I've pushed this to trunk. Tested x86_64-linux.
On Mon, 21 Nov 2022 at 20:44, Jonathan Wakely via Libstdc++
<libstd...@gcc.gnu.org> wrote:
>
> While finishing the time zone support for C++20, I noticed that the
> chrono::hh_mm_ss type is surprisingly large: 40 bytes. That's because
> we use duration<int64_t, ...> for each of the four members, _M_h, _M_m,
> _M_s and _M_ss. This is very wasteful. The patch below reduces it to 16
> bytes (or less for some targets) for most specializations using an
> integral type for the hh_mm_ss<D>::precision::rep type.
>
> Patrick, you added hh_mm_ss, but I assume you just implemented what is
> shown for the exposition-only members in the standard, rather than
> intentionally choosing this 40-byte representation? I don't recall
> discussing it at the time, but I might be forgetting something.
>
> Does the patch below make sense? I could remove the partial
> specialization that use a single byte for the subseconds, or only do
> that on 32-bit targets where alignof(int64_t) is 4. For targets where
> alignof(int64_t) is 8 we're always going to be 16 bytes minimum, so
> maybe there's no point squeezing everything into 12 bytes if we have a
> mandatory 4 bytes of padding.
>
> We could make those members use even less space, something like:
>
> hours _M_h;
> unsigned _M_m : 6;
> unsigned _M_s : 6;
> unsigned _M_is_neg : 1;
> unsigned _M_ss : 51; // good enough for femtoseconds
>
> This would only require 16 bytes even for hh_mm_ss<femtoseconds>, but
> requires extra instructions to read the values. That seems like the
> wrong trade off. A compact layout for hh_mm_ss<system_clock::duration>
> and hh_mm_ss<high_resolution_clock::duration> seems good enough. Paying
> extra instructions to access the members just to support incredibly high
> precision doesn't make sense to me.
>
> We could also elide the bool _M_is_neg member if _Duration::rep is
> unsigned, but it doesn't seem worth the metaprogramming/maintenance
> cost.
>
> We could consider using duration<uint_least32_t, ratio<3600>> for the
> _M_h member instead of chrono::hours (which uses int64_t). It seems
> unlikely (but not impossible) anybody will want to use hh_mm_ss for
> billions of hours past midnight. If we do that, then using a single byte
> for subseconds (when possible) does make sense, because the whole
> hh_mm_ss would fit in just 8 bytes (4 for hours, 1 for minutes, 1 for
> seconds, 1 for is_neg, and 1 for the subseconds). And then bit-fields
> start to look more appealing. Maybe just for the subseconds and is_neg
> flag, which would be 8 bytes for precision::period >= ratio<1,10'000>:
>
> uint_least32_t _M_h;
> uint_least8_t _M_m;
> uint_least8_t _M_s;
> uint_least16_t _M_ss : 15;
> uint_least16_t _M_is_neg : 1;
>
> Has anybody got any other suggestions for optimizing this or other
> chrono types? e.g. I think we should also consider using int32_t as the
> rep for chrono::{days,weeks,years} because a 64-bit type for years is
> unnecessary.
>
> It looks like other implementations don't bother trying to save space
> for chrono::hh_mm_ss, which makes me wonder if there's some reason not
> to do this. For MSVC it's only 32 bytes, which I think is because they
> use a 32-bit type for chrono::hours and chrono::minutes, not because
> they're doing anything special to make chrono::hh_mm_ss smaller.
>
>
> -- >8 --
>
> This uses a single byte for the minutes and seconds members, and places
> the bool member next to those single bytes. This means we do not need 40
> bytes to store a time that can fit in a single 8-byte integer.
>
> When there is no subsecond precision we can do away with the _M_ss
> member altogether. If the subsecond precision is coarse enough, we can
> use a smaller representation for _M_ss, e.g. hh_mm_ss<milliseconds> only
> needs uint_least32_t for _M_ss, and hh_mm_ss<duration<long, ratio<1,10>>
> and hh_mm_ss<duration<int8_t, nano>> each only needs a single byte. In
> the latter case the type can only ever represent up to 255ns anyway, so
> we don't need a larger representation type (in such cases, we could even
> remove the _M_h, _M_m and _M_s members, but it's a very unlikely
> scenario that isn't worth optimizing for).
>
> Except for specializations with a floating-point rep or using higher
> precision than nanoseconds, hh_mm_ss should now fit in 16 bytes, or even
> 12 bytes for x86-32 where alignof(long long) == 4.
>
> libstdc++-v3/ChangeLog:
>
> * include/std/chrono (chrono::hh_mm_ss): Do not use 64-bit
> representations for all four duration members. Reorder members.
> (hh_mm_ss::_S_fractional_width()): Optimize using countr_zero.
> (hh_mm_ss::hh_mm_ss()): Define as defaulted.
> (hh_mm_ss::hh_mm_ss(Duration)): Delegate to a new private
> constructor, instead of calling chrono::abs repeatedly.
> * testsuite/std/time/hh_mm_ss/1.cc: Check floating-point
> representations. Check default constructor. Check sizes.
> ---
> libstdc++-v3/include/std/chrono | 139 +++++++++++++-----
> libstdc++-v3/testsuite/std/time/hh_mm_ss/1.cc | 54 ++++++-
> 2 files changed, 158 insertions(+), 35 deletions(-)
>
> diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
> index 2468023f6c5..4ee163ba762 100644
> --- a/libstdc++-v3/include/std/chrono
> +++ b/libstdc++-v3/include/std/chrono
> @@ -41,6 +41,7 @@
> #include <bits/chrono.h>
>
> #if __cplusplus >= 202002L
> +# include <bit>
> # include <sstream>
> # include <string>
> # include <vector>
> @@ -2262,6 +2263,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> // HH_MM_SS
>
> + /// @cond undocumented
> namespace __detail
> {
> consteval long long
> @@ -2273,22 +2275,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> return __r;
> }
> }
> + /// @endcond
>
> + /// A duration representing a time since midnight (00h:00min:00s).
> template<typename _Duration>
> class hh_mm_ss
> {
> + static_assert( __is_duration<_Duration>::value );
> +
> private:
> - static constexpr int
> + static consteval int
> _S_fractional_width()
> {
> - int __multiplicity_2 = 0;
> - int __multiplicity_5 = 0;
> auto __den = _Duration::period::den;
> - while ((__den % 2) == 0)
> - {
> - ++__multiplicity_2;
> - __den /= 2;
> - }
> + const int __multiplicity_2 = std::__countr_zero((uintmax_t)__den);
> + __den >>= __multiplicity_2;
> + int __multiplicity_5 = 0;
> while ((__den % 5) == 0)
> {
> ++__multiplicity_5;
> @@ -2304,6 +2306,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> return __width;
> }
>
> + constexpr
> + hh_mm_ss(_Duration __d, bool __is_neg) noexcept
> + : _M_h (duration_cast<chrono::hours>(__d)),
> + _M_m (duration_cast<chrono::minutes>(__d - hours())),
> + _M_s (duration_cast<chrono::seconds>(__d - hours() - minutes())),
> + _M_is_neg(__is_neg)
> + {
> + auto __ss = __d - hours() - minutes() - seconds();
> + if constexpr (treat_as_floating_point_v<typename precision::rep>)
> + _M_ss._M_r = __ss.count();
> + else if constexpr (precision::period::den != 1)
> + _M_ss._M_r = duration_cast<precision>(__ss).count();
> + }
> +
> public:
> static constexpr unsigned fractional_width = {_S_fractional_width()};
>
> @@ -2312,28 +2328,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> chrono::seconds::rep>,
> ratio<1, __detail::__pow10(fractional_width)>>;
>
> - constexpr
> - hh_mm_ss() noexcept
> - : hh_mm_ss{_Duration::zero()}
> - { }
> + constexpr hh_mm_ss() noexcept = default;
>
> constexpr explicit
> hh_mm_ss(_Duration __d) noexcept
> - : _M_is_neg (__d < _Duration::zero()),
> - _M_h (duration_cast<chrono::hours>(abs(__d))),
> - _M_m (duration_cast<chrono::minutes>(abs(__d) - hours())),
> - _M_s (duration_cast<chrono::seconds>(abs(__d) - hours() -
> minutes()))
> - {
> - if constexpr (treat_as_floating_point_v<typename precision::rep>)
> - _M_ss = abs(__d) - hours() - minutes() - seconds();
> - else
> - _M_ss = duration_cast<precision>(abs(__d) - hours()
> - - minutes() - seconds());
> - }
> + : hh_mm_ss(chrono::abs(__d), __d < _Duration::zero())
> + { }
>
> constexpr bool
> is_negative() const noexcept
> - { return _M_is_neg; }
> + {
> + if constexpr (!__is_unsigned)
> + return _M_is_neg;
> + else
> + return false;
> + }
>
> constexpr chrono::hours
> hours() const noexcept
> @@ -2349,7 +2358,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>
> constexpr precision
> subseconds() const noexcept
> - { return _M_ss; }
> + { return static_cast<precision>(_M_ss); }
>
> constexpr explicit
> operator precision() const noexcept
> @@ -2358,20 +2367,82 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
> constexpr precision
> to_duration() const noexcept
> {
> - if (_M_is_neg)
> - return -(_M_h + _M_m + _M_s + _M_ss);
> - else
> - return _M_h + _M_m + _M_s + _M_ss;
> + if constexpr (!__is_unsigned)
> + if (_M_is_neg)
> + return -(_M_h + _M_m + _M_s + subseconds());
> + return _M_h + _M_m + _M_s + subseconds();
> }
>
> // TODO: Implement operator<<.
>
> private:
> - bool _M_is_neg;
> - chrono::hours _M_h;
> - chrono::minutes _M_m;
> - chrono::seconds _M_s;
> - precision _M_ss;
> + static constexpr bool __is_unsigned
> + = __and_v<is_integral<typename _Duration::rep>,
> + is_unsigned<typename _Duration::rep>>;
> +
> + template<typename _Ratio>
> + using __byte_duration = duration<unsigned char, _Ratio>;
> +
> + // The type of the _M_ss member that holds the subsecond precision.
> + template<typename _Dur>
> + struct __subseconds
> + {
> + typename _Dur::rep _M_r{};
> +
> + constexpr explicit
> + operator _Dur() const noexcept
> + { return _Dur(_M_r); }
> + };
> +
> + // An empty class if this precision doesn't need subseconds.
> + template<typename _Rep, typename _Period>
> + requires (!treat_as_floating_point_v<_Rep>) && (_Period::den == 1)
> + struct __subseconds<duration<_Rep, _Period>>
> + {
> + constexpr explicit
> + operator duration<_Rep, _Period>() const noexcept
> + { return {}; }
> + };
> +
> + // True if the maximum constructor argument can be represented in _Tp.
> + template<typename _Tp>
> + static constexpr bool __fits
> + = duration_values<typename _Duration::rep>::max()
> + <= duration_values<_Tp>::max();
> +
> + template<typename _Rep, typename _Period>
> + requires (!treat_as_floating_point_v<_Rep>)
> + && ratio_less_v<_Period, ratio<1, 1>>
> + && (ratio_greater_equal_v<_Period, ratio<1, 250>>
> + || __fits<unsigned char>)
> + struct __subseconds<duration<_Rep, _Period>>
> + {
> + unsigned char _M_r{};
> +
> + constexpr explicit
> + operator duration<_Rep, _Period>() const noexcept
> + { return duration<_Rep, _Period>(_M_r); }
> + };
> +
> + template<typename _Rep, typename _Period>
> + requires (!treat_as_floating_point_v<_Rep>)
> + && ratio_less_v<_Period, ratio<1, 250>>
> + && (ratio_greater_equal_v<_Period, ratio<1, 4'000'000'000>>
> + || __fits<uint_least32_t>)
> + struct __subseconds<duration<_Rep, _Period>>
> + {
> + uint_least32_t _M_r{};
> +
> + constexpr explicit
> + operator duration<_Rep, _Period>() const noexcept
> + { return duration<_Rep, _Period>(_M_r); }
> + };
> +
> + chrono::hours _M_h{};
> + __byte_duration<ratio<60>> _M_m{};
> + __byte_duration<ratio<1>> _M_s{};
> + bool _M_is_neg{};
> + __subseconds<precision> _M_ss{};
> };
>
> // 12/24 HOURS FUNCTIONS
> diff --git a/libstdc++-v3/testsuite/std/time/hh_mm_ss/1.cc
> b/libstdc++-v3/testsuite/std/time/hh_mm_ss/1.cc
> index 31dd1b2a8f3..1c63276a5f0 100644
> --- a/libstdc++-v3/testsuite/std/time/hh_mm_ss/1.cc
> +++ b/libstdc++-v3/testsuite/std/time/hh_mm_ss/1.cc
> @@ -59,5 +59,57 @@ constexpr_hh_mm_ss()
>
> static_assert(seconds{hh_mm_ss{100min}} == 100min);
>
> - // TODO: treat_as_floating_point_v
> + // treat_as_floating_point_v
> + using fseconds = duration<double, ratio<1>>;
> + constexpr hh_mm_ss<fseconds> fsec{0x123.0004p5s};
> + static_assert(std::is_same_v<hh_mm_ss<fseconds>::precision, fseconds>);
> + static_assert(fsec.hours() == 2h);
> + static_assert(fsec.minutes() == 35min);
> + static_assert(fsec.seconds() == 12s);
> + static_assert(fsec.subseconds() == 0x.0004p5s);
> + static_assert(!fsec.is_negative());
> +
> + using fminutes = duration<double, ratio<60>>;
> + constexpr hh_mm_ss<fminutes> fmin{-0x1.23p4min};
> + static_assert(std::is_same_v<hh_mm_ss<fminutes>::precision, fseconds>);
> + static_assert(fmin.hours() == 0h);
> + static_assert(fmin.minutes() == 18min);
> + static_assert(fmin.seconds() == 11s);
> + static_assert(fmin.subseconds() == 0.25s);
> + static_assert(fmin.is_negative());
> +}
> +
> +constexpr void
> +default_construction()
> +{
> + using namespace std::chrono;
> +
> + constexpr hh_mm_ss<seconds> s1;
> + static_assert(s1.to_duration() == s1.to_duration().zero());
> + constexpr hh_mm_ss<duration<char>> s2;
> + static_assert(s2.to_duration() == s2.to_duration().zero());
> + constexpr hh_mm_ss<duration<int, std::centi>> s3;
> + static_assert(s3.to_duration() == s3.to_duration().zero());
> + constexpr hh_mm_ss<duration<long long, std::femto>> s4;
> + static_assert(s4.to_duration() == s4.to_duration().zero());
> + constexpr hh_mm_ss<duration<double>> s5;
> + static_assert(s5.to_duration() == s5.to_duration().zero());
> +}
> +
> +constexpr void
> +size()
> +{
> + using namespace std::chrono;
> +
> + struct S0 { long long h; char m; char s; bool neg; };
> + static_assert(sizeof(hh_mm_ss<seconds>) == sizeof(S0));
> + struct S1 { long long h; char m; char s; bool neg; char ss; };
> + static_assert(sizeof(hh_mm_ss<duration<int, std::centi>>) == sizeof(S1));
> + struct S2 { long long h; char m, s; bool neg; int ss; };
> + static_assert(sizeof(hh_mm_ss<duration<int, std::milli>>) == sizeof(S2));
> + static_assert(sizeof(hh_mm_ss<duration<int, std::pico>>) == sizeof(S2));
> + struct S3 { long long h; char m, s; bool neg; long long ss; };
> + static_assert(sizeof(hh_mm_ss<duration<long long, std::pico>>) ==
> sizeof(S3));
> + struct S4 { long long h; char m, s; bool neg; double ss; };
> + static_assert(sizeof(hh_mm_ss<duration<double, std::micro>>) ==
> sizeof(S4));
> }
> --
> 2.38.1
>Re: [PATCH] libstdc++: Make chrono::hh_mm_ss more compact
Jonathan Wakely via Gcc-patches Tue, 06 Dec 2022 13:40:50 -0800
- [PATCH] libstdc++: Make chrono::hh_mm_ss ... Jonathan Wakely via Gcc-patches
- Re: [PATCH] libstdc++: Make chrono::... Jonathan Wakely via Gcc-patches
- Re: [PATCH] libstdc++: Make chrono::... Stephan Bergmann via Gcc-patches
- Re: [PATCH] libstdc++: Make chro... Jonathan Wakely via Gcc-patches
- Re: [PATCH] libstdc++: Make ... Jonathan Wakely via Gcc-patches
- Re: [PATCH] libstdc++: Make ... Stephan Bergmann via Gcc-patches
