     * We need a temporary array of 2 * len Value to hold the array elements
     * and the scratch space for merge sort. Check that its size does not
     * overflow size_t, which would allow for indexing beyond the end of the
     * malloc'd vector.
     */
#if JS_BITS_PER_WORD == 32
    if (size_t(len) > size_t(-1) / (2 * sizeof(Value))) {
        js_ReportAllocationOverflow(cx);
        return false;
    }
#endif

    /*
     * Initialize vec as a root. We will clear elements of vec one by
     * one while increasing the rooted amount of vec when we know that the
     * property at the corresponding index exists and its value must be rooted.
     *
     * In this way when sorting a huge mostly sparse array we will not
     * access the tail of vec corresponding to properties that do not
     * exist, allowing OS to avoiding committing RAM. See bug 330812.
     */
    {
        Value *vec = (Value *) cx->malloc(2 * size_t(len) * sizeof(Value));
        if (!vec)
            return false;

        DEFINE_LOCAL_CLASS_OF_STATIC_FUNCTION(AutoFreeVector) {
            JSContext *const cx;
            Value *&vec;
           public:
            AutoFreeVector(JSContext *cx, Value *&vec) : cx(cx), vec(vec) { }
            ~AutoFreeVector() {
                cx->free(vec);
            }
        } free(cx, vec);

        AutoArrayRooter tvr(cx, 0, vec);

        /*
         * By ECMA 262, 15.4.4.11, a property that does not exist (which we
         * call a "hole") is always greater than an existing property with
         * value undefined and that is always greater than any other property.
         * Thus to sort holes and undefs we simply count them, sort the rest
         * of elements, append undefs after them and then make holes after
         * undefs.
         */
        undefs = 0;
        newlen = 0;
        bool allStrings = true;
        for (i = 0; i < len; i++) {
            if (!JS_CHECK_OPERATION_LIMIT(cx))
                return false;

            /* Clear vec[newlen] before including it in the rooted set. */
            JSBool hole;
            vec[newlen].setNull();
            tvr.changeLength(newlen + 1);
            if (!GetElement(cx, obj, i, &hole, &vec[newlen]))
                return false;

            if (hole)
                continue;

            if (vec[newlen].isUndefined()) {
                ++undefs;
                continue;
            }

            allStrings = allStrings && vec[newlen].isString();

            ++newlen;
        }

        if (newlen == 0) {
            vp->setObject(*obj);
            return true; /* The array has only holes and undefs. */
        }

        /*
         * The first newlen elements of vec are copied from the array object
         * (above). The remaining newlen positions are used as GC-rooted scratch
         * space for mergesort. We must clear the space before including it to
         * the root set covered by tvr.count.
         */
        Value *mergesort_tmp = vec + newlen;
        MakeValueRangeGCSafe(mergesort_tmp, newlen);
        tvr.changeLength(newlen * 2);

        /* Here len == 2 * (newlen + undefs + number_of_holes). */
        if (fval.isNull()) {
            /*
             * Sort using the default comparator converting all elements to
             * strings.
             */
            if (allStrings) {
                elemsize = sizeof(Value);
            } else {
                /*
                 * To avoid string conversion on each compare we do it only once
                 * prior to sorting. But we also need the space for the original
                 * values to recover the sorting result. To reuse
                 * sort_compare_strings we move the original values to the odd
                 * indexes in vec, put the string conversion results in the even
                 * indexes and pass 2 * sizeof(Value) as an element size to the
                 * sorting function. In this way sort_compare_strings will only
                 * see the string values when it casts the compare arguments as
                 * pointers to Value.
                 *
                 * This requires doubling the temporary storage including the
                 * scratch space for the merge sort. Since vec already contains
                 * the rooted scratch space for newlen elements at the tail, we
                 * can use it to rearrange and convert to strings first and try
                 * realloc only when we know that we successfully converted all
                 * the elements.
                 */
#if JS_BITS_PER_WORD == 32
                if (size_t(newlen) > size_t(-1) / (4 * sizeof(Value))) {
                    js_ReportAllocationOverflow(cx);
                    return false;
                }
#endif

                /*
                 * Rearrange and string-convert the elements of the vector from
                 * the tail here and, after sorting, move the results back
                 * starting from the start to prevent overwrite the existing
                 * elements.
                 */
                i = newlen;
                do {
                    --i;
                    if (!JS_CHECK_OPERATION_LIMIT(cx))
                        return false;
                    const Value &v = vec[i];
                    str = js_ValueToString(cx, v);
                    if (!str)
                        return false;
                    // Copying v must come first, because the following line overwrites v
                    // when i == 0.
                    vec[2 * i + 1] = v;
                    vec[2 * i].setString(str);
                } while (i != 0);

                JS_ASSERT(tvr.array == vec);
                vec = (Value *) cx->realloc(vec, 4 * size_t(newlen) * sizeof(Value));
                if (!vec) {
                    vec = tvr.array;  /* N.B. AutoFreeVector */
                    return false;
                }
                mergesort_tmp = vec + 2 * newlen;
                MakeValueRangeGCSafe(mergesort_tmp, 2 * newlen);
                tvr.changeArray(vec, newlen * 4);
                elemsize = 2 * sizeof(Value);
            }
            if (!js_MergeSort(vec, size_t(newlen), elemsize,
                              sort_compare_strings, cx, mergesort_tmp,
                              JS_SORTING_GENERIC)) {
                return false;
            }
            if (!allStrings) {
                /*
                 * We want to make the following loop fast and to unroot the
                 * cached results of toString invocations before the operation
                 * callback has a chance to run the GC. For this reason we do
                 * not call JS_CHECK_OPERATION_LIMIT in the loop.
                 */
                i = 0;
                do {
                    vec[i] = vec[2 * i + 1];
                } while (++i != newlen);
            }
        } else {
            CompareArgs ca(cx);
            if (!ca.session.start(cx, fval, UndefinedValue(), 2))
                return false;

            if (!js_MergeSort(vec, size_t(newlen), sizeof(Value),
                              comparator_stack_cast(sort_compare),
                              &ca, mergesort_tmp,
                              JS_SORTING_VALUES)) {
                return false;
            }
        }

        /*
         * We no longer need to root the scratch space for the merge sort, so
         * unroot it now to make the job of a potential GC under
         * InitArrayElements easier.
         */
        tvr.changeLength(newlen);
        if (!InitArrayElements(cx, obj, 0, newlen, vec))
            return false;
    }

    /* Set undefs that sorted after the rest of elements. */
    while (undefs != 0) {
        --undefs;
        if (!JS_CHECK_OPERATION_LIMIT(cx) ||
            !SetArrayElement(cx, obj, newlen++, UndefinedValue())) {
            return false;
        }
    }

    /* Re-create any holes that sorted to the end of the array. */
    while (len > newlen) {
        if (!JS_CHECK_OPERATION_LIMIT(cx) || !DeleteArrayElement(cx, obj, --len, true))
            return false;
    }
    vp->setObject(*obj);
    return true;
}

/*
 * Perl-inspired push, pop, shift, unshift, and splice methods.
 */
static JSBool
array_push_slowly(JSContext *cx, JSObject *obj, uintN argc, Value *argv, Value *rval)
{
    jsuint length;

    if (!js_GetLengthProperty(cx, obj, &length))
        return JS_FALSE;
    if (!InitArrayElements(cx, obj, length, argc, argv))
        return JS_FALSE;

    /* Per ECMA-262, return the new array length. */
    jsdouble newlength = length + jsdouble(argc);
    rval->setNumber(newlength);
    return js_SetLengthProperty(cx, obj, newlength);
}

static JSBool
array_push1_dense(JSContext* cx, JSObject* obj, const Value &v, Value *rval)
{
    uint32 length = obj->getArrayLength();
    do {
        JSObject::EnsureDenseResult result = obj->ensureDenseArrayElements(cx, length, 1);
        if (result != JSObject::ED_OK) {
            if (result == JSObject::ED_FAILED)
                return false;
            JS_ASSERT(result == JSObject::ED_SPARSE);
            break;
        }

        obj->setArrayLength(length + 1);

        JS_ASSERT(obj->getDenseArrayElement(length).isMagic(JS_ARRAY_HOLE));
        obj->setDenseArrayElement(length, v);
        rval->setNumber(obj->getArrayLength());
        return true;
    } while (false);

    if (!obj->makeDenseArraySlow(cx))
        return false;
    Value tmp = v;
    return array_push_slowly(cx, obj, 1, &tmp, rval);
}

JS_ALWAYS_INLINE JSBool
ArrayCompPushImpl(JSContext *cx, JSObject *obj, const Value &v)
{
    JS_ASSERT(obj->isDenseArray());
    uint32_t length = obj->getArrayLength();
    JS_ASSERT(length <= obj->getDenseArrayCapacity());

    if (length == obj->getDenseArrayCapacity()) {
        if (length > JS_ARGS_LENGTH_MAX) {
            JS_ReportErrorNumberUC(cx, js_GetErrorMessage, NULL,
                                   JSMSG_ARRAY_INIT_TOO_BIG);
            return JS_FALSE;
        }

        /*
         * Array comprehension cannot add holes to the array and never leaks
         * the array before it is fully initialized. So we can use ensureSlots
         * instead of ensureDenseArrayElements.
         */
        if (!obj->ensureSlots(cx, length + 1))
            return false;
    }
    obj->setArrayLength(length + 1);
    obj->setDenseArrayElement(length, v);
    return JS_TRUE;
}

JSBool
js_ArrayCompPush(JSContext *cx, JSObject *obj, const Value &vp)
{
    return ArrayCompPushImpl(cx, obj, vp);
}

#ifdef JS_TRACER
JSBool JS_FASTCALL
js_ArrayCompPush_tn(JSContext *cx, JSObject *obj, ValueArgType v)
{
    if (!ArrayCompPushImpl(cx, obj, ValueArgToConstRef(v))) {
        SetBuiltinError(cx);
        return JS_FALSE;
    }

    return cx->tracerState->builtinStatus == 0;
}
JS_DEFINE_CALLINFO_3(extern, BOOL_FAIL, js_ArrayCompPush_tn, CONTEXT, OBJECT,
                     VALUE, 0, nanojit::ACCSET_STORE_ANY)
#endif

static JSBool
array_push(JSContext *cx, uintN argc, Value *vp)
{
    /* Insist on one argument and obj of the expected class. */
    JSObject *obj = ComputeThisFromVp(cx, vp);
    if (!obj)
        return JS_FALSE;
    if (argc != 1 || !obj->isDenseArray())
        return array_push_slowly(cx, obj, argc, vp + 2, vp);

    return array_push1_dense(cx, obj, vp[2], vp);
}

static JSBool
array_pop_slowly(JSContext *cx, JSObject* obj, Value *vp)
{
    jsuint index;
    JSBool hole;

    if (!js_GetLengthProperty(cx, obj, &index))
        return JS_FALSE;
    if (index == 0) {
        vp->setUndefined();
    } else {
        index--;

        /* Get the to-be-deleted property's value into vp. */
        if (!GetElement(cx, obj, index, &hole, vp))
            return JS_FALSE;
        if (!hole && !DeleteArrayElement(cx, obj, index, true))
            return JS_FALSE;
    }
    return js_SetLengthProperty(cx, obj, index);
}

static JSBool
array_pop_dense(JSContext *cx, JSObject* obj, Value *vp)
{
    jsuint index;
    JSBool hole;

    index = obj->getArrayLength();
    if (index == 0) {
        vp->setUndefined();
        return JS_TRUE;
    }
    index--;
    if (!GetElement(cx, obj, index, &hole, vp))
        return JS_FALSE;
    if (!hole && !DeleteArrayElement(cx, obj, index, true))
        return JS_FALSE;
    obj->setArrayLength(index);
    return JS_TRUE;
}

static JSBool
array_pop(JSContext *cx, uintN argc, Value *vp)
{
    JSObject *obj = ComputeThisFromVp(cx, vp);
    if (!obj)
        return JS_FALSE;
    if (obj->isDenseArray())
        return array_pop_dense(cx, obj, vp);
    return array_pop_slowly(cx, obj, vp);
}

static JSBool
array_shift(JSContext *cx, uintN argc, Value *vp)
{
    jsuint length, i;
    JSBool hole;

    JSObject *obj = ComputeThisFromVp(cx, vp);
    if (!obj || !js_GetLengthProperty(cx, obj, &length))
        return JS_FALSE;

    if (length == 0) {
        vp->setUndefined();
    } else {
        length--;

        if (obj->isDenseArray() && !js_PrototypeHasIndexedProperties(cx, obj) &&
            length < obj->getDenseArrayCapacity()) {
            *vp = obj->getDenseArrayElement(0);
            if (vp->isMagic(JS_ARRAY_HOLE))
                vp->setUndefined();
            Value *elems = obj->getDenseArrayElements();
            memmove(elems, elems + 1, length * sizeof(jsval));
            obj->setDenseArrayElement(length, MagicValue(JS_ARRAY_HOLE));
            obj->setArrayLength(length);
            if (!js_SuppressDeletedProperty(cx, obj, INT_TO_JSID(length)));
                return JS_FALSE;
            return JS_TRUE;
        }

        /* Get the to-be-deleted property's value into vp ASAP. */
        if (!GetElement(cx, obj, 0, &hole, vp))
            return JS_FALSE;

        /* Slide down the array above the first element. */
        AutoValueRooter tvr(cx);
        for (i = 0; i != length; i++) {
            if (!JS_CHECK_OPERATION_LIMIT(cx) ||
                !GetElement(cx, obj, i + 1, &hole, tvr.addr()) ||
                !SetOrDeleteArrayElement(cx, obj, i, hole, tvr.value())) {
                return JS_FALSE;
            }
        }

        /* Delete the only or last element when it exists. */
        if (!hole && !DeleteArrayElement(cx, obj, length, true))
            return JS_FALSE;
    }
    return js_SetLengthProperty(cx, obj, length);
}

static JSBool
array_unshift(JSContext *cx, uintN argc, Value *vp)
{
    Value *argv;
    jsuint length;
    JSBool hole;
    jsdouble last, newlen;

    JSObject *obj = ComputeThisFromVp(cx, vp);
    if (!obj || !js_GetLengthProperty(cx, obj, &length))
        return JS_FALSE;
    newlen = length;
    if (argc > 0) {
        /* Slide up the array to make room for argc at the bottom. */
        argv = JS_ARGV(cx, vp);
        if (length > 0) {
            bool optimized = false;
            do {
                if (!obj->isDenseArray())
                    break;
                if (js_PrototypeHasIndexedProperties(cx, obj))
                    break;
                JSObject::EnsureDenseResult result = obj->ensureDenseArrayElements(cx, length, argc);
                if (result != JSObject::ED_OK) {
                    if (result == JSObject::ED_FAILED)
                        return false;
                    JS_ASSERT(result == JSObject::ED_SPARSE);
                    break;
                }
                Value *elems = obj->getDenseArrayElements();
                memmove(elems + argc, elems, length * sizeof(jsval));
                for (uint32 i = 0; i < argc; i++)
                    obj->setDenseArrayElement(i, MagicValue(JS_ARRAY_HOLE));
                optimized = true;
            } while (false);

            if (!optimized) {
                last = length;
                jsdouble upperIndex = last + argc;
                AutoValueRooter tvr(cx);
                do {
                    --last, --upperIndex;
                    if (!JS_CHECK_OPERATION_LIMIT(cx) ||
                        !GetElement(cx, obj, last, &hole, tvr.addr()) ||
                        !SetOrDeleteArrayElement(cx, obj, upperIndex, hole, tvr.value())) {
                        return JS_FALSE;
                    }
                } while (last != 0);
            }
        }

        /* Copy from argv to the bottom of the array. */
        if (!InitArrayElements(cx, obj, 0, argc, argv))
            return JS_FALSE;

        newlen += argc;
    }
    if (!js_SetLengthProperty(cx, obj, newlen))
        return JS_FALSE;

    /* Follow Perl by returning the new array length. */
    vp->setNumber(newlen);
    return JS_TRUE;
}

static JSBool
array_splice(JSContext *cx, uintN argc, Value *vp)
{
    jsuint length, begin, end, count, delta, last;
    JSBool hole;

    /*
     * Create a new array value to return.  Our ECMA v2 proposal specs
     * that splice always returns an array value, even when given no
     * arguments.  We think this is best because it eliminates the need
     * for callers to do an extra test to handle the empty splice case.
     */
    JSObject *obj2 = NewDenseEmptyArray(cx);
    if (!obj2)
        return JS_FALSE;
    vp->setObject(*obj2);

    /* Nothing to do if no args.  Otherwise get length. */
    if (argc == 0)
        return JS_TRUE;
    Value *argv = JS_ARGV(cx, vp);
    JSObject *obj = ComputeThisFromVp(cx, vp);
    if (!obj || !js_GetLengthProperty(cx, obj, &length))
        return JS_FALSE;
    jsuint origlength = length;

    /* Convert the first argument into a starting index. */
    jsdouble d;
    if (!ValueToNumber(cx, *argv, &d))
        return JS_FALSE;
    d = js_DoubleToInteger(d);
    if (d < 0) {
        d += length;
        if (d < 0)
            d = 0;
    } else if (d > length) {
        d = length;
    }
    begin = (jsuint)d; /* d has been clamped to uint32 */
    argc--;
    argv++;

    /* Convert the second argument from a count into a fencepost index. */
    delta = length - begin;
    if (argc == 0) {
        count = delta;
        end = length;
    } else {
        if (!ValueToNumber(cx, *argv, &d))
            return JS_FALSE;
        d = js_DoubleToInteger(d);
        if (d < 0)
            d = 0;
        else if (d > delta)
            d = delta;
        count = (jsuint)d;
        end = begin + count;
        argc--;
        argv++;
    }

    AutoValueRooter tvr(cx);

    /* If there are elements to remove, put them into the return value. */
    if (count > 0) {
        if (obj->isDenseArray() && !js_PrototypeHasIndexedProperties(cx, obj) &&
            end <= obj->getDenseArrayCapacity()) {
            if (!InitArrayObject(cx, obj2, count, obj->getDenseArrayElements() + begin))
                return JS_FALSE;
        } else {
            for (last = begin; last < end; last++) {
                if (!JS_CHECK_OPERATION_LIMIT(cx) ||
                    !GetElement(cx, obj, last, &hole, tvr.addr())) {
                    return JS_FALSE;
                }

                /* Copy tvr.value() to the new array unless it's a hole. */
                if (!hole && !SetArrayElement(cx, obj2, last - begin, tvr.value()))
                    return JS_FALSE;
            }

            if (!js_SetLengthProperty(cx, obj2, count))
                return JS_FALSE;
        }
    }

    /* Find the direction (up or down) to copy and make way for argv. */
    if (argc > count) {
        delta = (jsuint)argc - count;
        last = length;
        bool optimized = false;
        do {
            if (!obj->isDenseArray())
                break;
            if (js_PrototypeHasIndexedProperties(cx, obj))
                break;
            if (length > obj->getDenseArrayCapacity())
                break;
            if (length != 0 && obj->getDenseArrayElement(length - 1).isMagic(JS_ARRAY_HOLE))
                break;
            JSObject::EnsureDenseResult result = obj->ensureDenseArrayElements(cx, length, delta);
            if (result != JSObject::ED_OK) {
                if (result == JSObject::ED_FAILED)
                    return false;
                JS_ASSERT(result == JSObject::ED_SPARSE);
                break;
            }
            Value *arraybeg = obj->getDenseArrayElements();
            Value *srcbeg = arraybeg + last - 1;
            Value *srcend = arraybeg + end - 1;
            Value *dstbeg = srcbeg + delta;
            for (Value *src = srcbeg, *dst = dstbeg; src > srcend; --src, --dst)
                *dst = *src;

            obj->setArrayLength(obj->getArrayLength() + delta);
            optimized = true;
        } while (false);

        if (!optimized) {
            /* (uint) end could be 0, so we can't use a vanilla >= test. */
            while (last-- > end) {
                if (!JS_CHECK_OPERATION_LIMIT(cx) ||
                    !GetElement(cx, obj, last, &hole, tvr.addr()) ||
                    !SetOrDeleteArrayElement(cx, obj, last + delta, hole, tvr.value())) {
                    return JS_FALSE;
                }
            }
        }
        length += delta;
    } else if (argc < count) {
        delta = count - (jsuint)argc;
        if (obj->isDenseArray() && !js_PrototypeHasIndexedProperties(cx, obj) &&
            length <= obj->getDenseArrayCapacity()) {

            Value *arraybeg = obj->getDenseArrayElements();
            Value *srcbeg = arraybeg + end;
            Value *srcend = arraybeg + length;
            Value *dstbeg = srcbeg - delta;
            for (Value *src = srcbeg, *dst = dstbeg; src < srcend; ++src, ++dst)
                *dst = *src;
        } else {
            for (last = end; last < length; last++) {
                if (!JS_CHECK_OPERATION_LIMIT(cx) ||
                    !GetElement(cx, obj, last, &hole, tvr.addr()) ||
                    !SetOrDeleteArrayElement(cx, obj, last - delta, hole, tvr.value())) {
                    return JS_FALSE;
                }
            }
        }
        length -= delta;
    }

    if (length < origlength && !js_SuppressDeletedIndexProperties(cx, obj, length, origlength))
        return JS_FALSE;

    /*
     * Copy from argv into the hole to complete the splice, and update length in
     * case we deleted elements from the end.
     */
    return InitArrayElements(cx, obj, begin, argc, argv) &&
           js_SetLengthProperty(cx, obj, length);
}

/*
 * Python-esque sequence operations.
 */
static JSBool
array_concat(JSContext *cx, uintN argc, Value *vp)
{
    /* Treat our |this| object as the first argument; see ECMA 15.4.4.4. */
    Value *p = JS_ARGV(cx, vp) - 1;

    /* Create a new Array object and root it using *vp. */
    JSObject *aobj = ComputeThisFromVp(cx, vp);
    JSObject *nobj;
    jsuint length;
    if (aobj->isDenseArray()) {
        /*
         * Clone aobj but pass the minimum of its length and capacity, to
         * handle a = [1,2,3]; a.length = 10000 "dense" cases efficiently. In
         * the normal case where length is <= capacity, nobj and aobj will have
         * the same capacity.
         */
        length = aobj->getArrayLength();
        jsuint capacity = aobj->getDenseArrayCapacity();
        nobj = NewDenseCopiedArray(cx, JS_MIN(length, capacity), aobj->getDenseArrayElements());
        if (!nobj)
            return JS_FALSE;
        nobj->setArrayLength(length);
        vp->setObject(*nobj);
        if (argc == 0)
            return JS_TRUE;
        argc--;
        p++;
    } else {
        nobj = NewDenseEmptyArray(cx);
        if (!nobj)
            return JS_FALSE;
        vp->setObject(*nobj);
        length = 0;
    }

    AutoValueRooter tvr(cx);

    /* Loop over [0, argc] to concat args into nobj, expanding all Arrays. */
    for (uintN i = 0; i <= argc; i++) {
        if (!JS_CHECK_OPERATION_LIMIT(cx))
            return false;
        const Value &v = p[i];
        if (v.isObject()) {
            aobj = &v.toObject();
            if (aobj->isArray() ||
                (aobj->isWrapper() && JSWrapper::wrappedObject(aobj)->isArray())) {
                jsid id = ATOM_TO_JSID(cx->runtime->atomState.lengthAtom);
                if (!aobj->getProperty(cx, id, tvr.addr()))
                    return false;
                jsuint alength;
                if (!ValueToLength(cx, tvr.addr(), &alength))
                    return false;
                for (jsuint slot = 0; slot < alength; slot++) {
                    JSBool hole;
                    if (!JS_CHECK_OPERATION_LIMIT(cx) ||
                        !GetElement(cx, aobj, slot, &hole, tvr.addr())) {
                        return false;
                    }

                    /*
                     * Per ECMA 262, 15.4.4.4, step 9, ignore nonexistent
                     * properties.
                     */
                    if (!hole &&
                        !SetArrayElement(cx, nobj, length+slot, tvr.value())) {
                        return false;
                    }
                }
                length += alength;
                continue;
            }
        }

        if (!SetArrayElement(cx, nobj, length, v))
            return false;
        length++;
    }

    return js_SetLengthProperty(cx, nobj, length);
}

static JSBool
array_slice(JSContext *cx, uintN argc, Value *vp)
{
    Value *argv;
    JSObject *nobj;
    jsuint length, begin, end, slot;
    JSBool hole;

    argv = JS_ARGV(cx, vp);

    JSObject *obj = ComputeThisFromVp(cx, vp);
    if (!obj || !js_GetLengthProperty(cx, obj, &length))
        return JS_FALSE;
    begin = 0;
    end = length;

    if (argc > 0) {
        jsdouble d;
        if (!ValueToNumber(cx, argv[0], &d))
            return JS_FALSE;
        d = js_DoubleToInteger(d);
        if (d < 0) {
            d += length;
            if (d < 0)
                d = 0;
        } else if (d > length) {
            d = length;
        }
        begin = (jsuint)d;

        if (argc > 1 && !argv[1].isUndefined()) {
            if (!ValueToNumber(cx, argv[1], &d))
                return JS_FALSE;
            d = js_DoubleToInteger(d);
            if (d < 0) {
                d += length;
                if (d < 0)
                    d = 0;
            } else if (d > length) {
                d = length;
            }
            end = (jsuint)d;
        }
    }

    if (begin > end)
        begin = end;

    if (obj->isDenseArray() && end <= obj->getDenseArrayCapacity() &&
        !js_PrototypeHasIndexedProperties(cx, obj)) {
        nobj = NewDenseCopiedArray(cx, end - begin, obj->getDenseArrayElements() + begin);
        if (!nobj)
            return JS_FALSE;
        vp->setObject(*nobj);
        return JS_TRUE;
    }

    /* Create a new Array object and root it using *vp. */
    nobj = NewDenseAllocatedArray(cx, end - begin);
    if (!nobj)
        return JS_FALSE;
    vp->setObject(*nobj);

    AutoValueRooter tvr(cx);
    for (slot = begin; slot < end; slot++) {
        if (!JS_CHECK_OPERATION_LIMIT(cx) ||
            !GetElement(cx, obj, slot, &hole, tvr.addr())) {
            return JS_FALSE;
        }
        if (!hole && !SetArrayElement(cx, nobj, slot - begin, tvr.value()))
            return JS_FALSE;
    }

    return JS_TRUE;
}

#if JS_HAS_ARRAY_EXTRAS

static JSBool
array_indexOfHelper(JSContext *cx, JSBool isLast, uintN argc, Value *vp)
{
    jsuint length, i, stop;
    Value tosearch;
    jsint direction;
    JSBool hole;

    JSObject *obj = ComputeThisFromVp(cx, vp);
    if (!obj || !js_GetLengthProperty(cx, obj, &length))
        return JS_FALSE;
    if (length == 0)
        goto not_found;

    if (argc <= 1) {
        i = isLast ? length - 1 : 0;
        tosearch = (argc != 0) ? vp[2] : UndefinedValue();
    } else {
        jsdouble start;

        tosearch = vp[2];
        if (!ValueToNumber(cx, vp[3], &start))
            return JS_FALSE;
        start = js_DoubleToInteger(start);
        if (start < 0) {
            start += length;
            if (start < 0) {
                if (isLast)
                    goto not_found;
                i = 0;
            } else {
                i = (jsuint)start;
            }
        } else if (start >= length) {
            if (!isLast)
                goto not_found;
            i = length - 1;
        } else {
            i = (jsuint)start;
        }
    }

    if (isLast) {
        stop = 0;
        direction = -1;
    } else {
        stop = length - 1;
        direction = 1;
    }

    for (;;) {
        if (!JS_CHECK_OPERATION_LIMIT(cx) ||
            !GetElement(cx, obj, (jsuint)i, &hole, vp)) {
            return JS_FALSE;
        }
        if (!hole) {
            JSBool equal;
            if (!StrictlyEqual(cx, *vp, tosearch, &equal))
                return JS_FALSE;
            if (equal) {
                vp->setNumber(i);
                return JS_TRUE;
            }
        }
        if (i == stop)
            goto not_found;
        i += direction;
    }

  not_found:
    vp->setInt32(-1);
    return JS_TRUE;
}

static JSBool
array_indexOf(JSContext *cx, uintN argc, Value *vp)
{
    return array_indexOfHelper(cx, JS_FALSE, argc, vp);
}

static JSBool
array_lastIndexOf(JSContext *cx, uintN argc, Value *vp)
{
    return array_indexOfHelper(cx, JS_TRUE, argc, vp);
}

/* Order is important; extras that take a predicate funarg must follow MAP. */
typedef enum ArrayExtraMode {
    FOREACH,
    REDUCE,
    REDUCE_RIGHT,
    MAP,
    FILTER,
    SOME,
    EVERY
} ArrayExtraMode;

#define REDUCE_MODE(mode) ((mode) == REDUCE || (mode) == REDUCE_RIGHT)

static JSBool
array_extra(JSContext *cx, ArrayExtraMode mode, uintN argc, Value *vp)
{
    JSObject *obj = ComputeThisFromVp(cx, vp);
    jsuint length;
    if (!obj || !js_GetLengthProperty(cx, obj, &length))
        return JS_FALSE;

    /*
     * First, get or compute our callee, so that we error out consistently
     * when passed a non-callable object.
     */
    if (argc == 0) {
        js_ReportMissingArg(cx, *vp, 0);
        return JS_FALSE;
    }
    Value *argv = vp + 2;
    JSObject *callable = js_ValueToCallableObject(cx, &argv[0], JSV2F_SEARCH_STACK);
    if (!callable)
        return JS_FALSE;

    /*
     * Set our initial return condition, used for zero-length array cases
     * (and pre-size our map return to match our known length, for all cases).
     */
    jsuint newlen;
    JSObject *newarr;
#ifdef __GNUC__ /* quell GCC overwarning */
    newlen = 0;
    newarr = NULL;
#endif
    jsint start = 0, end = length, step = 1;

    switch (mode) {
      case REDUCE_RIGHT:
        start = length - 1, end = -1, step = -1;
        /* FALL THROUGH */
      case REDUCE:
        if (length == 0 && argc == 1) {
            JS_ReportErrorNumber(cx, js_GetErrorMessage, NULL,
                                 JSMSG_EMPTY_ARRAY_REDUCE);
            return JS_FALSE;
        }
        if (argc >= 2) {
            *vp = argv[1];
        } else {
            JSBool hole;
            do {
                if (!GetElement(cx, obj, start, &hole, vp))
                    return JS_FALSE;
                start += step;
            } while (hole && start != end);

            if (hole && start == end) {
                JS_ReportErrorNumber(cx, js_GetErrorMessage, NULL,
                                     JSMSG_EMPTY_ARRAY_REDUCE);
                return JS_FALSE;
            }
        }
        break;
      case MAP:
      case FILTER:
        newlen = (mode == MAP) ? length : 0;
        newarr = NewDenseAllocatedArray(cx, newlen);
        if (!newarr)
            return JS_FALSE;
        vp->setObject(*newarr);
        break;
      case SOME:
        vp->setBoolean(false);
        break;
      case EVERY:
        vp->setBoolean(true);
        break;
      case FOREACH:
        vp->setUndefined();
        break;
    }

    if (length == 0)
        return JS_TRUE;

    Value thisv = (argc > 1 && !REDUCE_MODE(mode)) ? argv[1] : UndefinedValue();

    /*
     * For all but REDUCE, we call with 3 args (value, index, array). REDUCE
     * requires 4 args (accum, value, index, array).
     */
    argc = 3 + REDUCE_MODE(mode);

    InvokeSessionGuard session;
    if (!session.start(cx, ObjectValue(*callable), thisv, argc))
        return JS_FALSE;

    MUST_FLOW_THROUGH("out");
    JSBool ok = JS_TRUE;
    JSBool cond;

    Value objv = ObjectValue(*obj);
    AutoValueRooter tvr(cx);
    for (jsint i = start; i != end; i += step) {
        JSBool hole;
        ok = JS_CHECK_OPERATION_LIMIT(cx) &&
             GetElement(cx, obj, i, &hole, tvr.addr());
        if (!ok)
            goto out;
        if (hole)
            continue;

        /*
         * Push callable and 'this', then args. We must do this for every
         * iteration around the loop since Invoke clobbers its arguments.
         */
        uintN argi = 0;
        if (REDUCE_MODE(mode))
            session[argi++] = *vp;
        session[argi++] = tvr.value();
        session[argi++] = Int32Value(i);
        session[argi]   = objv;

        /* Do the call. */
        ok = session.invoke(cx);
        if (!ok)
            break;

        const Value &rval = session.rval();

        if (mode > MAP)
            cond = js_ValueToBoolean(rval);
#ifdef __GNUC__ /* quell GCC overwarning */
        else
            cond = JS_FALSE;
#endif

        switch (mode) {
          case FOREACH:
            break;
          case REDUCE:
          case REDUCE_RIGHT:
            *vp = rval;
            break;
          case MAP:
            ok = SetArrayElement(cx, newarr, i, rval);
            if (!ok)
                goto out;
            break;
          case FILTER:
            if (!cond)
                break;
            /* The element passed the filter, so push it onto our result. */
            ok = SetArrayElement(cx, newarr, newlen++, tvr.value());
            if (!ok)
                goto out;
            break;
          case SOME:
            if (cond) {
                vp->setBoolean(true);
                goto out;
            }
            break;
          case EVERY:
            if (!cond) {
                vp->setBoolean(false);
                goto out;
            }
            break;
        }
    }

  out:
    if (ok && mode == FILTER)
        ok = js_SetLengthProperty(cx, newarr, newlen);
    return ok;
}

static JSBool
array_forEach(JSContext *cx, uintN argc, Value *vp)
{
    return array_extra(cx, FOREACH, argc, vp);
}

static JSBool
array_map(JSContext *cx, uintN argc, Value *vp)
{
    return array_extra(cx, MAP, argc, vp);
}

static JSBool
array_reduce(JSContext *cx, uintN argc, Value *vp)
{
    return array_extra(cx, REDUCE, argc, vp);
}

static JSBool
array_reduceRight(JSContext *cx, uintN argc, Value *vp)
{
    return array_extra(cx, REDUCE_RIGHT, argc, vp);
}

static JSBool
array_filter(JSContext *cx, uintN argc, Value *vp)
{
    return array_extra(cx, FILTER, argc, vp);
}

static JSBool
array_some(JSContext *cx, uintN argc, Value *vp)
{
    return array_extra(cx, SOME, argc, vp);
}

static JSBool
array_every(JSContext *cx, uintN argc, Value *vp)
{
    return array_extra(cx, EVERY, argc, vp);
}
#endif

static JSBool
array_isArray(JSContext *cx, uintN argc, Value *vp)
{
    JSObject *obj;
    vp->setBoolean(argc > 0 &&
                   vp[2].isObject() &&
                   ((obj = &vp[2].toObject())->isArray() ||
                    (obj->isWrapper() && JSWrapper::wrappedObject(obj)->isArray())));
    return true;
}

static JSFunctionSpec array_methods[] = {
#if JS_HAS_TOSOURCE
    JS_FN(js_toSource_str,      array_toSource,     0,0),
#endif
    JS_FN(js_toString_str,      array_toString,     0,0),
    JS_FN(js_toLocaleString_str,array_toLocaleString,0,0),

    /* Perl-ish methods. */
    JS_FN("join",               array_join,         1,JSFUN_GENERIC_NATIVE),
    JS_FN("reverse",            array_reverse,      0,JSFUN_GENERIC_NATIVE),
    JS_FN("sort",               array_sort,         1,JSFUN_GENERIC_NATIVE),
    JS_FN("push",               array_push,         1,JSFUN_GENERIC_NATIVE),
    JS_FN("pop",                array_pop,          0,JSFUN_GENERIC_NATIVE),
    JS_FN("shift",              array_shift,        0,JSFUN_GENERIC_NATIVE),
    JS_FN("unshift",            array_unshift,      1,JSFUN_GENERIC_NATIVE),
    JS_FN("splice",             array_splice,       2,JSFUN_GENERIC_NATIVE),

    /* Pythonic sequence methods. */
    JS_FN("concat",             array_concat,       1,JSFUN_GENERIC_NATIVE),
    JS_FN("slice",              array_slice,        2,JSFUN_GENERIC_NATIVE),

#if JS_HAS_ARRAY_EXTRAS
    JS_FN("indexOf",            array_indexOf,      1,JSFUN_GENERIC_NATIVE),
    JS_FN("lastIndexOf",        array_lastIndexOf,  1,JSFUN_GENERIC_NATIVE),
    JS_FN("forEach",            array_forEach,      1,JSFUN_GENERIC_NATIVE),
    JS_FN("map",                array_map,          1,JSFUN_GENERIC_NATIVE),
    JS_FN("reduce",             array_reduce,       1,JSFUN_GENERIC_NATIVE),
    JS_FN("reduceRight",        array_reduceRight,  1,JSFUN_GENERIC_NATIVE),
    JS_FN("filter",             array_filter,       1,JSFUN_GENERIC_NATIVE),
    JS_FN("some",               array_some,         1,JSFUN_GENERIC_NATIVE),
    JS_FN("every",              array_every,        1,JSFUN_GENERIC_NATIVE),
#endif

    JS_FS_END
};

static JSFunctionSpec array_static_methods[] = {
    JS_FN("isArray",            array_isArray,      1,0),
    JS_FS_END
};

JSBool
js_Array(JSContext *cx, uintN argc, Value *vp)
{
    JSObject *obj;

    if (argc == 0) {
        obj = NewDenseEmptyArray(cx);
    } else if (argc > 1) {
        obj = NewDenseCopiedArray(cx, argc, vp + 2);
    } else if (!vp[2].isNumber()) {
        obj = NewDenseCopiedArray(cx, 1, vp + 2);
    } else {
        jsuint length;
        if (!ValueToLength(cx, vp + 2, &length))
            return JS_FALSE;
        obj = NewDenseUnallocatedArray(cx, length);
    }

    if (!obj)
        return JS_FALSE;
    vp->setObject(*obj);

    return JS_TRUE;
}

JSObject *
js_InitArrayClass(JSContext *cx, JSObject *obj)
{
    JSObject *proto = js_InitClass(cx, obj, NULL, &js_ArrayClass, js_Array, 1,
                                   NULL, array_methods, NULL, array_static_methods);
    if (!proto)
        return NULL;

    /*
     * Assert that js_InitClass used the correct (slow array, not dense array)
     * class for proto's emptyShape class.
     */
    JS_ASSERT(proto->emptyShapes && proto->emptyShapes[0]->getClass() == proto->getClass());

    proto->setArrayLength(0);
    return proto;
}

/*
 * Array allocation functions.
 */
namespace js {

template<bool allocateCapacity>
static JS_ALWAYS_INLINE JSObject *
NewArray(JSContext *cx, jsuint length, JSObject *proto)
{
    JS_ASSERT_IF(proto, proto->isArray());

    gc::FinalizeKind kind = GuessObjectGCKind(length, true);
    JSObject *obj = detail::NewObject<WithProto::Class, false>(cx, &js_ArrayClass, proto, NULL, kind);
    if (!obj)
        return NULL;

    obj->setArrayLength(length);

    if (allocateCapacity && !obj->ensureSlots(cx, length))
        return NULL;

    return obj;
}

JSObject * JS_FASTCALL
NewDenseEmptyArray(JSContext *cx, JSObject *proto)
{
    return NewArray<false>(cx, 0, proto);
}

JSObject * JS_FASTCALL
NewDenseAllocatedArray(JSContext *cx, uint32 length, JSObject *proto)
{
    return NewArray<true>(cx, length, proto);
}

JSObject * JS_FASTCALL
NewDenseUnallocatedArray(JSContext *cx, uint32 length, JSObject *proto)
{
    return NewArray<false>(cx, length, proto);
}

JSObject *
NewDenseCopiedArray(JSContext *cx, uintN length, Value *vp, JSObject *proto)
{
    JSObject* obj = NewArray<true>(cx, length, proto);
    if (!obj)
        return NULL;

    JS_ASSERT(obj->getDenseArrayCapacity() >= length);

    if (vp)
        memcpy(obj->getDenseArrayElements(), vp, length * sizeof(Value));

    return obj;
}

#ifdef JS_TRACER
JS_DEFINE_CALLINFO_2(extern, OBJECT, NewDenseEmptyArray, CONTEXT, OBJECT, 0,
                     nanojit::ACCSET_STORE_ANY)
JS_DEFINE_CALLINFO_3(extern, OBJECT, NewDenseAllocatedArray, CONTEXT, UINT32, OBJECT, 0,
                     nanojit::ACCSET_STORE_ANY)
JS_DEFINE_CALLINFO_3(extern, OBJECT, NewDenseUnallocatedArray, CONTEXT, UINT32, OBJECT, 0,
                     nanojit::ACCSET_STORE_ANY)
#endif



JSObject *
NewSlowEmptyArray(JSContext *cx)
{
    JSObject *obj = NewNonFunction<WithProto::Class>(cx, &js_SlowArrayClass, NULL, NULL);
    if (!obj)
        return NULL;

    obj->setArrayLength(0);
    return obj;
}

}


#ifdef DEBUG
JSBool
js_ArrayInfo(JSContext *cx, uintN argc, jsval *vp)
{
    uintN i;
    JSObject *array;

    for (i = 0; i < argc; i++) {
        Value arg = Valueify(JS_ARGV(cx, vp)[i]);

        char *bytes = DecompileValueGenerator(cx, JSDVG_SEARCH_STACK, arg, NULL);
        if (!bytes)
            return JS_FALSE;
        if (arg.isPrimitive() ||
            !(array = arg.toObjectOrNull())->isArray()) {
            fprintf(stderr, "%s: not array\n", bytes);
            cx->free(bytes);
            continue;
        }
        fprintf(stderr, "%s: %s (len %u", bytes,
                array->isDenseArray() ? "dense" : "sparse",
                array->getArrayLength());
        if (array->isDenseArray()) {
            fprintf(stderr, ", capacity %u",
                    array->getDenseArrayCapacity());
        }
        fputs(")\n", stderr);
        cx->free(bytes);
    }

    JS_SET_RVAL(cx, vp, JSVAL_VOID);
    return true;
}
#endif

JS_FRIEND_API(JSBool)
js_CoerceArrayToCanvasImageData(JSObject *obj, jsuint offset, jsuint count,
                                JSUint8 *dest)
{
    uint32 length;

    if (!obj || !obj->isDenseArray())
        return JS_FALSE;

    length = obj->getArrayLength();
    if (length < offset + count)
        return JS_FALSE;

    JSUint8 *dp = dest;
    for (uintN i = offset; i < offset+count; i++) {
        const Value &v = obj->getDenseArrayElement(i);
        if (v.isInt32()) {
            jsint vi = v.toInt32();
            if (jsuint(vi) > 255)
                vi = (vi < 0) ? 0 : 255;
            *dp++ = JSUint8(vi);
        } else if (v.isDouble()) {
            jsdouble vd = v.toDouble();
            if (!(vd >= 0)) /* Not < so that NaN coerces to 0 */
                *dp++ = 0;
            else if (vd > 255)
                *dp++ = 255;
            else {
                jsdouble toTruncate = vd + 0.5;
                JSUint8 val = JSUint8(toTruncate);

                /*
                 * now val is rounded to nearest, ties rounded up.  We want
                 * rounded to nearest ties to even, so check whether we had a
                 * tie.
                 */
                if (val == toTruncate) {
                  /*
                   * It was a tie (since adding 0.5 gave us the exact integer
                   * we want).  Since we rounded up, we either already have an
                   * even number or we have an odd number but the number we
                   * want is one less.  So just unconditionally masking out the
                   * ones bit should do the trick to get us the value we
                   * want.
                   */
                  *dp++ = (val & ~1);
                } else {
                  *dp++ = val;
                }
            }
        } else {
            return JS_FALSE;
        }
    }

    return JS_TRUE;
}

JS_FRIEND_API(JSBool)
js_IsDensePrimitiveArray(JSObject *obj)
{
    if (!obj || !obj->isDenseArray())
        return JS_FALSE;

    jsuint capacity = obj->getDenseArrayCapacity();
    for (jsuint i = 0; i < capacity; i++) {
        if (obj->getDenseArrayElement(i).isObject())
            return JS_FALSE;
    }

    return JS_TRUE;
}

JS_FRIEND_API(JSBool)
js_CloneDensePrimitiveArray(JSContext *cx, JSObject *obj, JSObject **clone)
{
    JS_ASSERT(obj);
    if (!obj->isDenseArray()) {
        /*
         * This wasn't a dense array. Return JS_TRUE but a NULL clone to signal
         * that no exception was encountered.
         */
        *clone = NULL;
        return JS_TRUE;
    }

    jsuint length = obj->getArrayLength();

    /*
     * Must use the minimum of original array's length and capacity, to handle
     * |a = [1,2,3]; a.length = 10000| "dense" cases efficiently. In the normal
     * case where length is <= capacity, the clone and original array will have
     * the same capacity.
     */
    jsuint jsvalCount = JS_MIN(obj->getDenseArrayCapacity(), length);

    js::AutoValueVector vector(cx);
    if (!vector.reserve(jsvalCount))
        return JS_FALSE;

    for (jsuint i = 0; i < jsvalCount; i++) {
        const Value &val = obj->getDenseArrayElement(i);

        if (val.isString()) {
            // Strings must be made immutable before being copied to a clone.
            if (!js_MakeStringImmutable(cx, val.toString()))
                return JS_FALSE;
        } else if (val.isObject()) {
            /*
             * This wasn't an array of primitives. Return JS_TRUE but a null
             * clone to signal that no exception was encountered.
             */
            *clone = NULL;
            return JS_TRUE;
        }

        vector.append(val);
    }

    *clone = NewDenseCopiedArray(cx, jsvalCount, vector.begin());
    if (!*clone)
        return JS_FALSE;

    /* The length will be set to the JS_MIN, above, but length might be larger. */
    (*clone)->setArrayLength(length);
    return JS_TRUE;
}