1 files changed, 41 insertions, 0 deletions
diff --git a/mfbt/FloatingPoint.cpp b/mfbt/FloatingPoint.cpp
new file mode 100644
index 0000000000..4d52ffaaf8
--- /dev/null
+++ b/mfbt/FloatingPoint.cpp
@@ -0,0 +1,41 @@
+/* -*- 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 (!std::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 */