[libcxx-commits] [libcxx] 347512f - [libc++] Revert fstream::read optimizations (#168894)

[via libcxx-commits]

Author: Jordan Rupprecht
Date: 2025-11-21T07:08:15-06:00
New Revision: 347512ff38748ac6ebfacbfda172edb5cf1edbe2

URL: https://github.com/llvm/llvm-project/commit/347512ff38748ac6ebfacbfda172edb5cf1edbe2
DIFF: https://github.com/llvm/llvm-project/commit/347512ff38748ac6ebfacbfda172edb5cf1edbe2.diff

LOG: [libc++] Revert fstream::read optimizations (#168894)

This causes various runtime failures, as reported in #168628.

This reverts both #165223 and #167779

Added: 
    libcxx/test/benchmarks/streams/ofstream.bench.cpp

Modified: 
    libcxx/docs/ReleaseNotes/22.rst
    libcxx/include/fstream
    libcxx/test/libcxx/input.output/file.streams/fstreams/filebuf/traits_mismatch.verify.cpp
    libcxx/test/libcxx/input.output/file.streams/fstreams/traits_mismatch.verify.cpp

Removed: 
    libcxx/test/benchmarks/streams/fstream.bench.cpp


################################################################################
diff  --git a/libcxx/docs/ReleaseNotes/22.rst b/libcxx/docs/ReleaseNotes/22.rst
index 7af1351e8cee6..b8e0e9b5a1814 100644
--- a/libcxx/docs/ReleaseNotes/22.rst
+++ b/libcxx/docs/ReleaseNotes/22.rst
@@ -67,8 +67,8 @@ Improvements and New Features
   by up to 2.5x
 - The performance of ``erase(iterator, iterator)`` in the unordered containers has been improved by up to 1.9x
 - The performance of ``map::insert_or_assign`` has been improved by up to 2x
-- ``ofstream::write`` and ``ifstream::read`` have been optimized to pass through large reads and writes to system calls
-  directly instead of copying them in chunks.
+- ``ofstream::write`` has been optimized to pass through large strings to system calls directly instead of copying them
+  in chunks into a buffer.
 - Multiple internal types have been refactored to use ``[[no_unique_address]]``, resulting in faster compile times and
   reduced debug information.
 

diff  --git a/libcxx/include/fstream b/libcxx/include/fstream
index 90e35740c17cf..1f88d134fe061 100644
--- a/libcxx/include/fstream
+++ b/libcxx/include/fstream
@@ -308,25 +308,6 @@ protected:
     return basic_streambuf<_CharT, _Traits>::xsputn(__str, __len);
   }
 
-  _LIBCPP_HIDE_FROM_ABI_VIRTUAL streamsize xsgetn(char_type* __str, streamsize __len) override {
-    if (__always_noconv_) {
-      const streamsize __n = std::min(this->egptr() - this->gptr(), __len);
-      if (__n != 0) {
-        traits_type::copy(__str, this->gptr(), __n);
-        this->__gbump_ptr
diff (__n);
-      }
-      const streamsize __remainder    = __len - __n;
-      const streamsize __buffer_space = this->egptr() - this->eback();
-
-      if (__remainder >= __buffer_space)
-        return std::fread(__str + __n, sizeof(char_type), __remainder, __file_) + __n;
-      else if (__remainder > 0)
-        return basic_streambuf<_CharT, _Traits>::xsgetn(__str + __n, __remainder) + __n;
-      return __n;
-    }
-    return basic_streambuf<_CharT, _Traits>::xsgetn(__str, __len);
-  }
-
 private:
   char* __extbuf_;
   const char* __extbufnext_;

diff  --git a/libcxx/test/benchmarks/streams/fstream.bench.cpp b/libcxx/test/benchmarks/streams/ofstream.bench.cpp
similarity index 54%
rename from libcxx/test/benchmarks/streams/fstream.bench.cpp
rename to libcxx/test/benchmarks/streams/ofstream.bench.cpp
index 3ca1801ca8d03..60606a9d67e2f 100644
--- a/libcxx/test/benchmarks/streams/fstream.bench.cpp
+++ b/libcxx/test/benchmarks/streams/ofstream.bench.cpp
@@ -11,7 +11,7 @@
 
 #include <benchmark/benchmark.h>
 
-static void bm_ofstream_write(benchmark::State& state) {
+static void bm_write(benchmark::State& state) {
   std::vector<char> buffer;
   buffer.resize(16384);
 
@@ -20,24 +20,6 @@ static void bm_ofstream_write(benchmark::State& state) {
   for (auto _ : state)
     stream.write(buffer.data(), buffer.size());
 }
-BENCHMARK(bm_ofstream_write);
-
-static void bm_ifstream_read(benchmark::State& state) {
-  std::vector<char> buffer;
-  buffer.resize(16384);
-
-  std::ofstream gen_testfile("testfile");
-  gen_testfile.write(buffer.data(), buffer.size());
-
-  std::ifstream stream("testfile");
-  assert(stream);
-
-  for (auto _ : state) {
-    stream.read(buffer.data(), buffer.size());
-    benchmark::DoNotOptimize(buffer);
-    stream.seekg(0);
-  }
-}
-BENCHMARK(bm_ifstream_read);
+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 30e7b66d42325..283adbc057d1e 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
@@ -19,4 +19,4 @@
 
 std::basic_filebuf<char, std::char_traits<wchar_t> > f;
 // expected-error-re@*:* {{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@*:* 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 daafb36f9151a..ba6f3c31d3e34 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
@@ -21,7 +21,7 @@ std::basic_fstream<char, std::char_traits<wchar_t> > f;
 // expected-error-re@*:* {{static assertion failed{{.*}}traits_type::char_type must be the same type as CharT}}
 // expected-error-re@*:* {{static assertion failed{{.*}}traits_type::char_type must be the same type as CharT}}
 
-// expected-error@*:* 13 {{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.