[libcxx-commits] [libcxx] f02b661 - [libc++] Add move constructor & assignment to `exception_ptr` (#164281)
via libcxx-commits
libcxx-commits at lists.llvm.org
Mon Nov 3 12:18:47 PST 2025
Author: Adrian Vogelsgesang
Date: 2025-11-03T21:18:43+01:00
New Revision: f02b661054547b423177c9498cdb554f5036a3e0
URL: https://github.com/llvm/llvm-project/commit/f02b661054547b423177c9498cdb554f5036a3e0
DIFF: https://github.com/llvm/llvm-project/commit/f02b661054547b423177c9498cdb554f5036a3e0.diff
LOG: [libc++] Add move constructor & assignment to `exception_ptr` (#164281)
This commit adds move constructor, move assignment and `swap`
to `exception_ptr`. Adding those operators allows us to avoid
unnecessary calls to `__cxa_{inc,dec}rement_refcount`.
Performance results (from libc++'s CI):
```
Benchmark Baseline Candidate Difference % Difference
------------------------------------ ---------- ----------- ------------ --------------
bm_exception_ptr_copy_assign_nonnull 9.77 9.94 0.18 1.79%
bm_exception_ptr_copy_assign_null 10.29 10.65 0.35 3.42%
bm_exception_ptr_copy_ctor_nonnull 7.02 7.01 -0.01 -0.13%
bm_exception_ptr_copy_ctor_null 10.54 10.60 0.06 0.56%
bm_exception_ptr_move_assign_nonnull 16.92 13.76 -3.16 -18.70%
bm_exception_ptr_move_assign_null 10.61 10.76 0.14 1.36%
bm_exception_ptr_move_ctor_nonnull 13.31 10.25 -3.06 -23.02%
bm_exception_ptr_move_ctor_null 10.28 7.30 -2.98 -28.95%
bm_exception_ptr_swap_nonnull 19.22 0.63 -18.59 -96.74%
bm_exception_ptr_swap_null 20.02 7.79 -12.23 -61.07%
```
As expected, the `bm_exception_ptr_copy_*` benchmarks are not influenced by
this change. `bm_exception_ptr_move_*` benefits between 18% and 30%. The
`bm_exception_ptr_swap_*` tests show the biggest improvements since multiple
calls to the copy constructor are replaced by a simple pointer swap.
While `bm_exception_ptr_move_assign_null` did not show a regression in the CI
measurements, local measurements showed a regression from 3.98 to 4.71, i.e. by
18%. This is due to the additional `__tmp` inside `operator=`. The destructor
of `__other` is a no-op after the move because `__other.__ptr` will be a
nullptr. However, the compiler does not realize this, since the destructor is
not inlined and is lacking a fast-path. As such, the swap-based implementation
leads to an additional destructor call. `bm_exception_ptr_move_assign_nonnull`
still benefits because the swap-based move constructor avoids unnecessary
__cxa_{in,de}crement_refcount calls. As soon as we inline the destructor, this
regression should disappear again.
Works towards #44892
Added:
libcxx/test/std/language.support/support.exception/propagation/exception_ptr_move_assignment.pass.cpp
libcxx/test/std/language.support/support.exception/propagation/exception_ptr_move_ctr.pass.cpp
libcxx/test/std/language.support/support.exception/propagation/exception_ptr_swap.pass.cpp
Modified:
libcxx/include/__exception/exception_ptr.h
libcxx/modules/std/exception.inc
libcxx/test/std/language.support/support.exception/propagation/exception_ptr.pass.cpp
Removed:
################################################################################
diff --git a/libcxx/include/__exception/exception_ptr.h b/libcxx/include/__exception/exception_ptr.h
index 796fa924be121..e78126ea23852 100644
--- a/libcxx/include/__exception/exception_ptr.h
+++ b/libcxx/include/__exception/exception_ptr.h
@@ -16,6 +16,8 @@
#include <__memory/construct_at.h>
#include <__type_traits/decay.h>
#include <__type_traits/is_pointer.h>
+#include <__utility/move.h>
+#include <__utility/swap.h>
#include <cstdlib>
#include <typeinfo>
@@ -23,6 +25,9 @@
# pragma GCC system_header
#endif
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
#ifndef _LIBCPP_ABI_MICROSOFT
# if _LIBCPP_AVAILABILITY_HAS_INIT_PRIMARY_EXCEPTION
@@ -57,6 +62,8 @@ _LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD
#ifndef _LIBCPP_ABI_MICROSOFT
+inline _LIBCPP_HIDE_FROM_ABI void swap(exception_ptr& __x, exception_ptr& __y) _NOEXCEPT;
+
class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
void* __ptr_;
@@ -75,7 +82,15 @@ class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
_LIBCPP_HIDE_FROM_ABI exception_ptr(nullptr_t) _NOEXCEPT : __ptr_() {}
exception_ptr(const exception_ptr&) _NOEXCEPT;
+ _LIBCPP_HIDE_FROM_ABI exception_ptr(exception_ptr&& __other) _NOEXCEPT : __ptr_(__other.__ptr_) {
+ __other.__ptr_ = nullptr;
+ }
exception_ptr& operator=(const exception_ptr&) _NOEXCEPT;
+ _LIBCPP_HIDE_FROM_ABI exception_ptr& operator=(exception_ptr&& __other) _NOEXCEPT {
+ exception_ptr __tmp(std::move(__other));
+ std::swap(__tmp, *this);
+ return *this;
+ }
~exception_ptr() _NOEXCEPT;
_LIBCPP_HIDE_FROM_ABI explicit operator bool() const _NOEXCEPT { return __ptr_ != nullptr; }
@@ -88,10 +103,16 @@ class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
return !(__x == __y);
}
+ friend _LIBCPP_HIDE_FROM_ABI void swap(exception_ptr& __x, exception_ptr& __y) _NOEXCEPT;
+
friend _LIBCPP_EXPORTED_FROM_ABI exception_ptr current_exception() _NOEXCEPT;
friend _LIBCPP_EXPORTED_FROM_ABI void rethrow_exception(exception_ptr);
};
+inline _LIBCPP_HIDE_FROM_ABI void swap(exception_ptr& __x, exception_ptr& __y) _NOEXCEPT {
+ std::swap(__x.__ptr_, __y.__ptr_);
+}
+
# if _LIBCPP_HAS_EXCEPTIONS
# if _LIBCPP_AVAILABILITY_HAS_INIT_PRIMARY_EXCEPTION
template <class _Ep>
@@ -201,4 +222,6 @@ _LIBCPP_HIDE_FROM_ABI exception_ptr make_exception_ptr(_Ep __e) _NOEXCEPT {
#endif // _LIBCPP_ABI_MICROSOFT
_LIBCPP_END_UNVERSIONED_NAMESPACE_STD
+_LIBCPP_POP_MACROS
+
#endif // _LIBCPP___EXCEPTION_EXCEPTION_PTR_H
diff --git a/libcxx/modules/std/exception.inc b/libcxx/modules/std/exception.inc
index 02b0f80190e5b..3dbc0112c15a0 100644
--- a/libcxx/modules/std/exception.inc
+++ b/libcxx/modules/std/exception.inc
@@ -18,6 +18,7 @@ export namespace std {
using std::rethrow_exception;
using std::rethrow_if_nested;
using std::set_terminate;
+ using std::swap;
using std::terminate;
using std::terminate_handler;
using std::throw_with_nested;
diff --git a/libcxx/test/std/language.support/support.exception/propagation/exception_ptr.pass.cpp b/libcxx/test/std/language.support/support.exception/propagation/exception_ptr.pass.cpp
index 0aded33e660d5..7e25d40dc8a7d 100644
--- a/libcxx/test/std/language.support/support.exception/propagation/exception_ptr.pass.cpp
+++ b/libcxx/test/std/language.support/support.exception/propagation/exception_ptr.pass.cpp
@@ -14,7 +14,6 @@
#include <exception>
#include <cassert>
-#include <type_traits>
#include "test_macros.h"
diff --git a/libcxx/test/std/language.support/support.exception/propagation/exception_ptr_move_assignment.pass.cpp b/libcxx/test/std/language.support/support.exception/propagation/exception_ptr_move_assignment.pass.cpp
new file mode 100644
index 0000000000000..6882bc6548da3
--- /dev/null
+++ b/libcxx/test/std/language.support/support.exception/propagation/exception_ptr_move_assignment.pass.cpp
@@ -0,0 +1,45 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: no-exceptions, c++03
+
+// <exception>
+
+// typedef unspecified exception_ptr;
+
+// Test the move assignment of exception_ptr
+
+#include <exception>
+#include <utility>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main(int, char**) {
+ std::exception_ptr p = std::make_exception_ptr(42);
+ std::exception_ptr p2{p};
+ assert(p2 == p);
+ // Under test: the move assignment
+ std::exception_ptr p3;
+ p3 = std::move(p2);
+ assert(p3 == p);
+// `p2` was moved from. In libc++ it will be nullptr, but
+// this is not guaranteed by the standard.
+#if defined(_LIBCPP_VERSION) && !defined(_LIBCPP_ABI_MICROSOFT)
+ assert(p2 == nullptr);
+ assert(p2 == nullptr);
+#endif
+
+ try {
+ std::rethrow_exception(p3);
+ } catch (int e) {
+ assert(e == 42);
+ }
+
+ return 0;
+}
diff --git a/libcxx/test/std/language.support/support.exception/propagation/exception_ptr_move_ctr.pass.cpp b/libcxx/test/std/language.support/support.exception/propagation/exception_ptr_move_ctr.pass.cpp
new file mode 100644
index 0000000000000..122e229fd6e47
--- /dev/null
+++ b/libcxx/test/std/language.support/support.exception/propagation/exception_ptr_move_ctr.pass.cpp
@@ -0,0 +1,43 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: no-exceptions, c++03
+
+// <exception>
+
+// typedef unspecified exception_ptr;
+
+// Test the move constructor of exception_ptr
+
+#include <exception>
+#include <utility>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main(int, char**) {
+ std::exception_ptr p = std::make_exception_ptr(42);
+ std::exception_ptr p2{p};
+ assert(p2 == p);
+ // Under test: The move constructor
+ std::exception_ptr p3{std::move(p2)};
+ assert(p3 == p);
+// `p2` was moved from. In libc++ it will be nullptr, but
+// this is not guaranteed by the standard.
+#if defined(_LIBCPP_VERSION) && !defined(_LIBCPP_ABI_MICROSOFT)
+ assert(p2 == nullptr);
+#endif
+
+ try {
+ std::rethrow_exception(p3);
+ } catch (int e) {
+ assert(e == 42);
+ }
+
+ return 0;
+}
diff --git a/libcxx/test/std/language.support/support.exception/propagation/exception_ptr_swap.pass.cpp b/libcxx/test/std/language.support/support.exception/propagation/exception_ptr_swap.pass.cpp
new file mode 100644
index 0000000000000..82b4713bed538
--- /dev/null
+++ b/libcxx/test/std/language.support/support.exception/propagation/exception_ptr_swap.pass.cpp
@@ -0,0 +1,40 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: no-exceptions
+
+// <exception>
+
+// typedef unspecified exception_ptr;
+
+// Test swapping of exception_ptr
+
+#include <exception>
+#include <utility>
+#include <cassert>
+
+#include "test_macros.h"
+
+int main(int, char**) {
+ std::exception_ptr p21 = std::make_exception_ptr(42);
+ std::exception_ptr p42 = std::make_exception_ptr(21);
+ std::swap(p42, p21);
+
+ try {
+ std::rethrow_exception(p21);
+ } catch (int e) {
+ assert(e == 21);
+ }
+ try {
+ std::rethrow_exception(p42);
+ } catch (int e) {
+ assert(e == 42);
+ }
+
+ return 0;
+}
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