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[libc++][format][2/7] Optimizes c-string arguments. #101805

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merged 1 commit into from
Oct 6, 2024
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[libc++][format][2/7] Optimizes c-string arguments. #101805

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32 changes: 7 additions & 25 deletions libcxx/include/__format/formatter_string.h
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Original file line number Diff line number Diff line change
Expand Up @@ -64,32 +64,14 @@ struct _LIBCPP_TEMPLATE_VIS formatter<const _CharT*, _CharT> : public __formatte
template <class _FormatContext>
_LIBCPP_HIDE_FROM_ABI typename _FormatContext::iterator format(const _CharT* __str, _FormatContext& __ctx) const {
_LIBCPP_ASSERT_INTERNAL(__str, "The basic_format_arg constructor should have prevented an invalid pointer.");

__format_spec::__parsed_specifications<_CharT> __specs = _Base::__parser_.__get_parsed_std_specifications(__ctx);
# if _LIBCPP_STD_VER >= 23
if (_Base::__parser_.__type_ == __format_spec::__type::__debug)
return __formatter::__format_escaped_string(basic_string_view<_CharT>{__str}, __ctx.out(), __specs);
# endif

// When using a center or right alignment and the width option the length
// of __str must be known to add the padding upfront. This case is handled
// by the base class by converting the argument to a basic_string_view.
// Converting the input to a basic_string_view means the data is looped over twice;
// - once to determine the length, and
// - once to process the data.
//
// When using left alignment and the width option the padding is added
// after outputting __str so the length can be determined while outputting
// __str. The same holds true for the precision, during outputting __str it
// can be validated whether the precision threshold has been reached. For
// now these optimizations aren't implemented. Instead the base class
// handles these options.
// TODO FMT Implement these improvements.
if (__specs.__has_width() || __specs.__has_precision())
return __formatter::__write_string(basic_string_view<_CharT>{__str}, __ctx.out(), __specs);

// No formatting required, copy the string to the output.
auto __out_it = __ctx.out();
while (*__str)
*__out_it++ = *__str++;
return __out_it;
// This sounds slower than writing the output directly. However internally
// the output algorithms have optimizations for "bulk" operations, which
// makes this faster than a single-pass character-by-character output.
return _Base::format(basic_string_view<_CharT>(__str), __ctx);
}
};

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