Pass std::unique_ptr, std::shared_ptr and std::weak_ptr in registers when possible.

[rsmith]

Summary

Currently, these types are all always passed indirectly, because they
have non-trivial special members. With this patch, we'll pass them in
registers instead under the libc++ unstable ABI when using a recent
Clang.

This has a small impact on observable behavior: the destructor will be
run in the callee rather than in the caller, which means

1. the pointee type of unique_ptr<T> must be complete in the context of a function definition that takes such a pointer by value, and
2. the order in which destructors for parameters are run is not strictly in reverse construction order.

This still needs some performance testing and (since it's technically
non-conforming due to the above) should be put behind a flag, but I'm
mailing it out now to test the waters for this kind of change.

This depends on D63744 (though that dependence could be removed with
some changes to __compressed_pair).

[EricWF]

libcxx/include/__config

1017

I'm trying to keep all of the ABI configuration in the same place at the top of the file.

Could you give this a descriptive name like _LIBCPP_ABI_ENABLE_TRIVIAL_ABI_ATTRIBUTE and then use that here to configure _LIBCPP_TRIVIAL_ABI?

[EricWF]

@rsmith Please address the inline comment. Otherwise this LGTM.

[rsmith]

Abandoned in favor of https://reviews.llvm.org/D82490

libcxx/include/__config

 #else
 #  define _LIBCPP_NODISCARD_AFTER_CXX17
 #endif
 
 #if __has_cpp_attribute(no_unique_address) && defined(_LIBCPP_ABI_UNSTABLE)
 #  define _LIBCPP_USE_NO_UNIQUE_ADDRESS 1
 #endif
 
+#if __has_cpp_attribute(clang::trivial_abi) && defined(_LIBCPP_ABI_UNSTABLE)
+#  define _LIBCPP_TRIVIAL_ABI [[clang::trivial_abi]]
+#else
+#  define _LIBCPP_TRIVIAL_ABI
+#endif
+
 #if _LIBCPP_STD_VER > 14 && defined(__cpp_inline_variables) && (__cpp_inline_variables >= 201606L)
 #  define _LIBCPP_INLINE_VAR inline
 #else
 #  define _LIBCPP_INLINE_VAR
 #endif
 
 #ifdef _LIBCPP_HAS_NO_RVALUE_REFERENCES
 #  define _LIBCPP_EXPLICIT_MOVE(x) _VSTD::move(x)

libcxx/include/memory

 template <class _Deleter>
 struct __unique_ptr_deleter_sfinae<_Deleter&> {
   typedef _Deleter& __lval_ref_type;
   typedef _Deleter&& __bad_rval_ref_type;
   typedef false_type __enable_rval_overload;
 };
 
 template <class _Tp, class _Dp = default_delete<_Tp> >
-class _LIBCPP_TEMPLATE_VIS unique_ptr {
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_TRIVIAL_ABI unique_ptr {
 public:
   typedef _Tp element_type;
   typedef _Dp deleter_type;
   typedef _LIBCPP_NODEBUG_TYPE typename __pointer_type<_Tp, deleter_type>::type pointer;
 
   static_assert(!is_rvalue_reference<deleter_type>::value,
                 "the specified deleter type cannot be an rvalue reference");
 

 #else
     __ptr_.swap(__u.__ptr_);
 #endif
   }
 };
 
 
 template <class _Tp, class _Dp>
-class _LIBCPP_TEMPLATE_VIS unique_ptr<_Tp[], _Dp> {
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_TRIVIAL_ABI unique_ptr<_Tp[], _Dp> {
 public:
   typedef _Tp element_type;
   typedef _Dp deleter_type;
   typedef typename __pointer_type<_Tp, deleter_type>::type pointer;
 
 private:
 #ifdef _LIBCPP_USE_NO_UNIQUE_ADDRESS
   pointer __ptr_;
public:
     {
         typedef allocator<_Other> other;
     };
 };
 
 template<class _Tp> class _LIBCPP_TEMPLATE_VIS enable_shared_from_this;
 
 template<class _Tp>
-class _LIBCPP_TEMPLATE_VIS shared_ptr
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_TRIVIAL_ABI shared_ptr
 {
 public:
     typedef _Tp element_type;
 
 #if _LIBCPP_STD_VER > 14
     typedef weak_ptr<_Tp> weak_type;
 #endif
 private:

 get_deleter(const shared_ptr<_Tp>& __p) _NOEXCEPT
 {
     return __p.template __get_deleter<_Dp>();
 }
 
 #endif  // _LIBCPP_NO_RTTI
 
 template<class _Tp>
-class _LIBCPP_TEMPLATE_VIS weak_ptr
+class _LIBCPP_TEMPLATE_VIS _LIBCPP_TRIVIAL_ABI weak_ptr
 {
 public:
     typedef _Tp element_type;
 private:
     element_type*        __ptr_;
     __shared_weak_count* __cntrl_;
 
 public: