https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109162
--- Comment #25 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Tomasz Kaminski <tkami...@gcc.gnu.org>: https://gcc.gnu.org/g:01e5ef3e8b91288f5d387a27708f9f8979a50edf commit r16-142-g01e5ef3e8b91288f5d387a27708f9f8979a50edf Author: Tomasz KamiÅski <tkami...@redhat.com> Date: Wed Apr 23 13:17:09 2025 +0200 libstdc++: Minimalize temporary allocations when width is specified [PR109162] When width parameter is specified for formatting range, tuple or escaped presentation of string, we used to format characters to temporary string, and write produce sequence padded according to the spec. However, once the estimated width of formatted representation of input is larger than the value of spec width, it can be written directly to the output. This limits size of required allocation, especially for large ranges. Similarly, if precision (maximum) width is provided for string presentation, only a prefix of sequence with estimated width not greater than precision, needs to be buffered. To realize above, this commit implements a new _Padding_sink specialization. This sink holds an output iterator, a value of padding width, (optionally) maximum width and a string buffer inherited from _Str_sink. Then any incoming characters are treated in one of following ways, depending of estimated width W of written sequence: * written to string if W is smaller than padding width and maximum width (if present) * ignored, if W is greater than maximum width * written to output iterator, if W is greater than padding width The padding sink is used instead of _Str_sink in __format::__format_padded, __formatter_str::_M_format_escaped functions. Furthermore __formatter_str::_M_format implementation was reworked, to: * reduce number of instantiations by delegating to _Rg& and const _Rg& overloads, * non-debug presentation is written to _Out directly or via _Padding_sink * if maximum width is specified for debug format with non-unicode encoding, string size is limited to that number. PR libstdc++/109162 libstdc++-v3/ChangeLog: * include/bits/formatfwd.h (__simply_formattable_range): Moved from std/format. * include/std/format (__formatter_str::_format): Extracted escaped string handling to separate method... (__formatter_str::_M_format_escaped): Use __Padding_sink. (__formatter_str::_M_format): Adjusted implementation. (__formatter_str::_S_trunc): Extracted as namespace function... (__format::_truncate): Extracted from __formatter_str::_S_trunc. (__format::_Seq_sink): Removed forward declarations, made members protected and non-final. (_Seq_sink::_M_trim): Define. (_Seq_sink::_M_span): Renamed from view. (_Seq_sink::view): Returns string_view instead of span. (__format::_Str_sink): Moved after _Seq_sink. (__format::__format_padded): Use _Padding_sink. * testsuite/std/format/debug.cc: Add timeout and new tests. * testsuite/std/format/ranges/sequence.cc: Specify unicode as encoding and new tests. * testsuite/std/format/ranges/string.cc: Likewise. * testsuite/std/format/tuple.cc: Likewise. Reviewed-by: Jonathan Wakely <jwak...@redhat.com> Signed-off-by: Tomasz KamiÅski <tkami...@redhat.com>
