[clang] [libcxx] [clang & libcxx] constexpr pointer tagging (DO NOT MERGE) (PR #111861)

[Hana Dusíková via cfe-commits]

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@@ -0,0 +1,517 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef _LIBCPP___TAGGED_PTR_H
+#define _LIBCPP___TAGGED_PTR_H
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+#if _LIBCPP_STD_VER >= 26
+  
+#include <__config>
+#include <__type_traits/is_trivially_copyable.h>
+#include <__assert>
+#include "__bit/has_single_bit.h"
+#include <__type_traits/rank.h>
+#include "pointer_traits.h"
+#include <compare>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+template <typename T, typename Y> concept convertible_to_from = std::convertible_to<Y, T> && std::convertible_to<T, Y>;
+  
+template <typename T> concept pointer_tagging_schema = requires(T::dirty_pointer payload, T::clean_pointer clean, T::tag_type tag) {
+  requires convertible_to_from<typename T::tag_type, uintptr_t>;
+  requires std::is_pointer_v<typename T::clean_pointer>;
+  
+  { T::encode_pointer_with_tag(clean, tag) } noexcept -> std::same_as<typename T::dirty_pointer>;
+  { T::recover_pointer(payload) } noexcept -> std::same_as<typename T::clean_pointer>;
+  { T::recover_value(payload) } noexcept -> std::same_as<typename T::tag_type>;
+};
+
+template <typename T> concept pointer_tagging_schema_with_aliasing = pointer_tagging_schema<T> && requires(T::dirty_pointer payload) {
+  { T::recover_aliasing_pointer(payload) } noexcept -> std::same_as<typename T::clean_pointer>;
+};
+
+// no-op schema so I can better explain how schemas work
+struct no_tag {
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hanickadot wrote:

For now I'm going with `memory`

https://github.com/llvm/llvm-project/pull/111861