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 4 uses _Streambuf_sink
in std::print; patch 1 adds tests.
Performance:
fmt_to (ostreambuf_iter) 4 B 16 B 64 B 256 B 1024 B 4096 B
v3: 0.97x 1.61x 4.15x 10.98x 51.97x 121.22x
print (uses _Streambuf_sink, per [ostream.formatted.print]):
4 B 16 B 64 B 256 B 1024 B 4096 B
before (ns/op): 87 114 112 118 190 286
after (ns/op): 78 78 77 86 82 134
ratio: 1.12x 1.47x 1.46x 1.37x 2.33x 2.14x
string / fmt_to_scanner unchanged (controls).
Changes in v3
==============
(Items marked with [R] were suggested by Tomasz Kaminski.)
- Extracted protected _M_flush(span) from _M_overflow so that both
_M_overflow and _M_finish share the same write path, and the
_Iter_sink specialization reuses it too. [R]
- Added _M_write_failed flag and _M_discarding() override to
_Streambuf_sink, so the format engine stops output early after a
write failure. [R]
- Changed _M_max from __diff_t to size_t with _S_no_limit sentinel. [R]
- Error propagation to the iterator moved to _Iter_sink::_M_finish
(checked just before returning std::move), so _M_overflow no longer
touches _M_out.
- Used _Streambuf_sink directly in vprint_nonunicode (patch 4),
replacing the _Str_sink + __ostream_write two-phase approach.
- Added _M_set_failed() to ostreambuf_iterator for the iterator's
error propagation path.
- Added try/catch in _M_flush to handle sputn I/O exceptions without
letting them propagate as vformat exceptions. Per
[ostream.formatted.print]/(4.2), vformat exceptions propagate without
badbit; I/O exceptions from sputn are not vformat exceptions and
are converted to _M_write_failed, then reported as badbit per (4.4).
- _M_finish() now ref-qualified (&&) on all sinks for consistency;
the print path uses std::move to call it. [R]
- Extracted _M_limit(span) helper in the _Iter_sink specialization for
shared counting/truncation between _M_overflow and _M_finish;
_M_finish now calls _M_flush directly, skipping unnecessary buffer
restoration. [R]
Not applied
============
1. Destructor flush [R]
Flush in destruction runs after the error check in print, so it
cannot help report I/O errors. And no other sink flushes in its
destructor.
2. _M_bump override [R]
The default advances _M_next within the current span; commit
happens via _M_flush. A comment explains this.
Anlai Lu (4):
libstdc++: Add tests for format_to with ostreambuf_iterator
libstdc++: Add _Streambuf_sink for direct streambuf formatting
libstdc++: Specialize _Iter_sink for ostreambuf_iterator
libstdc++: Optimize std::print using _Streambuf_sink
libstdc++-v3/include/bits/ostream_print.h | 12 +-
.../include/bits/streambuf_iterator.h | 11 +
libstdc++-v3/include/std/format | 199 ++++++++++++++
libstdc++-v3/include/std/streambuf | 4 +
.../format/functions/format_to_ostreambuf.cc | 244 ++++++++++++++++++
5 files changed, 463 insertions(+), 7 deletions(-)
create mode 100644
libstdc++-v3/testsuite/std/format/functions/format_to_ostreambuf.cc
base-commit: a0591d86bd2be838e30c132481a0b15a6ba1e112
--
2.34.1
