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diff --git a/mfbt/Casting.h b/mfbt/Casting.h
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+/* -*- 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/. */
+
+/* Cast operations to supplement the built-in casting operations. */
+
+#ifndef mozilla_Casting_h
+#define mozilla_Casting_h
+
+#include "mozilla/Assertions.h"
+
+#include <cstring>
+#include <type_traits>
+#include <limits>
+#include <cmath>
+
+namespace mozilla {
+
+/**
+ * Sets the outparam value of type |To| with the same underlying bit pattern of
+ * |aFrom|.
+ *
+ * |To| and |From| must be types of the same size; be careful of cross-platform
+ * size differences, or this might fail to compile on some but not all
+ * platforms.
+ *
+ * There is also a variant that returns the value directly.  In most cases, the
+ * two variants should be identical.  However, in the specific case of x86
+ * chips, the behavior differs: returning floating-point values directly is done
+ * through the x87 stack, and x87 loads and stores turn signaling NaNs into
+ * quiet NaNs... silently.  Returning floating-point values via outparam,
+ * however, is done entirely within the SSE registers when SSE2 floating-point
+ * is enabled in the compiler, which has semantics-preserving behavior you would
+ * expect.
+ *
+ * If preserving the distinction between signaling NaNs and quiet NaNs is
+ * important to you, you should use the outparam version.  In all other cases,
+ * you should use the direct return version.
+ */
+template <typename To, typename From>
+inline void BitwiseCast(const From aFrom, To* aResult) {
+  static_assert(sizeof(From) == sizeof(To),
+                "To and From must have the same size");
+
+  // We could maybe downgrade these to std::is_trivially_copyable, but the
+  // various STLs we use don't all provide it.
+  static_assert(std::is_trivial<From>::value,
+                "shouldn't bitwise-copy a type having non-trivial "
+                "initialization");
+  static_assert(std::is_trivial<To>::value,
+                "shouldn't bitwise-copy a type having non-trivial "
+                "initialization");
+
+  std::memcpy(static_cast<void*>(aResult), static_cast<const void*>(&aFrom),
+              sizeof(From));
+}
+
+template <typename To, typename From>
+inline To BitwiseCast(const From aFrom) {
+  To temp;
+  BitwiseCast<To, From>(aFrom, &temp);
+  return temp;
+}
+
+namespace detail {
+
+template <typename T>
+constexpr int64_t safe_integer() {
+  static_assert(std::is_floating_point_v<T>);
+  return std::pow(2, std::numeric_limits<T>::digits);
+}
+
+template <typename T>
+constexpr uint64_t safe_integer_unsigned() {
+  static_assert(std::is_floating_point_v<T>);
+  return std::pow(2, std::numeric_limits<T>::digits);
+}
+
+// This is working around https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81676,
+// fixed in gcc-10
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wunused-but-set-variable"
+template <typename In, typename Out>
+bool IsInBounds(In aIn) {
+  constexpr bool inSigned = std::is_signed_v<In>;
+  constexpr bool outSigned = std::is_signed_v<Out>;
+  constexpr bool bothSigned = inSigned && outSigned;
+  constexpr bool bothUnsigned = !inSigned && !outSigned;
+  constexpr bool inFloat = std::is_floating_point_v<In>;
+  constexpr bool outFloat = std::is_floating_point_v<Out>;
+  constexpr bool bothFloat = inFloat && outFloat;
+  constexpr bool noneFloat = !inFloat && !outFloat;
+  constexpr Out outMax = std::numeric_limits<Out>::max();
+  constexpr Out outMin = std::numeric_limits<Out>::lowest();
+
+  // This selects the widest of two types, and is used to cast throughout.
+  using select_widest = std::conditional_t<(sizeof(In) > sizeof(Out)), In, Out>;
+
+  if constexpr (bothFloat) {
+    if (aIn > select_widest(outMax) || aIn < select_widest(outMin)) {
+      return false;
+    }
+  }
+  // Normal casting applies, the floating point number is floored.
+  if constexpr (inFloat && !outFloat) {
+    static_assert(sizeof(aIn) <= sizeof(int64_t));
+    // Check if the input floating point is larger than the output bounds. This
+    // catches situations where the input is a float larger than the max of the
+    // output type.
+    if (aIn < static_cast<double>(outMin) ||
+        aIn > static_cast<double>(outMax)) {
+      return false;
+    }
+    // At this point we know that the input can be converted to an integer.
+    // Check if it's larger than the bounds of the target integer.
+    if (outSigned) {
+      int64_t asInteger = static_cast<int64_t>(aIn);
+      if (asInteger < outMin || asInteger > outMax) {
+        return false;
+      }
+    } else {
+      uint64_t asInteger = static_cast<uint64_t>(aIn);
+      if (asInteger > outMax) {
+        return false;
+      }
+    }
+  }
+
+  // Checks if the integer is representable exactly as a floating point value of
+  // a specific width.
+  if constexpr (!inFloat && outFloat) {
+    if constexpr (inSigned) {
+      if (aIn < -safe_integer<Out>() || aIn > safe_integer<Out>()) {
+        return false;
+      }
+    } else {
+      if (aIn >= safe_integer_unsigned<Out>()) {
+        return false;
+      }
+    }
+  }
+
+  if constexpr (noneFloat) {
+    if constexpr (bothUnsigned) {
+      if (aIn > select_widest(outMax)) {
+        return false;
+      }
+    }
+    if constexpr (bothSigned) {
+      if (aIn > select_widest(outMax) || aIn < select_widest(outMin)) {
+        return false;
+      }
+    }
+    if constexpr (inSigned && !outSigned) {
+      if (aIn < 0 || std::make_unsigned_t<In>(aIn) > outMax) {
+        return false;
+      }
+    }
+    if constexpr (!inSigned && outSigned) {
+      if (aIn > select_widest(outMax)) {
+        return false;
+      }
+    }
+  }
+  return true;
+}
+#pragma GCC diagnostic pop
+
+}  // namespace detail
+
+/**
+ * Cast a value of type |From| to a value of type |To|, asserting that the cast
+ * will be a safe cast per C++ (that is, that |to| is in the range of values
+ * permitted for the type |From|).
+ * In particular, this will fail if a integer cannot be represented exactly as a
+ * floating point value, because it's too large.
+ */
+template <typename To, typename From>
+inline To AssertedCast(const From aFrom) {
+  static_assert(std::is_arithmetic_v<To> && std::is_arithmetic_v<From>);
+  MOZ_ASSERT((detail::IsInBounds<From, To>(aFrom)));
+  return static_cast<To>(aFrom);
+}
+
+/**
+ * Cast a value of numeric type |From| to a value of numeric type |To|, release
+ * asserting that the cast will be a safe cast per C++ (that is, that |to| is in
+ * the range of values permitted for the type |From|).
+ * In particular, this will fail if a integer cannot be represented exactly as a
+ * floating point value, because it's too large.
+ */
+template <typename To, typename From>
+inline To ReleaseAssertedCast(const From aFrom) {
+  static_assert(std::is_arithmetic_v<To> && std::is_arithmetic_v<From>);
+  MOZ_RELEASE_ASSERT((detail::IsInBounds<From, To>(aFrom)));
+  return static_cast<To>(aFrom);
+}
+
+namespace detail {
+
+template <typename From>
+class LazyAssertedCastT final {
+  const From mVal;
+
+ public:
+  explicit LazyAssertedCastT(const From val) : mVal(val) {}
+
+  template <typename To>
+  operator To() const {
+    return AssertedCast<To>(mVal);
+  }
+};
+
+}  // namespace detail
+
+/**
+ * Like AssertedCast, but infers |To| for AssertedCast lazily based on usage.
+ * > uint8_t foo = LazyAssertedCast(1000);  // boom
+ */
+template <typename From>
+inline auto LazyAssertedCast(const From val) {
+  return detail::LazyAssertedCastT<From>(val);
+}
+
+}  // namespace mozilla
+
+#endif /* mozilla_Casting_h */