mfbt/FloatingPoint.cpp
author Sam Sneddon <me@gsnedders.com>
Tue, 05 Mar 2019 12:19:18 +0000
changeset 464592 ed02da4c6be28b064d9c6168f54c7f84036fa9e1
parent 448947 6f3709b3878117466168c40affa7bca0b60cf75b
permissions -rw-r--r--
Bug 1486514 [wpt PR 12695] - Make reftests 800x600 everywhere, a=testonly Automatic update from web-platform-tests Change reftest size to 800x600 everywhere (#12695) With Firefox, with the internal reftest runner (used by default), this doesn't actually change the reftest size in Firefox versions prior to 66 (currently on the beta channel); to get the correct behaviour with older versions of Firefox, you must pass `--reftest-external` to use the external reftest runner. -- wpt-commits: d5f07ed3b5186e553940a3d5c547067850a4e3f0 wpt-pr: 12695

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/* Implementations of FloatingPoint functions */

#include "mozilla/FloatingPoint.h"

#include <cfloat>  // for FLT_MAX

namespace mozilla {

bool IsFloat32Representable(double aValue) {
  // NaNs and infinities are representable.
  if (!IsFinite(aValue)) {
    return true;
  }

  // If it exceeds finite |float| range, casting to |double| is always undefined
  // behavior per C++11 [conv.double]p1 last sentence.
  if (Abs(aValue) > FLT_MAX) {
    return false;
  }

  // But if it's within finite range, then either it's 1) an exact value and so
  // representable, or 2) it's "between two adjacent destination values" and
  // safe to cast to "an implementation-defined choice of either of those
  // values".
  auto valueAsFloat = static_cast<float>(aValue);

  // Per [conv.fpprom] this never changes value.
  auto valueAsFloatAsDouble = static_cast<double>(valueAsFloat);

  // Finally, in 1) exact representable value equals exact representable value,
  // or 2) *changed* value does not equal original value, ergo unrepresentable.
  return valueAsFloatAsDouble == aValue;
}

} /* namespace mozilla */