mfbt/FloatingPoint.cpp

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /* Implementations of FloatingPoint functions */
   8
   9 #include "mozilla/FloatingPoint.h"
  10
  11 #include <cfloat>  // for FLT_MAX
  12
  13 namespace mozilla {
  14
  15 bool IsFloat32Representable(double aValue) {
  16   // NaNs and infinities are representable.
  17   if (!std::isfinite(aValue)) {
  18     return true;
  19   }
  20
  21   // If it exceeds finite |float| range, casting to |double| is always undefined
  22   // behavior per C++11 [conv.double]p1 last sentence.
  23   if (Abs(aValue) > FLT_MAX) {
  24     return false;
  25   }
  26
  27   // But if it's within finite range, then either it's 1) an exact value and so
  28   // representable, or 2) it's "between two adjacent destination values" and
  29   // safe to cast to "an implementation-defined choice of either of those
  30   // values".
  31   auto valueAsFloat = static_cast<float>(aValue);
  32
  33   // Per [conv.fpprom] this never changes value.
  34   auto valueAsFloatAsDouble = static_cast<double>(valueAsFloat);
  35
  36   // Finally, in 1) exact representable value equals exact representable value,
  37   // or 2) *changed* value does not equal original value, ergo unrepresentable.
  38   return valueAsFloatAsDouble == aValue;
  39 }
  40
  41 } /* namespace mozilla */