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Cleared Coverity finding CID 170383 (INTEGER_OVERFLOW) (295) 

ModPowerOf2 was changed to include a saturating subtract. Unintuitively, the code got faster rather than slower. The saturating operation appears to have helped the optimizer
pull/301/head
Jeffrey Walton 2016-09-21 21:50:02 -04:00
parent 9b27d6b83f
commit 6bfa0d9ab0
1 changed files with 31 additions and 30 deletions
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61
misc.h
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@ -827,6 +827,35 @@ inline bool IsPowerOf2<word64>(const word64 &value)
# endif
#endif
//! \brief Performs a saturating subtract clamped at 0
//! \param a the minuend
//! \param b the subtrahend
//! \returns the difference produced by the saturating subtract
//! \details Saturating arithmetic restricts results to a fixed range. Results that are less than 0 are clamped at 0.
//! \details Use of saturating arithmetic in places can be advantageous because it can
//! avoid a branch by using an instruction like a conditional move (<tt>CMOVE</tt>).
template <class T1, class T2>
inline T1 SaturatingSubtract(const T1 &a, const T2 &b)
{
// Generated ASM of a typical clamp, http://gcc.gnu.org/ml/gcc-help/2014-10/msg00112.html
return T1((a > b) ? (a - b) : 0);
}
//! \brief Performs a saturating subtract clamped at 1
//! \param a the minuend
//! \param b the subtrahend
//! \returns the difference produced by the saturating subtract
//! \details Saturating arithmetic restricts results to a fixed range. Results that are less than
//! 1 are clamped at 1.
//! \details Use of saturating arithmetic in places can be advantageous because it can
//! avoid a branch by using an instruction like a conditional move (<tt>CMOVE</tt>).
template <class T1, class T2>
inline T1 SaturatingSubtract1(const T1 &a, const T2 &b)
{
// Generated ASM of a typical clamp, http://gcc.gnu.org/ml/gcc-help/2014-10/msg00112.html
return T1((a > b) ? (a - b) : 1);
}
//! \brief Reduces a value to a power of 2
//! \param a the first value
//! \param b the second value
@ -837,7 +866,8 @@ template <class T1, class T2>
inline T2 ModPowerOf2(const T1 &a, const T2 &b)
{
CRYPTOPP_ASSERT(IsPowerOf2(b));
return T2(a) & (b-1);
// Coverity finding CID 170383 Overflowed return value (INTEGER_OVERFLOW)
return T2(a) & SaturatingSubtract(b,1);
}
//! \brief Rounds a value down to a multiple of a second value
@ -959,35 +989,6 @@ inline bool NativeByteOrderIs(ByteOrder order)
return order == GetNativeByteOrder();
}
//! \brief Performs a saturating subtract clamped at 0
//! \param a the minuend
//! \param b the subtrahend
//! \returns the difference produced by the saturating subtract
//! \details Saturating arithmetic restricts results to a fixed range. Results that are less than 0 are clamped at 0.
//! \details Use of saturating arithmetic in places can be advantageous because it can
//! avoid a branch by using an instruction like a conditional move (<tt>CMOVE</tt>).
template <class T1, class T2>
inline T1 SaturatingSubtract(const T1 &a, const T2 &b)
{
// Generated ASM of a typical clamp, http://gcc.gnu.org/ml/gcc-help/2014-10/msg00112.html
return T1((a > b) ? (a - b) : 0);
}
//! \brief Performs a saturating subtract clamped at 1
//! \param a the minuend
//! \param b the subtrahend
//! \returns the difference produced by the saturating subtract
//! \details Saturating arithmetic restricts results to a fixed range. Results that are less than
//! 1 are clamped at 1.
//! \details Use of saturating arithmetic in places can be advantageous because it can
//! avoid a branch by using an instruction like a conditional move (<tt>CMOVE</tt>).
template <class T1, class T2>
inline T1 SaturatingSubtract1(const T1 &a, const T2 &b)
{
// Generated ASM of a typical clamp, http://gcc.gnu.org/ml/gcc-help/2014-10/msg00112.html
return T1((a > b) ? (a - b) : 1);
}
//! \brief Returns the direction the cipher is being operated
//! \param obj the cipher object being queried
//! \returns \p ENCRYPTION if the cipher obj is being operated in its forward direction,