On Mon, Jul 6, 2026 at 6:29 AM Anlai Lu <[email protected]> wrote:
> This series replaces per-character sputc() with bulk sputn() and
> zero-copy put-area writes when formatting to ostreambuf_iterator.
> Patch 2 adds a _Streambuf_sink that writes directly to a streambuf;
> patch 3 specializes _Iter_sink to use it; patch 1 adds tests.
>
> The design follows review suggestions from Jonathan Wakely (PR111052)
> and Tomasz Kaminski: _Streambuf_sink is a standalone class accepting
> basic_streambuf* directly, and the _Iter_sink specialization is a
> separate commit.
>
Thank you for the patch. I have commented on the 2/3 patch. The
_Streambuf_sink
can expose _M_discarding (and I believe we should), however any _Sink use
with format_to_n cannot do so (as we need to count specialization of
output).
I believe that by moving the counting to the specialization, we can still
handle
that correctly.
Furthermore, if we use _Streambuf_sink, it does not need to count
characters.
>
> Performance (header-overlay A/B, single core, BABA interleave):
>
> scenario 16 B 64 B 256 B 1024 B 4096 B
> print(ostream, "{}", x) 0.96x 0.93x 0.94x 0.96x 0.97x
> format_to(back_inserter, ...) 1.02x 0.99x 1.00x 1.00x 1.01x
> format_to(ostreambuf_iter, ...) 1.92x 5.39x 17.52x 54.47x 115.14x
> with width/padding spec 1.09x 1.08x 1.09x 1.08x 1.27x
> multi-arg ("_{}_...") 1.16x 1.55x 2.50x 4.39x 14.13x
> append mode - 4.63x 13.69x 32.74x -
> to filebuf (/dev/null) 2.01x 5.40x 12.80x 19.73x 22.19x
> to streambuf w/o put area - - 15.95x 21.67x 28.06x
> format_to_n(ostreambuf_iter,
> max=len/4, ...) - 1.90x 4.87x 6.82x -
> max=len*2, ...) - 5.14x 17.46x 55.78x -
>
> Perf stat for the primary fmt_to path:
>
> len instr% cycles% b_IPC a_IPC branch-miss
> 16 66.5% 51.9% 2.24 2.87 44.1%
> 64 33.8% 18.8% 1.60 2.88 0.0%
> 256 12.0% 5.6% 1.36 2.92 0.0%
> 1024 4.1% 1.8% 1.35 3.06 0.0%
> 4096 1.8% 0.8% 1.33 2.93 0.0%
>
> Known limitations:
> - The print path (std::print to ostream) shows a 3-7% regression
> from <format> header inflation; it does not use _Streambuf_sink.
>
> Anlai Lu (3):
> libstdc++: Add tests for format_to with ostreambuf_iterator
> libstdc++: Add _Streambuf_sink for direct streambuf formatting
> libstdc++: Specialize _Iter_sink for ostreambuf_iterator
>
> .../include/bits/streambuf_iterator.h | 11 +
> libstdc++-v3/include/std/format | 212 ++++++++++++++++++
> libstdc++-v3/include/std/streambuf | 4 +
> .../format/functions/format_to_ostreambuf.cc | 244 +++++++++++++++++++++
> 4 files changed, 471 insertions(+)
> create mode 100644
> libstdc++-v3/testsuite/std/format/functions/format_to_ostreambuf.cc
>
> base-commit: 5be7b70e769e6e992dbf631cd37e24bd9da9e05e
> --
> 2.34.1
>
>
