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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 */
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