[PATCH v2 2/4] libstdc++: Add _Streambuf_sink for direct streambuf formatting
Anlai Lu
agicy@qq.com
Wed Aug 12 03:05:56 GMT 2026
> On Aug 11, 2026, at 16:35, Tomasz Kaminski <tkaminsk@redhat.com> wrote:
>
>
>
> On Mon, Aug 10, 2026 at 6:29 PM Tomasz Kaminski <tkaminsk@redhat.com <mailto:tkaminsk@redhat.com>> wrote:
>>
>>
>> On Mon, Aug 10, 2026 at 5:40 PM Tomasz Kamiński <tkaminsk@redhat.com <mailto:tkaminsk@redhat.com>> wrote:
>>> From: Anlai Lu <agicy@qq.com <mailto:agicy@qq.com>>
>>>
>>> Introduce _Streambuf_sink that writes directly to basic_streambuf
>>> via sputn, preferring zero-copy writes into the streambuf put area
>>> (pptr/epptr/pbump) and falling back to the stack buffer and bulk
>>> sputn.
>>>
>>> Streambuf I/O errors are tracked internally (_M_write_failed) and
>>> reported after formatting completes, so that format-phase exceptions
>>> (format_error, bad_alloc) propagate without setting badbit per
>>> [ostream.formatted.print]/(4.2).
>>>
>>> libstdc++-v3/ChangeLog:
>>>
>>> * include/std/format (__format::_Streambuf_sink): New class
>>> template.
>>> * include/std/streambuf: Include bits/version.h.
>>> (__format::_Streambuf_sink) [__glibcxx_format]: Declare.
>>> (std::basic_streambuf) [__glibcxx_format]: Befriend
>>> __format::_Streambuf_sink.
>>>
>>> Co-authored-by: Tomasz Kamiński <tkaminsk@redhat.com <mailto:tkaminsk@redhat.com>>
>>> Signed-off-by: Anlai Lu <agicy@qq.com <mailto:agicy@qq.com>>
>>> Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com <mailto:tkaminsk@redhat.com>>
>>> ---
>>> Changes in v2:
>>> - call _M_reserve_put_area in _Streambuf_sink constructor,
>>> poiting buffer to put area
>>> - ostreambuf_iterator: Remove the changes, that were unrelated
>>> to standard
>>> - remove _GLIBCXX_CONSTEXPR_FORMAT, as they classes cannot
>>> be used at compile time
>>> - remove and inline _M_pbump and _M_use_stackbuf, rename
>>> _M_use_put_area to _M_reserve_put_area
>>> - done some formatting changes
>>>
>>> Testing on powerprc64. Tested on x86_64, additionally tested
>>> *format* and *print* in all standard modes, assertions and
>>> debug. OK for trunk?
>>>
>>> libstdc++-v3/include/std/format | 110 +++++++++++++++++++++++++++++
>>> libstdc++-v3/include/std/streambuf | 9 +++
>>> 2 files changed, 119 insertions(+)
>>>
>>> diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
>>> index 729cb89ec60..9223cd2e78b 100644
>>> --- a/libstdc++-v3/include/std/format
>>> +++ b/libstdc++-v3/include/std/format
>>> @@ -3532,6 +3532,116 @@ namespace __format
>>> { }
>>> };
>>>
>>> + // A format sink that writes directly to a basic_streambuf.
>>> + // Prefers zero-copy writes into the streambuf's put area
>>> + // (pptr/epptr/pbump), falling back to the stack buffer
>>> + // (_M_buf) and bulk sputn.
>>> + template<typename _CharT, typename _Traits = char_traits<_CharT>>
>>> + class _Streambuf_sink : public _Buf_sink<_CharT>
>>> + {
>>> + protected:
>>> + basic_streambuf<_CharT, _Traits>* _M_sbuf;
>>> + bool _M_write_failed = false;
>>> +
>>> + bool
>>> + _M_on_stack() const noexcept
>>> + { return this->_M_used().data() == this->_M_buf; }
>>> +
>>> + // Try to point our span directly into the streambuf's put
>>> + // area for zero-copy writes. Caller must fall back to
>>> + // _M_use_stackbuf() on failure.
>>> + bool
>>> + _M_reserve_put_area(size_t __n = 0)
>>> + {
>>> + if (auto __p = _M_sbuf->pptr())
>>> + if (auto __e = _M_sbuf->epptr(); __e > __p)
>>> + if (size_t __a(__e - __p); __a >= __n)
>>> + {
>>> + this->_M_reset(span<_CharT>(__p, __a));
>>> + return true;
>>> + }
>>> + return false;
>>> + }
>>> +
>>> + // Write characters to the streambuf, tracking errors.
>>> + // I/O exceptions from sputn are caught and converted to
>>> + // _M_write_failed so they do not propagate as vformat exceptions
>>> + // per [ostream.formatted.print]/(4.2).
>>> + void
>>> + _M_flush()
>>> + {
>>> + span<const _CharT> __s = this->_M_used();
>>> + if (__s.empty() || _M_write_failed) [[unlikely]]
>>> + return;
>>> +
>>> + __try
>>> + {
>>> + if (!_M_on_stack())
>>> + _M_sbuf->__safe_pbump(__s.size());
>>> + else if (_M_sbuf->sputn(__s.data(), __s.size())
>>> + != static_cast<streamsize>(__s.size()))
>>> + _M_write_failed = true;
>>> + }
>>> + __catch(const __cxxabiv1::__forced_unwind&)
>>> + { throw; }
>>> + __catch(...)
>>> + { _M_write_failed = true; }
>>> + }
>>> +
>>> + void
>>> + _M_overflow() override
>>> + {
>>> + _M_flush();
>>> + if (!_M_reserve_put_area() || _M_write_failed)
>> We should check _M_write_failed first and make it unlikely.
> Changed it locally to:
> _M_overflow() override
> {
> _M_flush();
> - if (!_M_reserve_put_area() || _M_write_failed)
> - this->_M_reset(this->_M_buf);
> + if (!_M_write_failed) [[likely]]
> + if (_M_reserve_put_area())
> + return;
> + this->_M_reset(this->_M_buf);
> }
>
Done - _M_overflow now checks _M_write_failed first, per your version.
I will re-run the benchmarks after this change.
>>> + this->_M_reset(this->_M_buf);
>>> + }
>>> +
>>> + public:
>>> + [[__gnu__::__always_inline__]]
>>> + explicit
>>> + _Streambuf_sink(basic_streambuf<_CharT, _Traits>* __sbuf) noexcept
>>> + : _M_sbuf(__sbuf)
>>> + { _M_reserve_put_area(); }
>>> +
>>> + // _M_bump is not overridden: the default advances _M_next within
>>> + // the current span (stack buffer or put area). Actual commit
>>> + // to the streambuf happens via _M_flush, called from _M_overflow
>>> + // (buffer full) or _M_finish (end of formatting).
>>> +
>>> + typename _Sink<_CharT>::_Reservation
>>> + _M_reserve(size_t __n) override
>>> + {
>>> + if (__n <= this->_M_unused().size())
>>> + return { this };
>>> +
>>> + if (!this->_M_used().empty())
>>> + _M_flush();
>>> +
>>> + // Try to write directly into the streambuf's put area.
>>> + if (_M_reserve_put_area(__n))
>>> + return { this };
>>> +
>>> + // Otherwise reset to the stack buffer.
>>> + this->_M_reset(this->_M_buf);
>>> + if (__n <= this->_M_unused().size())
>>> + return { this };
>>> +
>>> + return { nullptr };
>>> + }
>>> +
>>> + bool
>>> + _M_discarding() const override
>>> + { return _M_write_failed; }
>>> +
>>> + void
>>> + _M_finish()
>>> + { _M_flush(); }
>>> +
>>> + bool
>>> + _M_failed() const noexcept
>>> + { return _M_write_failed; }
>>> + };
>>> +
>>> using _GLIBCXX_STD_C::vector;
>>>
>>> // A sink that fills a sequence (e.g. std::string, std::vector, std::deque).
>>> diff --git a/libstdc++-v3/include/std/streambuf b/libstdc++-v3/include/std/streambuf
>>> index 616e44f74a7..d273db2835f 100644
>>> --- a/libstdc++-v3/include/std/streambuf
>>> +++ b/libstdc++-v3/include/std/streambuf
>>> @@ -39,6 +39,8 @@
>>>
>>> #include <bits/requires_hosted.h> // iostreams
>>>
>>> +#include <bits/version.h>
>>> +
>>> #include <bits/c++config.h>
>>> #include <bits/iosfwd.h>
>>> #include <bits/localefwd.h>
>>> @@ -57,6 +59,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>> __copy_streambufs_eof(basic_streambuf<_CharT, _Traits>*,
>>> basic_streambuf<_CharT, _Traits>*, bool&);
>>>
>>> +#ifdef __glibcxx_format // C++ >= 20 && HOSTED
>>> + namespace __format { template<typename, typename> class _Streambuf_sink; }
>>> +#endif
>>> +
>>> /**
>>> * @brief The actual work of input and output (interface).
>>> * @ingroup io
>>> @@ -149,6 +155,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>>> friend class basic_ostream<char_type, traits_type>;
>>> friend class istreambuf_iterator<char_type, traits_type>;
>>> friend class ostreambuf_iterator<char_type, traits_type>;
>>> +#ifdef __glibcxx_format // C++ >= 20 && HOSTED
>>> + friend class __format::_Streambuf_sink<char_type, traits_type>;
>>> +#endif
>>>
>>> friend streamsize
>>> __copy_streambufs_eof<>(basic_streambuf*, basic_streambuf*, bool&);
>>> --
>>> 2.55.0
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