[PATCH] libstdc++: Use runtime format for internal format calls in chrono [PR110739]

[Tomasz Kaminski]

On Fri, Jun 27, 2025 at 10:31 AM Tomasz Kamiński <tkaminsk@redhat.com>
wrote:

> This patch adjust all internal std::format call inside of
> __formatter_chrono,
> to use runtime format stirng and thus avoid compile time checking of
> validity
> of the format string. Majority of cases are covered by calling newly
> introduced
> _S_empty_fs() function that returns __Runtime_format_string containing
> _S_empty_spec, instead of passing later directly.
>
> In case of _M_j we use _S_str_d3 function (extracted from _S_str_d2),
> eliminating
> call to std::format outside of unlikely scenario in which day of year is
> greater
> than 1000 (this may happen for year_month_day with month greater than 12).
> In
> consequence, outside of handling subseconds, we no longer delegate to
> std::format
> or construct temporary strings, when formatting chrono types with ok()
> values.
>
>         PR libstdc++/110739
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/chrono_io.h (__formatter_chrono::_S_empty_fs):
> Define.
>         (__formatter_chrono::_S_str_d2): Use _S_str_d3 for 3+ digits.
>         (__formatter_chrono::_S_str_d3): Extracted from _S_str_d2.
>         (__formatter_chrono::_M_H_I, __formatter_chrono::_M_R_X): Replace
>         _S_empty_spec with _S_empty_fs().
>         (__formatter_chrono::_M_j): Likewise and use _S_str_d3 in common
>         case.
> ---



I do not think this buys us much, but I think it is worth doing anyway.
It also finishes my side goal, of getting rid of temporary strings, and
using local buffers,
that I applied to other specifiers in previous commits.
Tested on x86_64-linux locally. The std/time* test passed with
-D_GLIBCXX_USE_CXX11_ABI=0 and -D_GLIBCXX_DEBUG.
OK for trunk?


>  libstdc++-v3/include/bits/chrono_io.h | 27 ++++++++++++++++++++++-----
>  1 file changed, 22 insertions(+), 5 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/chrono_io.h
> b/libstdc++-v3/include/bits/chrono_io.h
> index bcfd51b9866..d6bc6c7cf2a 100644
> --- a/libstdc++-v3/include/bits/chrono_io.h
> +++ b/libstdc++-v3/include/bits/chrono_io.h
> @@ -873,6 +873,11 @@ namespace __format
>        static constexpr const _CharT* _S_minus_empty_spec = _S_chars + 17;
>        static constexpr const _CharT* _S_empty_spec = _S_chars + 18;
>
> +      [[__gnu__::__always_inline__]]
> +      static _Runtime_format_string<_CharT>
> +      _S_empty_fs()
> +      { return _Runtime_format_string<_CharT>(_S_empty_spec); }
> +
>        // Return the formatting locale.
>        template<typename _FormatContext>
>         std::locale
> @@ -1411,7 +1416,7 @@ namespace __format
>                 __i = 12;
>             }
>           else if (__i >= 100) [[unlikely]]
> -           return std::format_to(std::move(__out), _S_empty_spec, __i);
> +           return std::format_to(std::move(__out), _S_empty_fs(), __i);
>
>           return __format::__write(std::move(__out), _S_two_digits(__i));
>         }
> @@ -1425,11 +1430,15 @@ namespace __format
>           {
>             // Decimal number of days, without padding.
>             auto __d = chrono::floor<chrono::days>(__t._M_hours).count();
> -           return std::format_to(std::move(__out), _S_empty_spec, __d);
> +           return std::format_to(std::move(__out), _S_empty_fs(), __d);
>           }
>
> -         return std::format_to(std::move(__out), _GLIBCXX_WIDEN("{:03d}"),
> -                               __t._M_day_of_year.count());
> +         auto __d = __t._M_day_of_year.count();
> +         if (__d >= 1000) [[unlikely]]
> +           return std::format_to(std::move(__out), _S_empty_fs(), __d);
> +
> +         _CharT __buf[3];
> +         return __format::__write(std::move(__out), _S_str_d3(__buf,
> __d));
>         }
>
>        template<typename _FormatContext>
> @@ -1534,7 +1543,7 @@ namespace __format
>
>           if (__hi >= 100) [[unlikely]]
>             {
> -             __out = std::format_to(std::move(__out), _S_empty_spec,
> __hi);
> +             __out = std::format_to(std::move(__out), _S_empty_fs(),
> __hi);
>               __sv.remove_prefix(2);
>             }
>           else
> @@ -1772,7 +1781,15 @@ namespace __format
>        {
>         if (__n < 100) [[likely]]
>           return _S_two_digits(__n);
> +        return _S_str_d3(__buf, __n);
> +      }
>
> +      [[__gnu__::__always_inline__]]
> +      // Returns decimal representation of __n, padded to 3 digits.
> +      // Returned string_view points to __buf.
> +      static basic_string_view<_CharT>
> +      _S_str_d3(span<_CharT, 3> __buf, unsigned __n)
> +      {
>         _S_fill_two_digits(__buf.data(), __n / 10);
>         __buf[2] = _S_chars[__n % 10];
>         return __string_view(__buf.data(), 3);
> --
> 2.49.0
>
>
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