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

[Nikolas Klauser via libcxx-commits]

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+// -*- 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 packs the bottom bits of the base pointer with an offset that tracks the iterator's current position.
+// To retrieve the current pointer, we mask off the bottom bits to get the base pointer, and add its offset.
+// This only applies if the container's maximum range can fit inside the representable range of those bottom
+// bits, so (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(alignof(iter_value_t<_Ptr>), _RangeCapacity))
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philnik777 wrote:

Why are these not just `static_assert`s? What's the point of making this SFINAE?

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