[libcxx-commits] [libcxx] [libc++] Add a compact, bounded iterator (PR #208271)

[Louis Dionne via libcxx-commits]

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@@ -0,0 +1,254 @@
+// -*- 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__STATIC_PACKED_BOUNDED_ITER_H
+#define _LIBCPP__STATIC_PACKED_BOUNDED_ITER_H
+
+#include <__assert>
+#include <__bit/bit_cast.h>
+#include <__bit/countr.h>
+#include <__compare/ordering.h>
+#include <__compare/three_way_comparable.h>
+#include <__config>
+#include <__cstddef/size_t.h>
+#include <__iterator/concepts.h>
+#include <__iterator/incrementable_traits.h>
+#include <__iterator/iterator_traits.h>
+#include <__type_traits/is_constructible.h>
+#include <__type_traits/is_pointer.h>
+
+#include <cstdint>
+
+#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#  pragma GCC system_header
+#endif
+
+_LIBCPP_PUSH_MACROS
+#include <__undef_macros>
+
+#if _LIBCPP_STD_VER >= 26
+
+// static_packed_bounded_iter is a bounded, contiguous iterator that is aware of its container's (compile-time) maximum
+// capacity. It reuses the bottom unused bits of a pointer to keep track of its current position.
+// This only applies if the container's maximum range can fit inside (2 ^ available_bits) - 1
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+consteval bool __range_fits_in_alignment(size_t __alignment, size_t __num_elems) {
+  size_t __bits = std::countr_zero(__alignment);
+
+  // Example: For alignof(T) == 4, we have two bits free, which has a range of 0-3. We need to
+  // reserve one for the end position, so __num_elems must be < 3.
+  size_t __allowed_range = (1 << __bits) - 1;
+  return __allowed_range > __num_elems;
+}
+
+template <class _Ptr, class _Tag, size_t _RangeCapacity>
+  requires(is_pointer_v<_Ptr> && std::__range_fits_in_alignment(_LIBCPP_ALIGNOF(iter_value_t<_Ptr>), _RangeCapacity))
+class __static_packed_bounded_iterator {
+public:
+  using iterator_category = iterator_traits<_Ptr>::iterator_category;
+  using iterator_concept  = contiguous_iterator_tag;
+  using difference_type   = iter_difference_t<_Ptr>;
+  using pointer           = iterator_traits<_Ptr>::pointer;
+  using reference         = iter_reference_t<_Ptr>;
+  using value_type        = iter_value_t<_Ptr>;
+
+private:
+  static constexpr uintptr_t __count_mask_ = (1 << std::countr_zero(_LIBCPP_ALIGNOF(value_type))) - 1;
+  static constexpr uintptr_t __ptr_mask_   = ~__count_mask_;
+
+  union {
+    pointer __ptr_;
+    alignas(pointer) uintptr_t __data_;
+  };
+
+  uintptr_t __count() const { return __data_ & __count_mask_; }
----------------
ldionne wrote:

Maybe `unsigned int` or `size_t` here? It seems a bit more natural for a quantity like that.

https://github.com/llvm/llvm-project/pull/208271