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/include/__system_error/system_error.h
===================================================================
--- libcxx/include/__system_error/system_error.h
+++ libcxx/include/__system_error/system_error.h
@@ -36,9 +36,6 @@
   ~system_error() _NOEXCEPT override;
 
   _LIBCPP_HIDE_FROM_ABI const error_code& code() const _NOEXCEPT { return __ec_; }
-
-private:
-  static string __init(const error_code&, string);
 };
 
 _LIBCPP_NORETURN _LIBCPP_EXPORTED_FROM_ABI void __throw_system_error(int __ev, const char* __what_arg);
Index: libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.abilist
===================================================================
--- libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.abilist
+++ libcxx/lib/abi/arm64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.abilist
@@ -1062,7 +1062,6 @@
 {'is_defined': True, 'name': '__ZNSt3__112strstreambufD0Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112strstreambufD1Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112strstreambufD2Ev', 'type': 'FUNC'}
-{'is_defined': True, 'name': '__ZNSt3__112system_error6__initERKNS_10error_codeENS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112system_errorC1ENS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112system_errorC1ENS_10error_codeEPKc', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112system_errorC1ENS_10error_codeERKNS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE', 'type': 'FUNC'}
Index: libcxx/lib/abi/powerpc-ibm-aix.libcxxabi.v1.stable.exceptions.nonew.abilist
===================================================================
--- libcxx/lib/abi/powerpc-ibm-aix.libcxxabi.v1.stable.exceptions.nonew.abilist
+++ libcxx/lib/abi/powerpc-ibm-aix.libcxxabi.v1.stable.exceptions.nonew.abilist
@@ -393,7 +393,6 @@
 {'import_export': 'EXP', 'is_defined': True, 'name': '_ZNSt3__112strstreambufD0Ev', 'storage_mapping_class': 'DS', 'type': 'FUNC'}
 {'import_export': 'EXP', 'is_defined': True, 'name': '_ZNSt3__112strstreambufD1Ev', 'storage_mapping_class': 'DS', 'type': 'FUNC'}
 {'import_export': 'EXP', 'is_defined': True, 'name': '_ZNSt3__112strstreambufD2Ev', 'storage_mapping_class': 'DS', 'type': 'FUNC'}
-{'import_export': 'EXP', 'is_defined': True, 'name': '_ZNSt3__112system_error6__initERKNS_10error_codeENS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE', 'storage_mapping_class': 'DS', 'type': 'FUNC'}
 {'import_export': 'EXP', 'is_defined': True, 'name': '_ZNSt3__112system_errorC1ENS_10error_codeE', 'storage_mapping_class': 'DS', 'type': 'FUNC'}
 {'import_export': 'EXP', 'is_defined': True, 'name': '_ZNSt3__112system_errorC1ENS_10error_codeEPKc', 'storage_mapping_class': 'DS', 'type': 'FUNC'}
 {'import_export': 'EXP', 'is_defined': True, 'name': '_ZNSt3__112system_errorC1ENS_10error_codeERKNS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE', 'storage_mapping_class': 'DS', 'type': 'FUNC'}
Index: libcxx/lib/abi/powerpc64-ibm-aix.libcxxabi.v1.stable.exceptions.nonew.abilist
===================================================================
--- libcxx/lib/abi/powerpc64-ibm-aix.libcxxabi.v1.stable.exceptions.nonew.abilist
+++ libcxx/lib/abi/powerpc64-ibm-aix.libcxxabi.v1.stable.exceptions.nonew.abilist
@@ -393,7 +393,6 @@
 {'import_export': 'EXP', 'is_defined': True, 'name': '_ZNSt3__112strstreambufD0Ev', 'storage_mapping_class': 'DS', 'type': 'FUNC'}
 {'import_export': 'EXP', 'is_defined': True, 'name': '_ZNSt3__112strstreambufD1Ev', 'storage_mapping_class': 'DS', 'type': 'FUNC'}
 {'import_export': 'EXP', 'is_defined': True, 'name': '_ZNSt3__112strstreambufD2Ev', 'storage_mapping_class': 'DS', 'type': 'FUNC'}
-{'import_export': 'EXP', 'is_defined': True, 'name': '_ZNSt3__112system_error6__initERKNS_10error_codeENS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE', 'storage_mapping_class': 'DS', 'type': 'FUNC'}
 {'import_export': 'EXP', 'is_defined': True, 'name': '_ZNSt3__112system_errorC1ENS_10error_codeE', 'storage_mapping_class': 'DS', 'type': 'FUNC'}
 {'import_export': 'EXP', 'is_defined': True, 'name': '_ZNSt3__112system_errorC1ENS_10error_codeEPKc', 'storage_mapping_class': 'DS', 'type': 'FUNC'}
 {'import_export': 'EXP', 'is_defined': True, 'name': '_ZNSt3__112system_errorC1ENS_10error_codeERKNS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE', 'storage_mapping_class': 'DS', 'type': 'FUNC'}
Index: libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.abilist
===================================================================
--- libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.abilist
+++ libcxx/lib/abi/x86_64-apple-darwin.libcxxabi.v1.stable.exceptions.nonew.abilist
@@ -1062,7 +1062,6 @@
 {'is_defined': True, 'name': '__ZNSt3__112strstreambufD0Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112strstreambufD1Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112strstreambufD2Ev', 'type': 'FUNC'}
-{'is_defined': True, 'name': '__ZNSt3__112system_error6__initERKNS_10error_codeENS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112system_errorC1ENS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112system_errorC1ENS_10error_codeEPKc', 'type': 'FUNC'}
 {'is_defined': True, 'name': '__ZNSt3__112system_errorC1ENS_10error_codeERKNS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE', 'type': 'FUNC'}
Index: libcxx/lib/abi/x86_64-unknown-freebsd.libcxxabi.v1.stable.exceptions.nonew.abilist
===================================================================
--- libcxx/lib/abi/x86_64-unknown-freebsd.libcxxabi.v1.stable.exceptions.nonew.abilist
+++ libcxx/lib/abi/x86_64-unknown-freebsd.libcxxabi.v1.stable.exceptions.nonew.abilist
@@ -757,7 +757,6 @@
 {'is_defined': True, 'name': '_ZNSt3__112strstreambufD0Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112strstreambufD1Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112strstreambufD2Ev', 'type': 'FUNC'}
-{'is_defined': True, 'name': '_ZNSt3__112system_error6__initERKNS_10error_codeENS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112system_errorC1ENS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112system_errorC1ENS_10error_codeEPKc', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112system_errorC1ENS_10error_codeERKNS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE', 'type': 'FUNC'}
Index: libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.abilist
===================================================================
--- libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.abilist
+++ libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.exceptions.nonew.abilist
@@ -755,7 +755,6 @@
 {'is_defined': True, 'name': '_ZNSt3__112strstreambufD0Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112strstreambufD1Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112strstreambufD2Ev', 'type': 'FUNC'}
-{'is_defined': True, 'name': '_ZNSt3__112system_error6__initERKNS_10error_codeENS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112system_errorC1ENS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112system_errorC1ENS_10error_codeEPKc', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112system_errorC1ENS_10error_codeERKNS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE', 'type': 'FUNC'}
Index: libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.noexceptions.nonew.abilist
===================================================================
--- libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.noexceptions.nonew.abilist
+++ libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.noexceptions.nonew.abilist
@@ -727,7 +727,6 @@
 {'is_defined': True, 'name': '_ZNSt3__112strstreambufD0Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112strstreambufD1Ev', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112strstreambufD2Ev', 'type': 'FUNC'}
-{'is_defined': True, 'name': '_ZNSt3__112system_error6__initERKNS_10error_codeENS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112system_errorC1ENS_10error_codeE', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112system_errorC1ENS_10error_codeEPKc', 'type': 'FUNC'}
 {'is_defined': True, 'name': '_ZNSt3__112system_errorC1ENS_10error_codeERKNS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE', 'type': 'FUNC'}
Index: libcxx/src/system_error.cpp
===================================================================
--- libcxx/src/system_error.cpp
+++ libcxx/src/system_error.cpp
@@ -58,8 +58,8 @@
     return *this == code.category() && code.value() == condition;
 }
 
-#if !defined(_LIBCPP_HAS_NO_THREADS)
 namespace {
+#if !defined(_LIBCPP_HAS_NO_THREADS)
 
 //  GLIBC also uses 1024 as the maximum buffer size internally.
 constexpr size_t strerror_buff_size = 1024;
@@ -127,8 +127,26 @@
     return string(error_message);
 }
 #endif
+
+#endif // !defined(_LIBCPP_HAS_NO_THREADS)
+
+string make_error_str(const error_code& ec, string what_arg) {
+    if (ec) {
+        if (!what_arg.empty()) {
+            what_arg += ": ";
+        }
+        what_arg += ec.message();
+    }
+    return what_arg;
+}
+
+string make_error_str(const error_code& ec) {
+    if (ec) {
+        return ec.message();
+    }
+    return string();
+}
 } // end namespace
-#endif
 
 string
 __do_message::message(int ev) const
@@ -231,50 +249,38 @@
 
 // system_error
 
-string
-system_error::__init(const error_code& ec, string what_arg)
-{
-    if (ec)
-    {
-        if (!what_arg.empty())
-            what_arg += ": ";
-        what_arg += ec.message();
-    }
-    return what_arg;
-}
-
 system_error::system_error(error_code ec, const string& what_arg)
-    : runtime_error(__init(ec, what_arg)),
+    : runtime_error(make_error_str(ec, what_arg)),
       __ec_(ec)
 {
 }
 
 system_error::system_error(error_code ec, const char* what_arg)
-    : runtime_error(__init(ec, what_arg)),
+    : runtime_error(make_error_str(ec, what_arg)),
       __ec_(ec)
 {
 }
 
 system_error::system_error(error_code ec)
-    : runtime_error(__init(ec, "")),
+    : runtime_error(make_error_str(ec)),
       __ec_(ec)
 {
 }
 
 system_error::system_error(int ev, const error_category& ecat, const string& what_arg)
-    : runtime_error(__init(error_code(ev, ecat), what_arg)),
+    : runtime_error(make_error_str(error_code(ev, ecat), what_arg)),
       __ec_(error_code(ev, ecat))
 {
 }
 
 system_error::system_error(int ev, const error_category& ecat, const char* what_arg)
-    : runtime_error(__init(error_code(ev, ecat), what_arg)),
+    : runtime_error(make_error_str(error_code(ev, ecat), what_arg)),
       __ec_(error_code(ev, ecat))
 {
 }
 
 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))
 {
 }