[libcxx-commits] [libcxx] [libcxx] Avoid __cxa_atexit with -O0 (PR #89882)

[via libcxx-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: Vitaly Buka (vitalybuka)

<details>
<summary>Changes</summary>

Empty destructor will register __cxa_atexit.
https://gcc.godbolt.org/z/hce587b65

We can not remove the destructor with union where
_Tp can have non-trivial destructor.

But we can remove destructor if we use in-place
new instead of union.
https://gcc.godbolt.org/z/Yqxx57eEd - empty even with -O0.


---
Full diff: https://github.com/llvm/llvm-project/pull/89882.diff


1 Files Affected:

- (modified) libcxx/include/__utility/no_destroy.h (+10-18) 


``````````diff
diff --git a/libcxx/include/__utility/no_destroy.h b/libcxx/include/__utility/no_destroy.h
index f9c1eb7bed4569..833e16116c42d7 100644
--- a/libcxx/include/__utility/no_destroy.h
+++ b/libcxx/include/__utility/no_destroy.h
@@ -29,33 +29,25 @@ struct __uninitialized_tag {};
 // initialization using __emplace.
 template <class _Tp>
 struct __no_destroy {
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI explicit __no_destroy(__uninitialized_tag) : __dummy_() {
-    if (__libcpp_is_constant_evaluated()) {
-      __dummy_ = char();
-    }
-  }
-  _LIBCPP_HIDE_FROM_ABI ~__no_destroy() {
-    // nothing
-  }
+  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI explicit __no_destroy(__uninitialized_tag) {}
 
   template <class... _Args>
-  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI explicit __no_destroy(_Args&&... __args)
-      : __obj_(std::forward<_Args>(__args)...) {}
+  _LIBCPP_CONSTEXPR _LIBCPP_HIDE_FROM_ABI explicit __no_destroy(_Args&&... __args) {
+    new (&__obj_) _Tp(std::forward<_Args>(__args)...);
+  }
 
   template <class... _Args>
   _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI _Tp& __emplace(_Args&&... __args) {
-    new (&__obj_) _Tp(std::forward<_Args>(__args)...);
-    return __obj_;
+    return *(new (&__obj_) _Tp(std::forward<_Args>(__args)...));
   }
 
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI _Tp& __get() { return __obj_; }
-  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI _Tp const& __get() const { return __obj_; }
+  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI _Tp& __get() { return *reinterpret_cast<_Tp*>(__obj_); }
+  _LIBCPP_CONSTEXPR_SINCE_CXX14 _LIBCPP_HIDE_FROM_ABI _Tp const& __get() const {
+    return *reinterpret_cast<const _Tp*>(__obj_);
+  }
 
 private:
-  union {
-    _Tp __obj_;
-    char __dummy_; // so we can initialize a member even with __uninitialized_tag for constexpr-friendliness
-  };
+  alignas(_Tp) unsigned char __obj_[sizeof(_Tp)] = {};
 };
 
 _LIBCPP_END_NAMESPACE_STD

``````````

</details>


https://github.com/llvm/llvm-project/pull/89882