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Diffstat (limited to 'mfbt/Casting.h')
| -rw-r--r-- | mfbt/Casting.h | 243 |
1 files changed, 243 insertions, 0 deletions
diff --git a/mfbt/Casting.h b/mfbt/Casting.h new file mode 100644 index 0000000000..a7d0fb50d6 --- /dev/null +++ b/mfbt/Casting.h @@ -0,0 +1,243 @@ +/* -*- 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 "mozilla/TypeTraits.h" + +#include <limits.h> + +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"); + union + { + From mFrom; + To mTo; + } u; + u.mFrom = aFrom; + *aResult = u.mTo; +} + +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 = IsSigned<From>::value ? FromIsSigned : FromIsUnsigned, + ToSignedness = IsSigned<To>::value ? 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 = IsIntegral<From>::value && + IsIntegral<To>::value> +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); +} + +} // namespace mozilla + +#endif /* mozilla_Casting_h */ |