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.
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
