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[libc++] Make forward_list constexpr as part of P3372R3 #129435

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[libc++] Make forward_list constexpr as part of P3372R3 #129435

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838c50f
Make forward_list constexpr as part of P3372R3
winner245 Feb 28, 2025
3d213b1
Address reviewers comments
winner245 Mar 2, 2025
b6fd6de
Fix constant evaluation due to non-constant pointer_traits::pointer_t…
winner245 Mar 3, 2025
a5b63dd
Revert to libc++ pointer_traits::pointer_to with necessary cast
winner245 Mar 4, 2025
7c90175
Add __static_fancy_pointer_cast
winner245 Mar 4, 2025
6b7800a
Fix ADL call
winner245 Mar 4, 2025
30daee8
Rebase onto main to resolve conflicts
winner245 May 8, 2025
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2 changes: 2 additions & 0 deletions libcxx/docs/FeatureTestMacroTable.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -420,6 +420,8 @@ Status
---------------------------------------------------------- -----------------
``__cpp_lib_constexpr_algorithms`` ``202306L``
---------------------------------------------------------- -----------------
``__cpp_lib_constexpr_forward_list`` ``202502L``
---------------------------------------------------------- -----------------
``__cpp_lib_constexpr_new`` ``202406L``
---------------------------------------------------------- -----------------
``__cpp_lib_constexpr_queue`` ``202502L``
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20 changes: 11 additions & 9 deletions libcxx/include/__memory/allocation_guard.h
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Expand Up @@ -49,24 +49,26 @@ struct __allocation_guard {
using _Size _LIBCPP_NODEBUG = typename allocator_traits<_Alloc>::size_type;

template <class _AllocT> // we perform the allocator conversion inside the constructor
_LIBCPP_HIDE_FROM_ABI explicit __allocation_guard(_AllocT __alloc, _Size __n)
_LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI explicit __allocation_guard(_AllocT __alloc, _Size __n)
: __alloc_(std::move(__alloc)),
__n_(__n),
__ptr_(allocator_traits<_Alloc>::allocate(__alloc_, __n_)) // initialization order is important
{}

_LIBCPP_HIDE_FROM_ABI ~__allocation_guard() _NOEXCEPT { __destroy(); }
_LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI ~__allocation_guard() _NOEXCEPT { __destroy(); }

_LIBCPP_HIDE_FROM_ABI __allocation_guard(const __allocation_guard&) = delete;
_LIBCPP_HIDE_FROM_ABI __allocation_guard(__allocation_guard&& __other) _NOEXCEPT
__allocation_guard(const __allocation_guard&) = delete;
__allocation_guard& operator=(const __allocation_guard& __other) = delete;

_LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI __allocation_guard(__allocation_guard&& __other) _NOEXCEPT
: __alloc_(std::move(__other.__alloc_)),
__n_(__other.__n_),
__ptr_(__other.__ptr_) {
__other.__ptr_ = nullptr;
}

_LIBCPP_HIDE_FROM_ABI __allocation_guard& operator=(const __allocation_guard& __other) = delete;
_LIBCPP_HIDE_FROM_ABI __allocation_guard& operator=(__allocation_guard&& __other) _NOEXCEPT {
_LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI __allocation_guard&
operator=(__allocation_guard&& __other) _NOEXCEPT {
if (std::addressof(__other) != this) {
__destroy();

Expand All @@ -79,17 +81,17 @@ struct __allocation_guard {
return *this;
}

_LIBCPP_HIDE_FROM_ABI _Pointer
_LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI _Pointer
__release_ptr() _NOEXCEPT { // not called __release() because it's a keyword in objective-c++
_Pointer __tmp = __ptr_;
__ptr_ = nullptr;
return __tmp;
}

_LIBCPP_HIDE_FROM_ABI _Pointer __get() const _NOEXCEPT { return __ptr_; }
_LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI _Pointer __get() const _NOEXCEPT { return __ptr_; }

private:
_LIBCPP_HIDE_FROM_ABI void __destroy() _NOEXCEPT {
_LIBCPP_CONSTEXPR_SINCE_CXX26 _LIBCPP_HIDE_FROM_ABI void __destroy() _NOEXCEPT {
if (__ptr_ != nullptr) {
allocator_traits<_Alloc>::deallocate(__alloc_, __ptr_, __n_);
}
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16 changes: 14 additions & 2 deletions libcxx/include/__memory/pointer_traits.h
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Expand Up @@ -245,8 +245,8 @@ inline _LIBCPP_HIDE_FROM_ABI constexpr auto to_address(_Tp* __p) noexcept {
}

template <class _Pointer>
inline _LIBCPP_HIDE_FROM_ABI constexpr auto
to_address(const _Pointer& __p) noexcept -> decltype(std::__to_address(__p)) {
inline _LIBCPP_HIDE_FROM_ABI constexpr auto to_address(const _Pointer& __p) noexcept
-> decltype(std::__to_address(__p)) {
return std::__to_address(__p);
}
#endif
Expand Down Expand Up @@ -302,6 +302,18 @@ concept __resettable_smart_pointer_with_args = requires(_Smart __s, _Pointer __p

#endif

// This function ensures safe conversions between fancy pointers at compile-time, where we avoid casts from/to
// `__void_pointer` by obtaining the underlying raw pointer from the fancy pointer using `std::to_address`,
// then dereferencing it to retrieve the pointed-to object, and finally constructing the target fancy pointer
// to that object using the `std::pointer_traits<>::pinter_to` function.
template <class _PtrTo, class _PtrFrom>
_LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI _PtrTo __static_fancy_pointer_cast(const _PtrFrom& __p) {
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Can you document what this function does with a short comment?

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Is there a reason we can't replace the cases like we did in list?

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Are you referring to #130310 ? If so, that PR (which hasn't landed yet) removes many instances of these casts, but not all of them.

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I guess so. I think my main question is whether this utility is still needed after that patch. If not, I think I'd like this patch to be rebased on the other instead of adding a utility we know for a fact is removed by a follow-up patch. That would simplify this patch, which I think is quite important. The "constexpr everything" patches are really difficult to go through due to all the changes (even though none of them are complicated), so every bit of simplification makes it significantly easier to look at.

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@winner245 winner245 Jun 5, 2025 •
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I think what #130310 really does is to inline the three cast functions __as_iter_node, __next_as_begin, __get_unsafe_node_pointer, which cast between __node_pointer and __begin_node_pointer via __void_pointer. During runtime evaluation, the intermediate __void_pointer is unnecessary. During compile time evaluation, it leads to compilation failure to use __void_pointer to cast between fancy pointers (e.g., min_pointer), which is why the new utility function __static_fancy_pointer_cast was introduced in this patch. __static_fancy_pointer_cast works by first extracting the underlying raw pointer from the fancy pointer using std::to_address, then dereferencing it to obtain the pointed-to object, and finally acquiring the target fancy pointer to that object. This ensures successful compile-time evaluation for fancy pointers.

That said, I’d like to land this patch first to guarantee that all fancy pointer-related casts function correctly. Once that’s done, #130310 can rebase and serve as a follow-up to inline the three cast functions.

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It doesn't just inline the calls. AFAICT there is no cast to __void_pointer left, which is the part that introduces the problem IIUC.

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The problem is that even without the cast to __void_pointer, the cast would fail in compile-time evaluation for fancy pointer (See: https://godbolt.org/z/E7jcezjnc). That's why I need __static_fancy_pointer_cast.

using __ptr_traits = pointer_traits<_PtrTo>;
using __element_type = typename __ptr_traits::element_type;
return __p ? __ptr_traits::pointer_to(*static_cast<__element_type*>(std::addressof(*__p)))
: static_cast<_PtrTo>(nullptr);
}

_LIBCPP_END_NAMESPACE_STD

_LIBCPP_POP_MACROS
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