[libc++] Optimize ofstream::write

[philnik777]

----------------------------
Benchmark       old      new
----------------------------
bm_write    1382 ns   521 ns

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

Changes

----------------------------
Benchmark       old      new
----------------------------
bm_write    1382 ns   521 ns

---

Full diff: https://github.com/llvm/llvm-project/pull/123337.diff

6 Files Affected:

• (modified) libcxx/docs/ReleaseNotes/20.rst (+3)
• (modified) libcxx/include/fstream (+12)
• (added) libcxx/test/benchmarks/streams/ofstream.bench.cpp (+25)
• (modified) libcxx/test/libcxx/input.output/file.streams/fstreams/filebuf/traits_mismatch.verify.cpp (+1-9)
• (modified) libcxx/test/libcxx/input.output/file.streams/fstreams/traits_mismatch.verify.cpp (+1-1)
• (modified) libcxx/test/std/input.output/file.streams/fstreams/filebuf.virtuals/seekoff.pass.cpp (-1)

diff --git a/libcxx/docs/ReleaseNotes/20.rst b/libcxx/docs/ReleaseNotes/20.rst
index 2736061544c531..50ab4aad48c24f 100644
--- a/libcxx/docs/ReleaseNotes/20.rst
+++ b/libcxx/docs/ReleaseNotes/20.rst
@@ -111,6 +111,9 @@ Improvements and New Features
   std::errc::not_a_directory``, or use ``err.default_error_condition()`` to map to an ``error_condition``, and then test
   its ``value()`` and ``category()``.
 
+- ``ofstream::write`` has been optimized to pass through large strings to system calls directly instead of copying them
+  in chunks into a buffer.
+
 Deprecations and Removals
 -------------------------
 
diff --git a/libcxx/include/fstream b/libcxx/include/fstream
index f0e9425e0a53d9..491a94aca73974 100644
--- a/libcxx/include/fstream
+++ b/libcxx/include/fstream
@@ -232,6 +232,8 @@ _LIBCPP_EXPORTED_FROM_ABI void* __filebuf_windows_native_handle(FILE* __file) no
 
 template <class _CharT, class _Traits>
 class _LIBCPP_TEMPLATE_VIS basic_filebuf : public basic_streambuf<_CharT, _Traits> {
+  using __base _LIBCPP_NODEBUG = basic_streambuf<_CharT, _Traits>;
+
 public:
   typedef _CharT char_type;
   typedef _Traits traits_type;
@@ -304,6 +306,16 @@ protected:
   int sync() override;
   void imbue(const locale& __loc) override;
 
+  _LIBCPP_HIDE_FROM_ABI_VIRTUAL streamsize xsputn(const char_type* __str, streamsize __len) override {
+    if (__always_noconv_ && __len >= (this->epptr() - this->pbase())) {
+      if (traits_type::eq_int_type(overflow(), traits_type::eof()))
+        return 0;
+
+      return std::fwrite(__str, sizeof(char_type), __len, __file_);
+    }
+    return __base::xsputn(__str, __len);
+  }
+
 private:
   char* __extbuf_;
   const char* __extbufnext_;
diff --git a/libcxx/test/benchmarks/streams/ofstream.bench.cpp b/libcxx/test/benchmarks/streams/ofstream.bench.cpp
new file mode 100644
index 00000000000000..60606a9d67e2f5
--- /dev/null
+++ b/libcxx/test/benchmarks/streams/ofstream.bench.cpp
@@ -0,0 +1,25 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <fstream>
+#include <vector>
+
+#include <benchmark/benchmark.h>
+
+static void bm_write(benchmark::State& state) {
+  std::vector<char> buffer;
+  buffer.resize(16384);
+
+  std::ofstream stream("/dev/null");
+
+  for (auto _ : state)
+    stream.write(buffer.data(), buffer.size());
+}
+BENCHMARK(bm_write);
+
+BENCHMARK_MAIN();
diff --git a/libcxx/test/libcxx/input.output/file.streams/fstreams/filebuf/traits_mismatch.verify.cpp b/libcxx/test/libcxx/input.output/file.streams/fstreams/filebuf/traits_mismatch.verify.cpp
index 37ab176ea26a0f..9df0cffa1d441a 100644
--- a/libcxx/test/libcxx/input.output/file.streams/fstreams/filebuf/traits_mismatch.verify.cpp
+++ b/libcxx/test/libcxx/input.output/file.streams/fstreams/filebuf/traits_mismatch.verify.cpp
@@ -21,12 +21,4 @@
 
 std::basic_filebuf<char, std::char_traits<wchar_t> > f;
 // expected-error-re@streambuf:* {{static assertion failed{{.*}}traits_type::char_type must be the same type as CharT}}
-// expected-error@fstream:* {{only virtual member functions can be marked 'override'}}
-// expected-error@fstream:* {{only virtual member functions can be marked 'override'}}
-// expected-error@fstream:* {{only virtual member functions can be marked 'override'}}
-// expected-error@fstream:* {{only virtual member functions can be marked 'override'}}
-// expected-error@fstream:* {{only virtual member functions can be marked 'override'}}
-// expected-error@fstream:* {{only virtual member functions can be marked 'override'}}
-// expected-error@fstream:* {{only virtual member functions can be marked 'override'}}
-// expected-error@fstream:* {{only virtual member functions can be marked 'override'}}
-// expected-error@fstream:* {{only virtual member functions can be marked 'override'}}
+// expected-error@*:* 10 {{only virtual member functions can be marked 'override'}}
diff --git a/libcxx/test/libcxx/input.output/file.streams/fstreams/traits_mismatch.verify.cpp b/libcxx/test/libcxx/input.output/file.streams/fstreams/traits_mismatch.verify.cpp
index f936d8db47af5b..007af7004dfc62 100644
--- a/libcxx/test/libcxx/input.output/file.streams/fstreams/traits_mismatch.verify.cpp
+++ b/libcxx/test/libcxx/input.output/file.streams/fstreams/traits_mismatch.verify.cpp
@@ -23,7 +23,7 @@ std::basic_fstream<char, std::char_traits<wchar_t> > f;
 // expected-error-re@ios:* {{static assertion failed{{.*}}traits_type::char_type must be the same type as CharT}}
 // expected-error-re@streambuf:* {{static assertion failed{{.*}}traits_type::char_type must be the same type as CharT}}
 
-// expected-error@*:* 11 {{only virtual member functions can be marked 'override'}}
+// expected-error@*:* 12 {{only virtual member functions can be marked 'override'}}
 
 // FIXME: As of commit r324062 Clang incorrectly generates a diagnostic about mismatching
 // exception specifications for types which are already invalid for one reason or another.
diff --git a/libcxx/test/std/input.output/file.streams/fstreams/filebuf.virtuals/seekoff.pass.cpp b/libcxx/test/std/input.output/file.streams/fstreams/filebuf.virtuals/seekoff.pass.cpp
index 8008901802e91e..f6378e7998ee98 100644
--- a/libcxx/test/std/input.output/file.streams/fstreams/filebuf.virtuals/seekoff.pass.cpp
+++ b/libcxx/test/std/input.output/file.streams/fstreams/filebuf.virtuals/seekoff.pass.cpp
@@ -29,7 +29,6 @@ int main(int, char**)
                                                        | std::ios_base::trunc) != 0);
         assert(f.is_open());
         f.sputn("abcdefghijklmnopqrstuvwxyz", 26);
-        LIBCPP_ASSERT(buf[0] == 'v');
         pos_type p = f.pubseekoff(-15, std::ios_base::cur);
         assert(p == 11);
         assert(f.sgetc() == 'l');