Casting.h

/* -*- 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 <limits.h>
#include <type_traits>

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 {

enum ToSignedness { ToIsSigned, ToIsUnsigned };
enum FromSignedness { FromIsSigned, FromIsUnsigned };

template <typename From, typename To,
          FromSignedness =
              std::is_signed_v<From> ? FromIsSigned : FromIsUnsigned,
          ToSignedness = std::is_signed_v<To> ? ToIsSigned : ToIsUnsigned>
struct BoundsCheckImpl;

// Implicit conversions on operands to binary operations make this all a bit
// hard to verify.  Attempt to ease the pain below by *only* comparing values
// that are obviously the same type (and will undergo no further conversions),
// even when it's not strictly necessary, for explicitness.

enum UUComparison { FromIsBigger, FromIsNotBigger };

// Unsigned-to-unsigned range check

template <typename From, typename To,
          UUComparison =
              (sizeof(From) > sizeof(To)) ? FromIsBigger : FromIsNotBigger>
struct UnsignedUnsignedCheck;

template <typename From, typename To>
struct UnsignedUnsignedCheck<From, To, FromIsBigger> {
 public:
  static bool checkBounds(const From aFrom) { return aFrom <= From(To(-1)); }
};

template <typename From, typename To>
struct UnsignedUnsignedCheck<From, To, FromIsNotBigger> {
 public:
  static bool checkBounds(const From aFrom) { return true; }
};

template <typename From, typename To>
struct BoundsCheckImpl<From, To, FromIsUnsigned, ToIsUnsigned> {
 public:
  static bool checkBounds(const From aFrom) {
    return UnsignedUnsignedCheck<From, To>::checkBounds(aFrom);
  }
};

// Signed-to-unsigned range check

template <typename From, typename To>
struct BoundsCheckImpl<From, To, FromIsSigned, ToIsUnsigned> {
 public:
  static bool checkBounds(const From aFrom) {
    if (aFrom < 0) {
      return false;
    }
    if (sizeof(To) >= sizeof(From)) {
      return true;
    }
    return aFrom <= From(To(-1));
  }
};

// Unsigned-to-signed range check

enum USComparison { FromIsSmaller, FromIsNotSmaller };

template <typename From, typename To,
          USComparison =
              (sizeof(From) < sizeof(To)) ? FromIsSmaller : FromIsNotSmaller>
struct UnsignedSignedCheck;

template <typename From, typename To>
struct UnsignedSignedCheck<From, To, FromIsSmaller> {
 public:
  static bool checkBounds(const From aFrom) { return true; }
};

template <typename From, typename To>
struct UnsignedSignedCheck<From, To, FromIsNotSmaller> {
 public:
  static bool checkBounds(const From aFrom) {
    const To MaxValue = To((1ULL << (CHAR_BIT * sizeof(To) - 1)) - 1);
    return aFrom <= From(MaxValue);
  }
};

template <typename From, typename To>
struct BoundsCheckImpl<From, To, FromIsUnsigned, ToIsSigned> {
 public:
  static bool checkBounds(const From aFrom) {
    return UnsignedSignedCheck<From, To>::checkBounds(aFrom);
  }
};

// Signed-to-signed range check

template <typename From, typename To>
struct BoundsCheckImpl<From, To, FromIsSigned, ToIsSigned> {
 public:
  static bool checkBounds(const From aFrom) {
    if (sizeof(From) <= sizeof(To)) {
      return true;
    }
    const To MaxValue = To((1ULL << (CHAR_BIT * sizeof(To) - 1)) - 1);
    const To MinValue = -MaxValue - To(1);
    return From(MinValue) <= aFrom && From(aFrom) <= From(MaxValue);
  }
};

template <typename From, typename To,
          bool TypesAreIntegral =
              std::is_integral_v<From>&& std::is_integral_v<To>>
class BoundsChecker;

template <typename From>
class BoundsChecker<From, From, true> {
 public:
  static bool checkBounds(const From aFrom) { return true; }
};

template <typename From, typename To>
class BoundsChecker<From, To, true> {
 public:
  static bool checkBounds(const From aFrom) {
    return BoundsCheckImpl<From, To>::checkBounds(aFrom);
  }
};

template <typename From, typename To>
inline bool IsInBounds(const From aFrom) {
  return BoundsChecker<From, To>::checkBounds(aFrom);
}

}  // namespace detail

/**
 * Cast a value of integral type |From| to a value of integral 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|).
 */
template <typename To, typename From>
inline To AssertedCast(const From aFrom) {
  MOZ_ASSERT((detail::IsInBounds<From, To>(aFrom)));
  return static_cast<To>(aFrom);
}

/**
 * Cast a value of integral type |From| to a value of integral 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|).
 */
template <typename To, typename From>
inline To ReleaseAssertedCast(const From aFrom) {
  MOZ_RELEASE_ASSERT((detail::IsInBounds<From, To>(aFrom)));
  return static_cast<To>(aFrom);
}

}  // namespace mozilla

#endif /* mozilla_Casting_h */