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diff --git a/Source/JavaScriptCore/runtime/PureNaN.h b/Source/JavaScriptCore/runtime/PureNaN.h
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+/*
+ * 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