Source/JavaScriptCore/runtime/PureNaN.h

/*
 * 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