[libc++] Optimize internal function in <system_error>

[diamante0018]

Summary

In the event the internal function __init is called with an empty string the code will take unnecessary extra steps, in addition, the code generated might be overall greater because, to my understanding, when initializing a string with an empty const char* "" (like in this case), the compiler might be unable to deduce the string is indeed empty at compile time and more code is generated.

The goal of this patch is to make a new internal function that will accept just an error code skipping the empty string argument. It should skip the unnecessary steps and in the event if (ec) is false, it will return an empty string using the correct ctor, avoiding any extra code generation issues.

After the conversation about this patch matured in the libcxx channel on the LLVM Discord server, the patch was analyzed quickly with "Compiler Explorer" and other tools and it was discovered that it does indeed reduce the amount of code generated when using the latest stable clang version (16) which in turn produces faster code.

This patch targets LLVM 18 as it will break the ABI by addressing https://github.com/llvm/llvm-project/issues/63985

Benchmark tests run on other machines as well show in the best case, that the new version without the extra string as an argument performs 10 times faster.
On the buildkite CI run it shows the new code takes less CPU time as well.
In conclusion, the new code should also just appear cleaner because there are fewer checks to do when there is no message.

[philnik]

This definitely needs some benchmark, or at least show the difference in code gen. I would be very surprised if the compiler isn't able to optimize this case.

[diamante0018]

After the conversation about this patch matured in the libcxx channel on the LLVM Discord server, the patch was analyzed quickly with "Compiler Explorer" and other tools and it was discovered that it does indeed reduce the amount of code generated when using the latest stable clang version (16) which in turn produces faster code.
In this updated diff, a new benchmark test file was added.

[diamante0018]

Use {} to construct system_error object.

[diamante0018]

Removed unused header include directive

[diamante0018]

Avoid breaking the ABI!

[diamante0018]

fix compilation when threads are disabled

[ldionne]

libcxx/src/system_error.cpp

53

Just to put things in perspective, system_error is an exception type. I don't think we should pursue changes just for the sake of optimizing the construction of exception types.

However, mixed with https://github.com/llvm/llvm-project/issues/63985, I think this change could be worthwhile since we would both make this faster (ok, whatever) but also remove an unused function from our ABI surface (which is nice).

So personally I think the approach we should take here is to remove __init from our ABI and replace it with a function that is internal to system_error.cpp like you did here. Removing __init from the ABI will require a bit of preparation work to make sure it's safe but that's something we can help with if you update this patch with the full changes.

Just to set expectations, I would target this for LLVM 18, not LLVM 17.

[diamante0018]

As suggested, I will commit to making a diff that will address the issue with __init altogether.
Removing it should be the best option, I might need a few attempts to see how I can make CI turn green.

[diamante0018]

libcxx/src/system_error.cpp

53

Sounds good to me! I can wait until LLVM 18, no problem.
I pushed the full changes.

[diamante0018]

Try to fix tests

[ldionne]

This basically LGTM with updated changelog and release note. I did an internal check and couldn't find anything that referenced this symbol, which seems to support the reasoning that this was never used outside of the dylib.

libcxx/benchmarks/CMakeLists.txt

188

Can you please also update libcxx/lib/abi/CHANGELOG.TXT and libcxx/docs/ReleaseNotes/18.rst (in the ABI affecting changes section).

Please make sure to mention the reasoning for why this removal from the ABI should not be a breaking change!

[diamante0018]

Address review comments: Add this patch to the changelog

[diamante0018]

In D155820#4567036, @ldionne wrote:

This basically LGTM with updated changelog and release note. I did an internal check and couldn't find anything that referenced this symbol, which seems to support the reasoning that this was never used outside of the dylib.

Hi, Thanks for your guidance. This is my first interaction with the LLVM docs and changelogs so I hope I got it right. Let me know if my additions are fine, thanks.

[diamante0018]

Rebase

[diamante0018]

Thanks for approving, if you can could you land it for me?
My details are:

Edoardo Sanguineti <[e-mail]>

libcxx/benchmarks/CMakeLists.txt

set(BENCHMARK_TESTS
     format_to_n.bench.cpp
     format_to.bench.cpp
     format.bench.cpp
     formatted_size.bench.cpp
     formatter_float.bench.cpp
     formatter_int.bench.cpp
     function.bench.cpp
     join_view.bench.cpp
     lexicographical_compare_three_way.bench.cpp
     map.bench.cpp
     monotonic_buffer.bench.cpp
     ordered_set.bench.cpp
     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
     variant_visit_1.bench.cpp
     variant_visit_2.bench.cpp
     variant_visit_3.bench.cpp
     vector_operations.bench.cpp
     )

libcxx/benchmarks/system_error.bench.cpp

+//===----------------------------------------------------------------------===//
+//
+// 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();

libcxx/include/__system_error/system_error.h

public:
   system_error(error_code __ec);
   system_error(int __ev, const error_category& __ecat, const string& __what_arg);
   system_error(int __ev, const error_category& __ecat, const char* __what_arg);
   system_error(int __ev, const error_category& __ecat);
   _LIBCPP_HIDE_FROM_ABI system_error(const system_error&) _NOEXCEPT = default;
   ~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);
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP___SYSTEM_ERROR_SYSTEM_ERROR_H

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 No newline at end of file

libcxx/lib/abi/x86_64-unknown-linux-gnu.libcxxabi.v1.stable.noexceptions.nonew.abilist

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 No newline at end of file

libcxx/src/system_error.cpp

 {
     return error_condition(ev, *this);
 }
 
 bool
 error_category::equivalent(int code, const error_condition& condition) const noexcept
 {
     return default_error_condition(code) == condition;
 }
 
 bool
 error_category::equivalent(const error_code& code, int condition) const noexcept
 {
     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;
 
 string do_strerror_r(int ev);
 
 #if defined(_LIBCPP_MSVCRT_LIKE)
 string do_strerror_r(int ev) {
string do_strerror_r(int ev) {
     if (!error_message[0]) {
       std::snprintf(buffer, strerror_buff_size, "Unknown error %d", ev);
       error_message = buffer;
     }
     errno = old_errno;
     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
 {
 #if defined(_LIBCPP_HAS_NO_THREADS)
     return string(::strerror(ev));
 #else
     return do_strerror_r(ev);

 string
 error_code::message() const
 {
     return __cat_->message(__val_);
 }
 
 // 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))
 {
 }
 
 system_error::~system_error() noexcept
 {
 }