[libcxx-commits] [libcxx] [libc++] Optimize ofstream::write (PR #123337)

[Nikolas Klauser via libcxx-commits]

https://github.com/philnik777 created https://github.com/llvm/llvm-project/pull/123337

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


>From 502f8f2578c19820af71f639ac156528da9b8988 Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Fri, 17 Jan 2025 13:26:38 +0100
Subject: [PATCH] [libc++] Optimize ofstream::write

---
 libcxx/docs/ReleaseNotes/20.rst               |  3 +++
 libcxx/include/fstream                        | 12 +++++++++
 .../benchmarks/streams/ofstream.bench.cpp     | 25 +++++++++++++++++++
 3 files changed, 40 insertions(+)
 create mode 100644 libcxx/test/benchmarks/streams/ofstream.bench.cpp

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..61cf396a661822 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 = 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_;
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();