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 +1,31 @@
+//===----------------------------------------------------------------------===//
+//
+// 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 "Utilities.h"
+#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/include/__system_error/system_error.h
===================================================================
--- libcxx/include/__system_error/system_error.h
+++ libcxx/include/__system_error/system_error.h
@@ -39,6 +39,7 @@
 
 private:
   static string __init(const error_code&, string);
+  static string __init(const error_code&);
 };
 
 _LIBCPP_NORETURN _LIBCPP_EXPORTED_FROM_ABI void __throw_system_error(int __ev, const char* __what_arg);
Index: libcxx/src/system_error.cpp
===================================================================
--- libcxx/src/system_error.cpp
+++ libcxx/src/system_error.cpp
@@ -243,6 +243,16 @@
     return what_arg;
 }
 
+string
+system_error::__init(const error_code& ec)
+{
+    if (ec)
+    {
+        return ec.message();
+    }
+    return string();
+}
+
 system_error::system_error(error_code ec, const string& what_arg)
     : runtime_error(__init(ec, what_arg)),
       __ec_(ec)
@@ -256,7 +266,7 @@
 }
 
 system_error::system_error(error_code ec)
-    : runtime_error(__init(ec, "")),
+    : runtime_error(__init(ec)),
       __ec_(ec)
 {
 }
@@ -274,7 +284,7 @@
 }
 
 system_error::system_error(int ev, const error_category& ecat)
-    : runtime_error(__init(error_code(ev, ecat), "")),
+    : runtime_error(__init(error_code(ev, ecat))),
       __ec_(error_code(ev, ecat))
 {
 }