[libcxx] Remove empty ~__no_destroy

[vitalybuka]

Primary motivation: is that after #84651 msan will
complain if fields accessed after ~__no_destroy.

My understanding of the https://eel.is/c++draft/basic.life#10
Static object with trivial destruction has program lifetime.
Static object with empty destuctor has implicit lifetime, and
accessing the object after lifetime is UB.

It was UB before #84651, it's just msan ignored union members.

Existing code with unions uses empty destructor, so accessing after
the main() can cause UB.

"placement new" version can have trivial destructor, so there is no end
of lifetime.

Secondary motivation: empty destructor will register __cxa_atexit with -O0.
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.

New test fails without the patch on
https://lab.llvm.org/buildbot/#/builders/sanitizer-x86_64-linux-bootstrap-msan

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Vitaly Buka (vitalybuka)

Changes

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 --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

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