Index: libcxx/benchmarks/CMakeLists.txt
===================================================================
--- libcxx/benchmarks/CMakeLists.txt
+++ libcxx/benchmarks/CMakeLists.txt
@@ -192,6 +192,7 @@
     std_format_spec_string_unicode.bench.cpp
     string.bench.cpp
     stringstream.bench.cpp
+    system_error.bench.cpp
     to_chars.bench.cpp
     unordered_set_operations.bench.cpp
     util_smartptr.bench.cpp
Index: libcxx/benchmarks/system_error.bench.cpp
===================================================================
--- /dev/null
+++ libcxx/benchmarks/system_error.bench.cpp
@@ -0,0 +1,30 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 <string>
+#include <system_error>
+
+#include "benchmark/benchmark.h"
+
+static void BM_SystemErrorWithMessage(benchmark::State& state) {
+  for (auto _ : state) {
+    std::error_code ec{};
+    benchmark::DoNotOptimize(std::system_error{ec, ""});
+  }
+}
+BENCHMARK(BM_SystemErrorWithMessage);
+
+static void BM_SystemErrorWithoutMessage(benchmark::State& state) {
+  for (auto _ : state) {
+    std::error_code ec{};
+    benchmark::DoNotOptimize(std::system_error{ec});
+  }
+}
+BENCHMARK(BM_SystemErrorWithoutMessage);
+
+BENCHMARK_MAIN();
Index: libcxx/src/system_error.cpp
===================================================================
--- libcxx/src/system_error.cpp
+++ libcxx/src/system_error.cpp
@@ -127,6 +127,13 @@
     return string(error_message);
 }
 #endif
+
+string make_error_str(const error_code& ec) {
+    if (ec) {
+        return ec.message();
+    }
+    return string();
+}
 } // end namespace
 #endif
 
@@ -256,7 +263,7 @@
 }
 
 system_error::system_error(error_code ec)
-    : runtime_error(__init(ec, "")),
+    : runtime_error(make_error_str(ec)),
       __ec_(ec)
 {
 }
@@ -274,7 +281,7 @@
 }
 
 system_error::system_error(int ev, const error_category& ecat)
-    : runtime_error(__init(error_code(ev, ecat), "")),
+    : runtime_error(make_error_str(error_code(ev, ecat))),
       __ec_(error_code(ev, ecat))
 {
 }