[libcxx-commits] [libcxx] [libc++] Add move constructor & assignment to `exception_ptr` (PR #164281)
Adrian Vogelsgesang via libcxx-commits
libcxx-commits at lists.llvm.org
Mon Oct 27 13:26:35 PDT 2025
https://github.com/vogelsgesang updated https://github.com/llvm/llvm-project/pull/164281
>From 5527b28f53ee2bab3171f7c32152d52c21b9c960 Mon Sep 17 00:00:00 2001
From: Adrian Vogelsgesang <avogelsgesang at salesforce.com>
Date: Mon, 20 Oct 2025 16:07:33 +0000
Subject: [PATCH 1/6] [libc++] Add move constructor & assignment to
`exception_ptr`
This commit adds move constructor and move assignment to
`exception_ptr`. Adding those operators allows us to avoid unnecessary
calls to `__cxa_{inc,dec}rement_refcount`.
Performance results:
```
Benchmark Baseline Candidate Difference % Difference
------------------------------- ---------- ----------- ------------ --------------
bm_nonnull_exception_ptr 52.22 40.92 -11.31 -21.65
bm_null_exception_ptr 31.41 23.29 -8.12 -25.85
bm_optimized_null_exception_ptr 28.69 20.50 -8.19 -28.55
```
This commit does not add a `swap` specialization. Thanks to the added
move-assignment, we already save a couple of increments/decrements also
in the default `swap` implementation. The default `swap` is still not
perfect, as it calls the desctructor on `tmp`. As soon as we also
inlined the `~exception_ptr` destructor fast-path for `__ptr ==
nullptr`, the optimizer should be able to optimize the default `swap`
just as well as a specialized `swap`, though.
---
libcxx/include/__exception/exception_ptr.h | 16 ++++++++++++++++
.../support/runtime/exception_pointer_cxxabi.ipp | 4 ++++
.../runtime/exception_pointer_glibcxx.ipp | 6 ++++++
.../runtime/exception_pointer_unimplemented.ipp | 5 +++++
4 files changed, 31 insertions(+)
diff --git a/libcxx/include/__exception/exception_ptr.h b/libcxx/include/__exception/exception_ptr.h
index 796fa924be121..38c8036112be4 100644
--- a/libcxx/include/__exception/exception_ptr.h
+++ b/libcxx/include/__exception/exception_ptr.h
@@ -60,6 +60,9 @@ _LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD
class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
void* __ptr_;
+ static void __do_decrement_refcount(void* __ptr) _NOEXCEPT;
+ _LIBCPP_HIDE_FROM_ABI static void __decrement_refcount(void* __ptr) _NOEXCEPT { if (__ptr) __do_decrement_refcount(__ptr); }
+
static exception_ptr __from_native_exception_pointer(void*) _NOEXCEPT;
template <class _Ep>
@@ -75,7 +78,9 @@ 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&&) _NOEXCEPT;
exception_ptr& operator=(const exception_ptr&) _NOEXCEPT;
+ _LIBCPP_HIDE_FROM_ABI exception_ptr& operator=(exception_ptr&&) _NOEXCEPT;
~exception_ptr() _NOEXCEPT;
_LIBCPP_HIDE_FROM_ABI explicit operator bool() const _NOEXCEPT { return __ptr_ != nullptr; }
@@ -92,6 +97,17 @@ class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
friend _LIBCPP_EXPORTED_FROM_ABI void rethrow_exception(exception_ptr);
};
+_LIBCPP_HIDE_FROM_ABI inline exception_ptr::exception_ptr(exception_ptr&& __other) _NOEXCEPT : __ptr_(__other.__ptr_) {
+ __other.__ptr_ = nullptr;
+}
+
+_LIBCPP_HIDE_FROM_ABI inline exception_ptr& exception_ptr::operator=(exception_ptr&& __other) _NOEXCEPT {
+ __decrement_refcount(__ptr_);
+ __ptr_ = __other.__ptr_;
+ __other.__ptr_ = nullptr;
+ return *this;
+}
+
# if _LIBCPP_HAS_EXCEPTIONS
# if _LIBCPP_AVAILABILITY_HAS_INIT_PRIMARY_EXCEPTION
template <class _Ep>
diff --git a/libcxx/src/support/runtime/exception_pointer_cxxabi.ipp b/libcxx/src/support/runtime/exception_pointer_cxxabi.ipp
index 8f5c2060bb06c..a10c06457a998 100644
--- a/libcxx/src/support/runtime/exception_pointer_cxxabi.ipp
+++ b/libcxx/src/support/runtime/exception_pointer_cxxabi.ipp
@@ -13,6 +13,10 @@
namespace std {
+void exception_ptr::__do_decrement_refcount(void* __ptr) noexcept {
+ __cxa_decrement_exception_refcount(__ptr);
+}
+
exception_ptr::~exception_ptr() noexcept { __cxa_decrement_exception_refcount(__ptr_); }
exception_ptr::exception_ptr(const exception_ptr& other) noexcept : __ptr_(other.__ptr_) {
diff --git a/libcxx/src/support/runtime/exception_pointer_glibcxx.ipp b/libcxx/src/support/runtime/exception_pointer_glibcxx.ipp
index 174b44ce0e6f7..c10004ead4f7c 100644
--- a/libcxx/src/support/runtime/exception_pointer_glibcxx.ipp
+++ b/libcxx/src/support/runtime/exception_pointer_glibcxx.ipp
@@ -27,12 +27,18 @@ struct exception_ptr {
exception_ptr(const exception_ptr&) noexcept;
exception_ptr& operator=(const exception_ptr&) noexcept;
~exception_ptr() noexcept;
+
+ void _M_release() noexcept;
};
} // namespace __exception_ptr
[[noreturn]] void rethrow_exception(__exception_ptr::exception_ptr);
+void exception_ptr::__do_decrement_refcount(void* __ptr) noexcept {
+ reinterpret_cast<__exception_ptr::exception_ptr*>(this)->_M_release();
+}
+
exception_ptr::~exception_ptr() noexcept { reinterpret_cast<__exception_ptr::exception_ptr*>(this)->~exception_ptr(); }
exception_ptr::exception_ptr(const exception_ptr& other) noexcept : __ptr_(other.__ptr_) {
diff --git a/libcxx/src/support/runtime/exception_pointer_unimplemented.ipp b/libcxx/src/support/runtime/exception_pointer_unimplemented.ipp
index 05a71ce34e5ac..fdf2076c64386 100644
--- a/libcxx/src/support/runtime/exception_pointer_unimplemented.ipp
+++ b/libcxx/src/support/runtime/exception_pointer_unimplemented.ipp
@@ -11,6 +11,11 @@
namespace std {
+void exception_ptr::__do_decrement_refcount(void* __ptr) noexcept {
+#warning exception_ptr not yet implemented
+ __libcpp_verbose_abort("exception_ptr not yet implemented\n");
+}
+
exception_ptr::~exception_ptr() noexcept {
#warning exception_ptr not yet implemented
__libcpp_verbose_abort("exception_ptr not yet implemented\n");
>From 36757da3a161148b953ef65ab2fdd635ace5d1f4 Mon Sep 17 00:00:00 2001
From: Adrian Vogelsgesang <avogelsgesang at salesforce.com>
Date: Tue, 21 Oct 2025 11:50:47 +0000
Subject: [PATCH 2/6] Implement using `swap`
---
libcxx/include/__exception/exception_ptr.h | 15 +++++++++------
.../support/runtime/exception_pointer_cxxabi.ipp | 4 ----
.../support/runtime/exception_pointer_glibcxx.ipp | 4 ----
.../runtime/exception_pointer_unimplemented.ipp | 5 -----
4 files changed, 9 insertions(+), 19 deletions(-)
diff --git a/libcxx/include/__exception/exception_ptr.h b/libcxx/include/__exception/exception_ptr.h
index 38c8036112be4..ca02a079642c5 100644
--- a/libcxx/include/__exception/exception_ptr.h
+++ b/libcxx/include/__exception/exception_ptr.h
@@ -16,6 +16,7 @@
#include <__memory/construct_at.h>
#include <__type_traits/decay.h>
#include <__type_traits/is_pointer.h>
+#include <__utility/move.h>
#include <cstdlib>
#include <typeinfo>
@@ -60,9 +61,6 @@ _LIBCPP_BEGIN_UNVERSIONED_NAMESPACE_STD
class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
void* __ptr_;
- static void __do_decrement_refcount(void* __ptr) _NOEXCEPT;
- _LIBCPP_HIDE_FROM_ABI static void __decrement_refcount(void* __ptr) _NOEXCEPT { if (__ptr) __do_decrement_refcount(__ptr); }
-
static exception_ptr __from_native_exception_pointer(void*) _NOEXCEPT;
template <class _Ep>
@@ -93,6 +91,12 @@ class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
return !(__x == __y);
}
+ friend _LIBCPP_HIDE_FROM_ABI void swap(exception_ptr& __x, exception_ptr& __y) _NOEXCEPT {
+ void* __tmp = __x.__ptr_;
+ __x.__ptr_ = __y.__ptr_;
+ __y.__ptr_ = __tmp;
+ }
+
friend _LIBCPP_EXPORTED_FROM_ABI exception_ptr current_exception() _NOEXCEPT;
friend _LIBCPP_EXPORTED_FROM_ABI void rethrow_exception(exception_ptr);
};
@@ -102,9 +106,8 @@ _LIBCPP_HIDE_FROM_ABI inline exception_ptr::exception_ptr(exception_ptr&& __othe
}
_LIBCPP_HIDE_FROM_ABI inline exception_ptr& exception_ptr::operator=(exception_ptr&& __other) _NOEXCEPT {
- __decrement_refcount(__ptr_);
- __ptr_ = __other.__ptr_;
- __other.__ptr_ = nullptr;
+ exception_ptr __tmp(std::move(__other));
+ swap(__tmp, *this);
return *this;
}
diff --git a/libcxx/src/support/runtime/exception_pointer_cxxabi.ipp b/libcxx/src/support/runtime/exception_pointer_cxxabi.ipp
index a10c06457a998..8f5c2060bb06c 100644
--- a/libcxx/src/support/runtime/exception_pointer_cxxabi.ipp
+++ b/libcxx/src/support/runtime/exception_pointer_cxxabi.ipp
@@ -13,10 +13,6 @@
namespace std {
-void exception_ptr::__do_decrement_refcount(void* __ptr) noexcept {
- __cxa_decrement_exception_refcount(__ptr);
-}
-
exception_ptr::~exception_ptr() noexcept { __cxa_decrement_exception_refcount(__ptr_); }
exception_ptr::exception_ptr(const exception_ptr& other) noexcept : __ptr_(other.__ptr_) {
diff --git a/libcxx/src/support/runtime/exception_pointer_glibcxx.ipp b/libcxx/src/support/runtime/exception_pointer_glibcxx.ipp
index c10004ead4f7c..a5861dac3469b 100644
--- a/libcxx/src/support/runtime/exception_pointer_glibcxx.ipp
+++ b/libcxx/src/support/runtime/exception_pointer_glibcxx.ipp
@@ -35,10 +35,6 @@ struct exception_ptr {
[[noreturn]] void rethrow_exception(__exception_ptr::exception_ptr);
-void exception_ptr::__do_decrement_refcount(void* __ptr) noexcept {
- reinterpret_cast<__exception_ptr::exception_ptr*>(this)->_M_release();
-}
-
exception_ptr::~exception_ptr() noexcept { reinterpret_cast<__exception_ptr::exception_ptr*>(this)->~exception_ptr(); }
exception_ptr::exception_ptr(const exception_ptr& other) noexcept : __ptr_(other.__ptr_) {
diff --git a/libcxx/src/support/runtime/exception_pointer_unimplemented.ipp b/libcxx/src/support/runtime/exception_pointer_unimplemented.ipp
index fdf2076c64386..05a71ce34e5ac 100644
--- a/libcxx/src/support/runtime/exception_pointer_unimplemented.ipp
+++ b/libcxx/src/support/runtime/exception_pointer_unimplemented.ipp
@@ -11,11 +11,6 @@
namespace std {
-void exception_ptr::__do_decrement_refcount(void* __ptr) noexcept {
-#warning exception_ptr not yet implemented
- __libcpp_verbose_abort("exception_ptr not yet implemented\n");
-}
-
exception_ptr::~exception_ptr() noexcept {
#warning exception_ptr not yet implemented
__libcpp_verbose_abort("exception_ptr not yet implemented\n");
>From 740129e4a94d35d2439b8aef8121329dedfbf042 Mon Sep 17 00:00:00 2001
From: Adrian Vogelsgesang <avogelsgesang at salesforce.com>
Date: Tue, 21 Oct 2025 12:50:38 +0000
Subject: [PATCH 3/6] Define functions inline
---
libcxx/include/__exception/exception_ptr.h | 20 ++++++++-----------
.../runtime/exception_pointer_glibcxx.ipp | 2 --
2 files changed, 8 insertions(+), 14 deletions(-)
diff --git a/libcxx/include/__exception/exception_ptr.h b/libcxx/include/__exception/exception_ptr.h
index ca02a079642c5..45da9af66ab4c 100644
--- a/libcxx/include/__exception/exception_ptr.h
+++ b/libcxx/include/__exception/exception_ptr.h
@@ -76,9 +76,15 @@ 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&&) _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&&) _NOEXCEPT;
+ _LIBCPP_HIDE_FROM_ABI exception_ptr& operator=(exception_ptr&& __other) _NOEXCEPT {
+ exception_ptr __tmp(std::move(__other));
+ swap(__tmp, *this);
+ return *this;
+ }
~exception_ptr() _NOEXCEPT;
_LIBCPP_HIDE_FROM_ABI explicit operator bool() const _NOEXCEPT { return __ptr_ != nullptr; }
@@ -101,16 +107,6 @@ class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
friend _LIBCPP_EXPORTED_FROM_ABI void rethrow_exception(exception_ptr);
};
-_LIBCPP_HIDE_FROM_ABI inline exception_ptr::exception_ptr(exception_ptr&& __other) _NOEXCEPT : __ptr_(__other.__ptr_) {
- __other.__ptr_ = nullptr;
-}
-
-_LIBCPP_HIDE_FROM_ABI inline exception_ptr& exception_ptr::operator=(exception_ptr&& __other) _NOEXCEPT {
- exception_ptr __tmp(std::move(__other));
- swap(__tmp, *this);
- return *this;
-}
-
# if _LIBCPP_HAS_EXCEPTIONS
# if _LIBCPP_AVAILABILITY_HAS_INIT_PRIMARY_EXCEPTION
template <class _Ep>
diff --git a/libcxx/src/support/runtime/exception_pointer_glibcxx.ipp b/libcxx/src/support/runtime/exception_pointer_glibcxx.ipp
index a5861dac3469b..174b44ce0e6f7 100644
--- a/libcxx/src/support/runtime/exception_pointer_glibcxx.ipp
+++ b/libcxx/src/support/runtime/exception_pointer_glibcxx.ipp
@@ -27,8 +27,6 @@ struct exception_ptr {
exception_ptr(const exception_ptr&) noexcept;
exception_ptr& operator=(const exception_ptr&) noexcept;
~exception_ptr() noexcept;
-
- void _M_release() noexcept;
};
} // namespace __exception_ptr
>From 8d90812e90b1a8e7f49f2d15badaf672f740cd1f Mon Sep 17 00:00:00 2001
From: Adrian Vogelsgesang <avogelsgesang at salesforce.com>
Date: Mon, 27 Oct 2025 20:02:21 +0000
Subject: [PATCH 4/6] Undef `move` to fix build
---
libcxx/include/__exception/exception_ptr.h | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/libcxx/include/__exception/exception_ptr.h b/libcxx/include/__exception/exception_ptr.h
index 45da9af66ab4c..2d86a15f19ead 100644
--- a/libcxx/include/__exception/exception_ptr.h
+++ b/libcxx/include/__exception/exception_ptr.h
@@ -24,6 +24,9 @@
# pragma GCC system_header
#endif
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
#ifndef _LIBCPP_ABI_MICROSOFT
# if _LIBCPP_AVAILABILITY_HAS_INIT_PRIMARY_EXCEPTION
@@ -216,4 +219,6 @@ _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
>From b106dc9fcccdad9c8745c7602360e9d40bf04175 Mon Sep 17 00:00:00 2001
From: Adrian Vogelsgesang <avogelsgesang at salesforce.com>
Date: Mon, 27 Oct 2025 20:09:31 +0000
Subject: [PATCH 5/6] Make `swap` a non-hidden friend
---
libcxx/include/__exception/exception_ptr.h | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)
diff --git a/libcxx/include/__exception/exception_ptr.h b/libcxx/include/__exception/exception_ptr.h
index 2d86a15f19ead..d0da2a688977e 100644
--- a/libcxx/include/__exception/exception_ptr.h
+++ b/libcxx/include/__exception/exception_ptr.h
@@ -100,16 +100,18 @@ class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
return !(__x == __y);
}
- friend _LIBCPP_HIDE_FROM_ABI void swap(exception_ptr& __x, exception_ptr& __y) _NOEXCEPT {
- void* __tmp = __x.__ptr_;
- __x.__ptr_ = __y.__ptr_;
- __y.__ptr_ = __tmp;
- }
+ 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 {
+ void* __tmp = __x.__ptr_;
+ __x.__ptr_ = __y.__ptr_;
+ __y.__ptr_ = __tmp;
+}
+
# if _LIBCPP_HAS_EXCEPTIONS
# if _LIBCPP_AVAILABILITY_HAS_INIT_PRIMARY_EXCEPTION
template <class _Ep>
>From 1a26712aa3574024e70aa21af483fb3ad83e3e44 Mon Sep 17 00:00:00 2001
From: Adrian Vogelsgesang <adrian.vogelsgesang at tum.de>
Date: Mon, 27 Oct 2025 21:26:25 +0100
Subject: [PATCH 6/6] Update libcxx/include/__exception/exception_ptr.h
Co-authored-by: Nikolas Klauser <nikolasklauser at berlin.de>
---
libcxx/include/__exception/exception_ptr.h | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)
diff --git a/libcxx/include/__exception/exception_ptr.h b/libcxx/include/__exception/exception_ptr.h
index d0da2a688977e..73e5027b33b80 100644
--- a/libcxx/include/__exception/exception_ptr.h
+++ b/libcxx/include/__exception/exception_ptr.h
@@ -107,9 +107,7 @@ class _LIBCPP_EXPORTED_FROM_ABI exception_ptr {
};
inline _LIBCPP_HIDE_FROM_ABI void swap(exception_ptr& __x, exception_ptr& __y) _NOEXCEPT {
- void* __tmp = __x.__ptr_;
- __x.__ptr_ = __y.__ptr_;
- __y.__ptr_ = __tmp;
+ swap(__x.__ptr_, __y.__ptr_);
}
# if _LIBCPP_HAS_EXCEPTIONS
More information about the libcxx-commits
mailing list