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

[Tomasz Kamiński]

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.
---
 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