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

[Louis Dionne via libcxx-commits]

================
@@ -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_; }
+
+  constexpr _Ptr __current() const {
+    if consteval {
+      return __ptr_;
+    } else {
+      return std::bit_cast<pointer>(__data_ & __ptr_mask_) + __count();
+    }
+  }
+
+  constexpr void __update(difference_type __n) {
+    if consteval {
+      __ptr_ += __n;
+    } else {
+      __data_ += __n;
+    }
+  }
+
+  constexpr explicit __static_packed_bounded_iterator(_Ptr __p) noexcept : __ptr_(__p) {
+    if !consteval {
+      _LIBCPP_ASSERT_INTERNAL(
+          (__data_ & __count_mask_) == 0, "__static_packed_bounded_iterator: Expected alignment bits of ptr to be 0");
----------------
ldionne wrote:

```suggestion
          (reinterpret_cast<uintptr_t>(__p) & __count_mask_) == 0, "__static_packed_bounded_iterator: Expected alignment bits of ptr to be 0");
```

And since this is actually a precondition, one option would be to do:

```
constexpr explicit __static_packed_bounded_iterator(_Ptr __p) noexcept : __ptr_([&]{
  _LIBCPP_ASSERT_INTERNAL(...);
  return __p;
}())
{
}
```

Honestly, I think this is much less readable so I would stick with what you have modulo the `reinterpret_cast`, but it's an option.

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