1.1 --- /dev/null Thu Jan 01 00:00:00 1970 +0000
1.2 +++ b/mfbt/Casting.h Wed Dec 31 06:09:35 2014 +0100
1.3 @@ -0,0 +1,202 @@
1.4 +/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
1.5 +/* vim: set ts=8 sts=2 et sw=2 tw=80: */
1.6 +/* This Source Code Form is subject to the terms of the Mozilla Public
1.7 + * License, v. 2.0. If a copy of the MPL was not distributed with this
1.8 + * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
1.9 +
1.10 +/* Cast operations to supplement the built-in casting operations. */
1.11 +
1.12 +#ifndef mozilla_Casting_h
1.13 +#define mozilla_Casting_h
1.14 +
1.15 +#include "mozilla/Assertions.h"
1.16 +#include "mozilla/TypeTraits.h"
1.17 +
1.18 +#include <limits.h>
1.19 +
1.20 +namespace mozilla {
1.21 +
1.22 +/**
1.23 + * Return a value of type |To|, containing the underlying bit pattern of |from|.
1.24 + *
1.25 + * |To| and |From| must be types of the same size; be careful of cross-platform
1.26 + * size differences, or this might fail to compile on some but not all
1.27 + * platforms.
1.28 + */
1.29 +template<typename To, typename From>
1.30 +inline To
1.31 +BitwiseCast(const From from)
1.32 +{
1.33 + static_assert(sizeof(From) == sizeof(To),
1.34 + "To and From must have the same size");
1.35 + union {
1.36 + From from;
1.37 + To to;
1.38 + } u;
1.39 + u.from = from;
1.40 + return u.to;
1.41 +}
1.42 +
1.43 +namespace detail {
1.44 +
1.45 +enum ToSignedness { ToIsSigned, ToIsUnsigned };
1.46 +enum FromSignedness { FromIsSigned, FromIsUnsigned };
1.47 +
1.48 +template<typename From,
1.49 + typename To,
1.50 + FromSignedness = IsSigned<From>::value ? FromIsSigned : FromIsUnsigned,
1.51 + ToSignedness = IsSigned<To>::value ? ToIsSigned : ToIsUnsigned>
1.52 +struct BoundsCheckImpl;
1.53 +
1.54 +// Implicit conversions on operands to binary operations make this all a bit
1.55 +// hard to verify. Attempt to ease the pain below by *only* comparing values
1.56 +// that are obviously the same type (and will undergo no further conversions),
1.57 +// even when it's not strictly necessary, for explicitness.
1.58 +
1.59 +enum UUComparison { FromIsBigger, FromIsNotBigger };
1.60 +
1.61 +// Unsigned-to-unsigned range check
1.62 +
1.63 +template<typename From, typename To,
1.64 + UUComparison = (sizeof(From) > sizeof(To)) ? FromIsBigger : FromIsNotBigger>
1.65 +struct UnsignedUnsignedCheck;
1.66 +
1.67 +template<typename From, typename To>
1.68 +struct UnsignedUnsignedCheck<From, To, FromIsBigger>
1.69 +{
1.70 + public:
1.71 + static bool checkBounds(const From from) {
1.72 + return from <= From(To(-1));
1.73 + }
1.74 +};
1.75 +
1.76 +template<typename From, typename To>
1.77 +struct UnsignedUnsignedCheck<From, To, FromIsNotBigger>
1.78 +{
1.79 + public:
1.80 + static bool checkBounds(const From from) {
1.81 + return true;
1.82 + }
1.83 +};
1.84 +
1.85 +template<typename From, typename To>
1.86 +struct BoundsCheckImpl<From, To, FromIsUnsigned, ToIsUnsigned>
1.87 +{
1.88 + public:
1.89 + static bool checkBounds(const From from) {
1.90 + return UnsignedUnsignedCheck<From, To>::checkBounds(from);
1.91 + }
1.92 +};
1.93 +
1.94 +// Signed-to-unsigned range check
1.95 +
1.96 +template<typename From, typename To>
1.97 +struct BoundsCheckImpl<From, To, FromIsSigned, ToIsUnsigned>
1.98 +{
1.99 + public:
1.100 + static bool checkBounds(const From from) {
1.101 + if (from < 0)
1.102 + return false;
1.103 + if (sizeof(To) >= sizeof(From))
1.104 + return true;
1.105 + return from <= From(To(-1));
1.106 + }
1.107 +};
1.108 +
1.109 +// Unsigned-to-signed range check
1.110 +
1.111 +enum USComparison { FromIsSmaller, FromIsNotSmaller };
1.112 +
1.113 +template<typename From, typename To,
1.114 + USComparison = (sizeof(From) < sizeof(To)) ? FromIsSmaller : FromIsNotSmaller>
1.115 +struct UnsignedSignedCheck;
1.116 +
1.117 +template<typename From, typename To>
1.118 +struct UnsignedSignedCheck<From, To, FromIsSmaller>
1.119 +{
1.120 + public:
1.121 + static bool checkBounds(const From from) {
1.122 + return true;
1.123 + }
1.124 +};
1.125 +
1.126 +template<typename From, typename To>
1.127 +struct UnsignedSignedCheck<From, To, FromIsNotSmaller>
1.128 +{
1.129 + public:
1.130 + static bool checkBounds(const From from) {
1.131 + const To MaxValue = To((1ULL << (CHAR_BIT * sizeof(To) - 1)) - 1);
1.132 + return from <= From(MaxValue);
1.133 + }
1.134 +};
1.135 +
1.136 +template<typename From, typename To>
1.137 +struct BoundsCheckImpl<From, To, FromIsUnsigned, ToIsSigned>
1.138 +{
1.139 + public:
1.140 + static bool checkBounds(const From from) {
1.141 + return UnsignedSignedCheck<From, To>::checkBounds(from);
1.142 + }
1.143 +};
1.144 +
1.145 +// Signed-to-signed range check
1.146 +
1.147 +template<typename From, typename To>
1.148 +struct BoundsCheckImpl<From, To, FromIsSigned, ToIsSigned>
1.149 +{
1.150 + public:
1.151 + static bool checkBounds(const From from) {
1.152 + if (sizeof(From) <= sizeof(To))
1.153 + return true;
1.154 + const To MaxValue = To((1ULL << (CHAR_BIT * sizeof(To) - 1)) - 1);
1.155 + const To MinValue = -MaxValue - To(1);
1.156 + return From(MinValue) <= from &&
1.157 + From(from) <= From(MaxValue);
1.158 + }
1.159 +};
1.160 +
1.161 +template<typename From, typename To,
1.162 + bool TypesAreIntegral = IsIntegral<From>::value && IsIntegral<To>::value>
1.163 +class BoundsChecker;
1.164 +
1.165 +template<typename From>
1.166 +class BoundsChecker<From, From, true>
1.167 +{
1.168 + public:
1.169 + static bool checkBounds(const From from) { return true; }
1.170 +};
1.171 +
1.172 +template<typename From, typename To>
1.173 +class BoundsChecker<From, To, true>
1.174 +{
1.175 + public:
1.176 + static bool checkBounds(const From from) {
1.177 + return BoundsCheckImpl<From, To>::checkBounds(from);
1.178 + }
1.179 +};
1.180 +
1.181 +template<typename From, typename To>
1.182 +inline bool
1.183 +IsInBounds(const From from)
1.184 +{
1.185 + return BoundsChecker<From, To>::checkBounds(from);
1.186 +}
1.187 +
1.188 +} // namespace detail
1.189 +
1.190 +/**
1.191 + * Cast a value of integral type |From| to a value of integral type |To|,
1.192 + * asserting that the cast will be a safe cast per C++ (that is, that |to| is in
1.193 + * the range of values permitted for the type |From|).
1.194 + */
1.195 +template<typename To, typename From>
1.196 +inline To
1.197 +SafeCast(const From from)
1.198 +{
1.199 + MOZ_ASSERT((detail::IsInBounds<From, To>(from)));
1.200 + return static_cast<To>(from);
1.201 +}
1.202 +
1.203 +} // namespace mozilla
1.204 +
1.205 +#endif /* mozilla_Casting_h */
