On Fri, Jun 27, 2025 at 10:31 AM Tomasz Kamiński <tkami...@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
>
>
