[libcxx-commits] [libcxx] [libc++] Deprecate the C++20 synchronization library before C++20 (PR #86410)
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libcxx-commits at lists.llvm.org
Sat Mar 23 13:05:12 PDT 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libcxx
Author: Louis Dionne (ldionne)
<details>
<summary>Changes</summary>
When we initially implemented the C++20 synchronization library, we reluctantly accepted for the implementation to be backported to C++03 upon request from the person who provided the patch. This was when we were only starting to have experience with the issues this can create, so we flinched. Nowadays, we have a much stricter stance about not backporting features to previous standards.
We have recently started fixing several bugs (and near bugs) in our implementation of the synchronization library. A recurring theme during these reviews has been how difficult to understand the current code is, and upon inspection it becomes clear that being able to use a few recent C++ features (in particular lambdas) would help a great deal. The code would still be pretty intricate, but it would be a lot easier to reason about the flow of callbacks through things like __thread_poll_with_backoff.
As a result, this patch deprecates support for the synchronization library before C++20. In the next release, we can remove that support entirely.
---
Full diff: https://github.com/llvm/llvm-project/pull/86410.diff
8 Files Affected:
- (modified) libcxx/.clang-format (+1)
- (modified) libcxx/docs/ReleaseNotes/19.rst (+4)
- (modified) libcxx/include/__atomic/atomic.h (+8-4)
- (modified) libcxx/include/__atomic/atomic_flag.h (+10-6)
- (modified) libcxx/include/__config (+8)
- (modified) libcxx/include/barrier (+1-1)
- (modified) libcxx/include/latch (+1-1)
- (modified) libcxx/include/semaphore (+1-1)
``````````diff
diff --git a/libcxx/.clang-format b/libcxx/.clang-format
index 39ae1322ffa8a6..c37ab817bca906 100644
--- a/libcxx/.clang-format
+++ b/libcxx/.clang-format
@@ -24,6 +24,7 @@ AttributeMacros: [
'_LIBCPP_CONSTEXPR_SINCE_CXX23',
'_LIBCPP_CONSTEXPR',
'_LIBCPP_CONSTINIT',
+ '_LIBCPP_DEPRECATED_ATOMIC_SYNC',
'_LIBCPP_DEPRECATED_IN_CXX11',
'_LIBCPP_DEPRECATED_IN_CXX14',
'_LIBCPP_DEPRECATED_IN_CXX17',
diff --git a/libcxx/docs/ReleaseNotes/19.rst b/libcxx/docs/ReleaseNotes/19.rst
index cac42f9c3c3f79..1d9efa08550dc6 100644
--- a/libcxx/docs/ReleaseNotes/19.rst
+++ b/libcxx/docs/ReleaseNotes/19.rst
@@ -55,6 +55,10 @@ Improvements and New Features
Deprecations and Removals
-------------------------
+- The C++20 synchronization library (``<barrier>``, ``<latch>``, ``atomic::wait``, etc.) has been deprecated
+ in language modes prior to C++20. If you are using these features prior to C++20, please update to ``-std=c++20``.
+ In LLVM 20, the C++20 synchronization library will be removed entirely in language modes prior to C++20.
+
- TODO: The ``LIBCXX_ENABLE_ASSERTIONS`` CMake variable that was used to enable the safe mode has been deprecated and setting
it triggers an error; use the ``LIBCXX_HARDENING_MODE`` CMake variable with the value ``extensive`` instead. Similarly,
the ``_LIBCPP_ENABLE_ASSERTIONS`` macro has been deprecated (setting it to ``1`` still enables the extensive mode in
diff --git a/libcxx/include/__atomic/atomic.h b/libcxx/include/__atomic/atomic.h
index 3dfb6937d0325e..bd3f659c22df01 100644
--- a/libcxx/include/__atomic/atomic.h
+++ b/libcxx/include/__atomic/atomic.h
@@ -462,22 +462,26 @@ atomic_wait_explicit(const atomic<_Tp>* __o, typename atomic<_Tp>::value_type __
// atomic_notify_one
template <class _Tp>
-_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void atomic_notify_one(volatile atomic<_Tp>* __o) _NOEXCEPT {
+_LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
+atomic_notify_one(volatile atomic<_Tp>* __o) _NOEXCEPT {
__o->notify_one();
}
template <class _Tp>
-_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void atomic_notify_one(atomic<_Tp>* __o) _NOEXCEPT {
+_LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
+atomic_notify_one(atomic<_Tp>* __o) _NOEXCEPT {
__o->notify_one();
}
// atomic_notify_all
template <class _Tp>
-_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void atomic_notify_all(volatile atomic<_Tp>* __o) _NOEXCEPT {
+_LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
+atomic_notify_all(volatile atomic<_Tp>* __o) _NOEXCEPT {
__o->notify_all();
}
template <class _Tp>
-_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void atomic_notify_all(atomic<_Tp>* __o) _NOEXCEPT {
+_LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
+atomic_notify_all(atomic<_Tp>* __o) _NOEXCEPT {
__o->notify_all();
}
diff --git a/libcxx/include/__atomic/atomic_flag.h b/libcxx/include/__atomic/atomic_flag.h
index 084366237c16eb..f063c56cf3051e 100644
--- a/libcxx/include/__atomic/atomic_flag.h
+++ b/libcxx/include/__atomic/atomic_flag.h
@@ -49,22 +49,26 @@ struct atomic_flag {
__cxx_atomic_store(&__a_, _LIBCPP_ATOMIC_FLAG_TYPE(false), __m);
}
- _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void wait(bool __v, memory_order __m = memory_order_seq_cst) const
- volatile _NOEXCEPT {
+ _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
+ wait(bool __v, memory_order __m = memory_order_seq_cst) const volatile _NOEXCEPT {
std::__atomic_wait(*this, _LIBCPP_ATOMIC_FLAG_TYPE(__v), __m);
}
- _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
+ _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void
wait(bool __v, memory_order __m = memory_order_seq_cst) const _NOEXCEPT {
std::__atomic_wait(*this, _LIBCPP_ATOMIC_FLAG_TYPE(__v), __m);
}
- _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_one() volatile _NOEXCEPT {
+ _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_one() volatile _NOEXCEPT {
+ std::__atomic_notify_one(*this);
+ }
+ _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_one() _NOEXCEPT {
std::__atomic_notify_one(*this);
}
- _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_one() _NOEXCEPT { std::__atomic_notify_one(*this); }
_LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_all() volatile _NOEXCEPT {
std::__atomic_notify_all(*this);
}
- _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_all() _NOEXCEPT { std::__atomic_notify_all(*this); }
+ _LIBCPP_DEPRECATED_ATOMIC_SYNC _LIBCPP_AVAILABILITY_SYNC _LIBCPP_HIDE_FROM_ABI void notify_all() _NOEXCEPT {
+ std::__atomic_notify_all(*this);
+ }
#if _LIBCPP_STD_VER >= 20
_LIBCPP_HIDE_FROM_ABI constexpr atomic_flag() _NOEXCEPT : __a_(false) {}
diff --git a/libcxx/include/__config b/libcxx/include/__config
index 132cace3d5b2c4..27c653838ba63a 100644
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -954,6 +954,14 @@ typedef __char32_t char32_t;
# define _LIBCPP_DEPRECATED_(m)
# endif
+# if _LIBCPP_STD_VER < 20
+# define _LIBCPP_DEPRECATED_ATOMIC_SYNC \
+ _LIBCPP_DEPRECATED_("The C++20 synchronization library has been deprecated prior to C++20. Please update to " \
+ "using -std=c++20 if you need to use these facilities")
+# else
+# define _LIBCPP_DEPRECATED_ATOMIC_SYNC /* nothing */
+# endif
+
# if !defined(_LIBCPP_CXX03_LANG)
# define _LIBCPP_DEPRECATED_IN_CXX11 _LIBCPP_DEPRECATED
# else
diff --git a/libcxx/include/barrier b/libcxx/include/barrier
index c5fd84b91925b1..d776078267625a 100644
--- a/libcxx/include/barrier
+++ b/libcxx/include/barrier
@@ -257,7 +257,7 @@ public:
# endif // !_LIBCPP_HAS_NO_TREE_BARRIER
template <class _CompletionF = __empty_completion>
-class barrier {
+class _LIBCPP_DEPRECATED_ATOMIC_SYNC barrier {
__barrier_base<_CompletionF> __b_;
public:
diff --git a/libcxx/include/latch b/libcxx/include/latch
index 3cc72583811434..1937617f7dcc61 100644
--- a/libcxx/include/latch
+++ b/libcxx/include/latch
@@ -66,7 +66,7 @@ _LIBCPP_PUSH_MACROS
_LIBCPP_BEGIN_NAMESPACE_STD
-class latch {
+class _LIBCPP_DEPRECATED_ATOMIC_SYNC latch {
__atomic_base<ptrdiff_t> __a_;
public:
diff --git a/libcxx/include/semaphore b/libcxx/include/semaphore
index 1375ec3f7c04b1..0b1bb9525d5f0c 100644
--- a/libcxx/include/semaphore
+++ b/libcxx/include/semaphore
@@ -127,7 +127,7 @@ private:
};
template <ptrdiff_t __least_max_value = _LIBCPP_SEMAPHORE_MAX>
-class counting_semaphore {
+class _LIBCPP_DEPRECATED_ATOMIC_SYNC counting_semaphore {
__atomic_semaphore_base __semaphore_;
public:
``````````
</details>
https://github.com/llvm/llvm-project/pull/86410
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