`<filesystem>`: Improve `symlink_status` performance (#5071)

Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>

benchmarks/CMakeLists.txt

@@ -110,6 +110,7 @@ add_benchmark(adjacent_difference src/adjacent_difference.cpp)
 add_benchmark(bitset_from_string src/bitset_from_string.cpp)
 add_benchmark(bitset_to_string src/bitset_to_string.cpp)
 add_benchmark(efficient_nonlocking_print src/efficient_nonlocking_print.cpp)
+add_benchmark(filesystem src/filesystem.cpp)
 add_benchmark(find_and_count src/find_and_count.cpp)
 add_benchmark(find_first_of src/find_first_of.cpp)
 add_benchmark(iota src/iota.cpp)

benchmarks/src/filesystem.cpp

@@ -0,0 +1,22 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <benchmark/benchmark.h>
+#include <filesystem>
+#include <system_error>
+
+void symlink_status(benchmark::State& state) {
+    const auto path = std::filesystem::temp_directory_path();
+
+    for (auto _ : state) {
+        std::error_code ec;
+        benchmark::DoNotOptimize(path);
+        const auto status = std::filesystem::symlink_status(path, ec);
+        benchmark::DoNotOptimize(status);
+        benchmark::DoNotOptimize(ec);
+    }
+}
+
+BENCHMARK(symlink_status);
+
+BENCHMARK_MAIN();

stl/src/filesystem.cpp

@@ -900,6 +900,13 @@ namespace {
 
                 _Flags &= ~_Get_file_attributes_data;
             }
+
+            if (!_STD _Bitmask_includes_any(_Attributes, __std_fs_file_attr::_Reparse_point)
+                && _STD _Bitmask_includes_any(_Flags, __std_fs_stats_flags::_Reparse_tag)) {
+                // if reparse tag is requested by caller but the file is not a reparse point, set tag to _None
+                _Stats->_Reparse_point_tag = __std_fs_reparse_tag::_None;
+                _Flags &= ~__std_fs_stats_flags::_Reparse_tag;
+            }
         }
     }