/* * Copyright (C) 2014 Apple Inc. All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * 1. Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */ #pragma once #include <wtf/Assertions.h> #include <wtf/StdLibExtras.h> namespace JSC { // NaN (not-a-number) double values are central to how JavaScriptCore encodes JavaScript // values (JSValues). All values, including integers and non-numeric values, are always // encoded using the IEEE 754 binary double format. Non-double values are encoded using // a NaN with the sign bit set. The 51-bit payload is then used for encoding the actual // value - be it an integer or a pointer to an object, or something else. But we only // make use of the low 49 bits and the top 15 bits being all set to 1 is the indicator // that a value is not a double. Top 15 bits being set to 1 also indicate a signed // signaling NaN with some additional NaN payload bits. // // Our use of NaN encoding means that we have to be careful with how we use NaNs for // ordinary doubles. For example, it would be wrong to ever use a NaN that has the top // 15 bits set, as that would look like a non-double value to JSC. // // We can trust that on all of the hardware/OS combinations that we care about, // NaN-producing math operations never produce a NaN that looks like a tagged value. But // if we're ever in a situation where we worry about it, we can use purifyNaN() to get a // NaN that doesn't look like a tagged non-double value. The JavaScript language doesn't // distinguish between different flavors of NaN and there is no way to detect what kind // of NaN you have - hence so long as all double NaNs are purified then our tagging // scheme remains sound. // // It's worth noting that there are cases, like sin(), that will almost produce a NaN // that breaks us. sin(-inf) returns 0xfff8000000000000. This doesn't break us because // not all of the top 15 bits are set. But it's very close. Hence our assumptions about // NaN are just about the most aggressive assumptions we could possibly make without // having to call purifyNaN() in surprising places. // // For naming purposes, we say that a NaN is "pure" if it is safe to tag, in the sense // that doing so would result in a tagged value that would pass the "are you a double" // test. We say that a NaN is "impure" if attempting to tag it would result in a value // that would look like something other than a double. // Returns some kind of pure NaN. inline double pureNaN() { // Be sure that we return exactly the kind of NaN that is safe. We engineer the bits // ourselves to ensure that it's !isImpureNaN(). FWIW, this is what // numeric_limits<double>::quiet_NaN() returns on Mac/X86_64. But AFAICT there is // no guarantee that quiet_NaN would return a pureNaN on all platforms. For example, // the docs appear to imply that quiet_NaN could even return a double with the // signaling bit set on hardware that doesn't do signaling. That would probably // never happen, but it's healthy to be paranoid. return bitwise_cast<double>(0x7ff8000000000000ll); } #define PNaN (pureNaN()) inline bool isImpureNaN(double value) { // Tests if the double value would break JSVALUE64 encoding, which is the most // aggressive kind of encoding that we currently use. return bitwise_cast<uint64_t>(value) >= 0xfffe000000000000llu; } // If the given value is NaN then return a NaN that is known to be pure. inline double purifyNaN(double value) { if (value != value) return PNaN; return value; } } // namespace JSC