// ********************************************************* -*- C++ -*-
/*
* Copyright (C) 2004-2018 Exiv2 authors
* This program is part of the Exiv2 distribution.
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version 2
* of the License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, 5th Floor, Boston, MA 02110-1301 USA.
*/
/*!
@file safe_op.hpp
@brief Overflow checks for integers
@author Dan Čermák (D4N)
<a href="mailto:dan.cermak@cgc-instruments.com">dan.cermak@cgc-instruments.com</a>
@date 14-Dec-17, D4N: created
*/
#ifndef SAFE_OP_HPP_
#define SAFE_OP_HPP_
#include <limits>
#include <stdexcept>
#ifdef _MSC_VER
#include <Intsafe.h>
#endif
/*!
* @brief Arithmetic operations with overflow checks
*/
namespace Safe
{
/*!
* @brief Helper functions for providing integer overflow checks.
*
* This namespace contains internal helper functions fallback_$op_overflow
* and builtin_$op_overflow (where $op is an arithmetic operation like add,
* subtract, etc.). Both provide the following interface:
*
* bool fallback/builtin_$op_overflow(T first, T second, T& result);
*
* where T is an integer type.
*
* Each function performs checks whether first $op second can be safely
* performed without overflows. If yes, the result is saved in result and
* false is returned. Otherwise true is returned and the contents of result
* are unspecified.
*
* fallback_$op_overflow implements a portable but slower overflow check.
* builtin_$op_overflow uses compiler builtins (when available) and should
* be faster. As builtins are not available for all types,
* builtin_$op_overflow falls back to fallback_$op_overflow when no builtin
* is available.
*/
namespace Internal
{
/*!
* @brief Helper struct to determine whether a type is signed or unsigned
*
* This struct is a backport of std::is_signed from C++11. It has a public
* enum with the property VALUE which is true when the type is signed or
* false if it is unsigned.
*/
template <typename T>
struct is_signed
{
enum
{
VALUE = T(-1) < T(0)
};
};
/*!
* @brief Helper struct for SFINAE, from C++11
* This struct has a public typedef called type typedef'd to T if B is
* true. Otherwise there is no typedef.
*/
template <bool B, class T = void>
struct enable_if
{
};
/*!
* @brief Specialization of enable_if for the case B == true
*/
template <class T>
struct enable_if<true, T>
{
typedef T type;
};
/*!
* @brief Check the addition of two numbers for overflows for signed
* integer types larger than int or with the same size as int.
*
* This function performs a check if summand_1 + summand_2 would
* overflow and returns true in that case. If no overflow occurs,
* the sum is saved in result and false is returned.
*
* @return true on overflow, false on no overflow
*
* @param[in] summand_1, summand_2 The summands with are added
* @param[out] result Result of the addition, only populated when no
* overflow occurs.
*
* Further information:
* https://wiki.sei.cmu.edu/confluence/display/c/INT32-C.+Ensure+that+operations+on+signed+integers+do+not+result+in+overflow
*/
template <typename T>
typename enable_if<is_signed<T>::VALUE && sizeof(T) >= sizeof(int), bool>::type fallback_add_overflow(
T summand_1, T summand_2, T& result)
{
if (((summand_2 >= 0) && (summand_1 > std::numeric_limits<T>::max() - summand_2)) ||
((summand_2 < 0) && (summand_1 < std::numeric_limits<T>::min() - summand_2))) {
return true;
} else {
result = summand_1 + summand_2;
return false;
}
}
/*!
* @brief Check the addition of two numbers for overflows for signed
* integer types smaller than int.
*
* This function adds summand_1 and summand_2 exploiting integer
* promotion rules, thereby not causing undefined behavior. The
* result is checked against the limits of T and true is returned if
* they are exceeded. Otherwise the sum is saved in result and false
* is returned.
*
* @return true on overflow, false on no overflow
*
* @param[in] summand_1, summand_2 The summands with are added
* @param[out] result Result of the addition, only populated when no
* overflow occurs.
*
* Further information:
* https://wiki.sei.cmu.edu/confluence/display/c/INT02-C.+Understand+integer+conversion+rules
*/
template <typename T>
typename enable_if<is_signed<T>::VALUE && sizeof(T) < sizeof(int), bool>::type fallback_add_overflow(
T summand_1, T summand_2, T& result)
{
const int res = summand_1 + summand_2;
if ((res > std::numeric_limits<T>::max()) || (res < std::numeric_limits<T>::min())) {
return true;
} else {
result = static_cast<T>(res);
return false;
}
}
/*!
* @brief Check the addition of two numbers for overflows for unsigned
* integer types.
*
* This function adds summand_1 and summand_2 and checks after that if
* the operation overflowed. Since these are unsigned integers, no
* undefined behavior is invoked.
*
* @return true on overflow, false on no overflow
*
* @param[in] summand_1, summand_2 The summands with are added
* @param[out] result Result of the addition
*
* Further information:
* https://wiki.sei.cmu.edu/confluence/display/c/INT30-C.+Ensure+that+unsigned+integer+operations+do+not+wrap
*/
template <typename T>
typename enable_if<!is_signed<T>::VALUE, bool>::type fallback_add_overflow(T summand_1, T summand_2, T& result)
{
result = summand_1 + summand_2;
return result < summand_1;
}
/*!
* @brief Overflow addition check using compiler intrinsics.
*
* This function behaves exactly like fallback_add_overflow() but it
* relies on compiler intrinsics instead. This version should be faster
* than the fallback version as it can fully utilize available CPU
* instructions & the compiler's diagnostic.
*
* However, as some compilers don't provide intrinsics for certain
* types, the default implementation is the version from fallback.
*
* This function is fully specialized for each compiler.
*/
template <typename T>
bool builtin_add_overflow(T summand_1, T summand_2, T& result)
{
return fallback_add_overflow(summand_1, summand_2, result);
}
#if defined(__GNUC__) || defined(__clang__)
#if __GNUC__ >= 5 || __clang_major__ >= 3
/*!
* This macro pastes a specialization of builtin_add_overflow using gcc's &
* clang's __builtin_(s/u)add(l)(l)_overlow()
*
* The add function is implemented by forwarding the parameters to the intrinsic
* and returning its value.
*
* The intrinsics are documented here:
* https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html#Integer-Overflow-Builtins
*/
#define SPECIALIZE_builtin_add_overflow(type, builtin_name) \
/* Full specialization of builtin_add_overflow for type using the */ \
/* builtin_name intrinsic */ \
template <> \
inline bool builtin_add_overflow<type>(type summand_1, type summand_2, type & result) \
{ \
return builtin_name(summand_1, summand_2, &result); \
}
SPECIALIZE_builtin_add_overflow(int, __builtin_sadd_overflow);
SPECIALIZE_builtin_add_overflow(long, __builtin_saddl_overflow);
SPECIALIZE_builtin_add_overflow(long long, __builtin_saddll_overflow);
SPECIALIZE_builtin_add_overflow(unsigned int, __builtin_uadd_overflow);
SPECIALIZE_builtin_add_overflow(unsigned long, __builtin_uaddl_overflow);
SPECIALIZE_builtin_add_overflow(unsigned long long, __builtin_uaddll_overflow);
#undef SPECIALIZE_builtin_add_overflow
#endif // __GNUC__ >= 5 || __clang_major >= 3
#elif defined(_MSC_VER)
// intrinsics are not in available in MSVC 2005 and earlier
#if _MSC_VER >= 1400
/*!
* This macro pastes a specialization of builtin_add_overflow using MSVC's
* U(Int/Long/LongLong)Add.
*
* The add function is implemented by forwarding the parameters to the
* intrinsic. As MSVC's intrinsics return S_OK on success, this specialization
* returns whether the intrinsics return value does not equal S_OK. This ensures
* a uniform interface of the add function (false is returned when no overflow
* occurs, true on overflow).
*
* The intrinsics are documented here:
* https://msdn.microsoft.com/en-us/library/windows/desktop/ff516460(v=vs.85).aspx
*/
#define SPECIALIZE_builtin_add_overflow_WIN(type, builtin_name) \
template <> \
inline bool builtin_add_overflow(type summand_1, type summand_2, type& result) \
{ \
return builtin_name(summand_1, summand_2, &result) != S_OK; \
}
SPECIALIZE_builtin_add_overflow_WIN(unsigned int, UIntAdd);
SPECIALIZE_builtin_add_overflow_WIN(unsigned long, ULongAdd);
SPECIALIZE_builtin_add_overflow_WIN(unsigned long long, ULongLongAdd);
#undef SPECIALIZE_builtin_add_overflow_WIN
#endif // _MSC_VER >= 1400
#endif // defined(_MSC_VER)
} // namespace Internal
/*!
* @brief Safe addition, throws an exception on overflow.
*
* This function returns the result of summand_1 and summand_2 only when the
* operation would not overflow, otherwise an exception of type
* std::overflow_error is thrown.
*
* @param[in] summand_1, summand_2 summands to be summed up
* @return the sum of summand_1 and summand_2
* @throws std::overflow_error if the addition would overflow
*
* This function utilizes compiler builtins when available and should have a
* very small performance hit then. When builtins are unavailable, a more
* extensive check is required.
*
* Builtins are available for the following configurations:
* - GCC/Clang for signed and unsigned int, long and long long (not char & short)
* - MSVC for unsigned int, long and long long
*/
template <typename T>
T add(T summand_1, T summand_2)
{
T res = 0;
if (Internal::builtin_add_overflow(summand_1, summand_2, res)) {
throw std::overflow_error("Overflow in addition");
}
return res;
}
/*!
* @brief Calculates the absolute value of a number without producing
* negative values.
*
* The "standard" implementation of `abs(num)` (`num < 0 ? -num : num`)
* produces negative values when `num` is the smallest negative number. This
* is caused by `-1 * INTMAX = INTMIN + 1`, i.e. the real result of
* `abs(INTMIN)` overflows the integer type and results in `INTMIN` again
* (this is not guaranteed as it invokes undefined behavior).
*
* This function does not exhibit this behavior, it returns
* `std::numeric_limits<T>::max()` when the input is
* `std::numeric_limits<T>::min()`. The downside of this is that two
* negative values produce the same absolute value:
* `std::numeric_limits<T>::min()` and `std::numeric_limits<T>::min() + 1`.
*
* @tparam T a signed integer type
* @param[in] num The number which absolute value should be computed.
* @throws Never throws an exception.
* @return The absolute value of `num` or `std::numeric_limits<T>::max()`
* when `num == std::numeric_limits<T>::min()`.
*/
template <typename T>
typename Internal::enable_if<Internal::is_signed<T>::VALUE, T>::type abs(T num) throw()
{
if (num == std::numeric_limits<T>::min()) {
return std::numeric_limits<T>::max();
}
return num < 0 ? -num : num;
}
} // namespace Safe
#endif // SAFE_OP_HPP_