[libc++] Optimize ofstream::write

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
 -------------------------
 

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 = 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 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_;

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();