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[libc++] std::cmp_less and other integer comparison functions could be improved #151332

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[libc++] std::cmp_less and other integer comparison functions could be improved #151332

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[libc++] std::cmp_less and other integer comparison functions could b…
kisuhorikka Jul 30, 2025
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[libc++] std::cmp_less and other integer comparison functions could b…
kisuhorikka Jul 31, 2025
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12 changes: 12 additions & 0 deletions libcxx/include/__utility/cmp.h
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Original file line number Diff line number Diff line change
Expand Up @@ -26,10 +26,18 @@ _LIBCPP_BEGIN_NAMESPACE_STD

#if _LIBCPP_STD_VER >= 20

template <typename _Tp, typename _Ip>
inline constexpr bool __comparison_can_promote_to =
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Couldn't we use a concept here?

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The concept version was submitted several times, but it always failed the check due to timeout. The typename version, otherwise, passed without a hitch. I guess that when cmp.h is directly or indirectly referenced by other files, instantiation of the concept may take too much time.

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concepts take pretty much exactly the same time as variables. You should investigate further whether there is a problem in the compiler or something. I doubt it though.

__signed_integer<_Tp> ? sizeof(_Tp) <= sizeof(_Ip) : sizeof(_Tp) < sizeof(_Ip);

template <__signed_or_unsigned_integer _Tp, __signed_or_unsigned_integer _Up>
_LIBCPP_HIDE_FROM_ABI constexpr bool cmp_equal(_Tp __t, _Up __u) noexcept {
if constexpr (is_signed_v<_Tp> == is_signed_v<_Up>)
return __t == __u;
else if constexpr (__comparison_can_promote_to<_Tp, int> && __comparison_can_promote_to<_Up, int>)
return static_cast<int>(__t) == static_cast<int>(__u);
else if constexpr (__comparison_can_promote_to<_Tp, long long> && __comparison_can_promote_to<_Up, long long>)
return static_cast<long long>(__t) == static_cast<long long>(__u);
else if constexpr (is_signed_v<_Tp>)
return __t < 0 ? false : make_unsigned_t<_Tp>(__t) == __u;
else
Expand All @@ -45,6 +53,10 @@ template <__signed_or_unsigned_integer _Tp, __signed_or_unsigned_integer _Up>
_LIBCPP_HIDE_FROM_ABI constexpr bool cmp_less(_Tp __t, _Up __u) noexcept {
if constexpr (is_signed_v<_Tp> == is_signed_v<_Up>)
return __t < __u;
else if constexpr (__comparison_can_promote_to<_Tp, int> && __comparison_can_promote_to<_Up, int>)
return static_cast<int>(__t) < static_cast<int>(__u);
else if constexpr (__comparison_can_promote_to<_Tp, long long> && __comparison_can_promote_to<_Up, long long>)
return static_cast<long long>(__t) < static_cast<long long>(__u);
else if constexpr (is_signed_v<_Tp>)
return __t < 0 ? true : make_unsigned_t<_Tp>(__t) < __u;
else
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