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<numeric> Optimize gcd to use builtins #665

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merged 6 commits into from
Apr 11, 2020
Merged

<numeric> Optimize gcd to use builtins #665

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06e6d39
[numerics] Optimize gcd to use builtins
miscco Mar 9, 2020
d08ea4b
Fix template argument deduction for _countl_zero
miscco Apr 1, 2020
6a87c38
Use the same casts as the other bit operations
miscco Apr 2, 2020
a269a90
Update stl/inc/numeric
CaseyCarter Apr 2, 2020
c8961dd
Remove whitespace
miscco Apr 3, 2020
c833caa
_BitScanForward is countr_zero(x), not countl_zero(~x)
CaseyCarter Apr 11, 2020
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20 changes: 3 additions & 17 deletions stl/inc/bit
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Original file line number Diff line number Diff line change
Expand Up @@ -23,8 +23,6 @@ _STL_DISABLE_CLANG_WARNINGS
#undef new

_STD_BEGIN
enum class endian { little = 0, big = 1, native = little };
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#ifdef __cpp_lib_bit_cast // TRANSITION, VSO-1041044
template <class _To, class _From,
enable_if_t<conjunction_v<bool_constant<sizeof(_To) == sizeof(_From)>, is_trivially_copyable<_To>,
Expand All @@ -36,11 +34,6 @@ _NODISCARD constexpr _To bit_cast(const _From& _Val) noexcept {
#endif // TRANSITION, VSO-1041044

#ifdef __cpp_lib_bitops // TRANSITION, VSO-1020212
template <class _Ty>
inline constexpr bool _Is_standard_unsigned_integer =
_Is_any_of_v<remove_cv_t<_Ty>, unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long>;


template <class _Ty, enable_if_t<_Is_standard_unsigned_integer<_Ty>, int> = 0>
_NODISCARD constexpr int countl_zero(_Ty _Val) noexcept;

Expand Down Expand Up @@ -124,20 +117,12 @@ _NODISCARD constexpr int countl_one(const _Ty _Val) noexcept {

template <class _Ty, enable_if_t<_Is_standard_unsigned_integer<_Ty>, int> = 0>
_NODISCARD constexpr int countr_zero(const _Ty _Val) noexcept {
if (_Val == 0) {
return numeric_limits<_Ty>::digits;
}

if constexpr (sizeof(_Ty) <= sizeof(unsigned int)) {
return __builtin_ctz(_Val);
} else {
return __builtin_ctzll(_Val);
}
return _Countr_zero(_Val);
}

template <class _Ty, enable_if_t<_Is_standard_unsigned_integer<_Ty>, int> _Enabled = 0>
_NODISCARD constexpr int countr_one(const _Ty _Val) noexcept {
return _STD countr_zero(static_cast<_Ty>(~_Val));
return _Countr_zero(static_cast<_Ty>(~_Val));
}

template <class _Ty, enable_if_t<_Is_standard_unsigned_integer<_Ty>, int> _Enabled = 0>
Expand All @@ -150,6 +135,7 @@ _NODISCARD constexpr int popcount(const _Ty _Val) noexcept {
}
#endif // __cpp_lib_bitops

enum class endian { little = 0, big = 1, native = little };
_STD_END

#pragma pop_macro("new")
Expand Down
20 changes: 20 additions & 0 deletions stl/inc/limits
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Original file line number Diff line number Diff line change
Expand Up @@ -1011,6 +1011,26 @@ public:
static constexpr int min_exponent = LDBL_MIN_EXP;
static constexpr int min_exponent10 = LDBL_MIN_10_EXP;
};

#ifdef __cpp_lib_bitops // TRANSITION, VSO-1020212
template <class _Ty>
inline constexpr bool _Is_standard_unsigned_integer =
_Is_any_of_v<remove_cv_t<_Ty>, unsigned char, unsigned short, unsigned int, unsigned long, unsigned long long>;

template <class _Ty, enable_if_t<_Is_standard_unsigned_integer<_Ty>, int> = 0>
_NODISCARD constexpr int _Countr_zero(const _Ty _Val) noexcept {
if (_Val == 0) {
return numeric_limits<_Ty>::digits;
}

if constexpr (sizeof(_Ty) <= sizeof(unsigned int)) {
return __builtin_ctz(_Val);
} else {
return __builtin_ctzll(_Val);
}
}
#endif // __cpp_lib_bitops

_STD_END
#pragma pop_macro("new")
_STL_RESTORE_CLANG_WARNINGS
Expand Down
6 changes: 5 additions & 1 deletion stl/inc/numeric
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Original file line number Diff line number Diff line change
Expand Up @@ -859,7 +859,10 @@ _NODISCARD constexpr auto _Abs_u(const _Arithmetic _Val) noexcept {

// FUNCTION TEMPLATE _Stl_bitscan_forward
template <class _Unsigned>
constexpr unsigned long _Stl_bitscan_forward(_Unsigned _Mask) noexcept {
_NODISCARD constexpr unsigned long _Stl_bitscan_forward(_Unsigned _Mask) noexcept {
#ifdef __cpp_lib_bitops // TRANSITION, VSO-1020212
return static_cast<unsigned long>(_Countr_zero(_Mask));
#else // ^^^ __cpp_lib_bitops / !__cpp_lib_bitops vvv
// find the index of the least significant set bit (_BitScanForward isn't constexpr... yet :))
static_assert(is_unsigned_v<_Unsigned>, "Bitscan only works on bits");
unsigned long _Count = 0;
Expand All @@ -871,6 +874,7 @@ constexpr unsigned long _Stl_bitscan_forward(_Unsigned _Mask) noexcept {
}

return _Count;
#endif // !__cpp_lib_bitops
}

// FUNCTION TEMPLATE gcd
Expand Down