base/containers/vector_buffer.h

// Copyright 2017 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_CONTAINERS_VECTOR_BUFFER_H_
#define BASE_CONTAINERS_VECTOR_BUFFER_H_

#include <stdlib.h>
#include <string.h>

#include <type_traits>
#include <utility>

#include "base/check.h"
#include "base/check_op.h"
#include "base/compiler_specific.h"
#include "base/containers/util.h"
#include "base/memory/raw_ptr_exclusion.h"
#include "base/numerics/checked_math.h"

namespace base::internal {

// Internal implementation detail of base/containers.
//
// Implements a vector-like buffer that holds a certain capacity of T. Unlike
// std::vector, VectorBuffer never constructs or destructs its arguments, and
// can't change sizes. But it does implement templates to assist in efficient
// moving and destruction of those items manually.
//
// In particular, the destructor function does not iterate over the items if
// there is no destructor. Moves should be implemented as a memcpy/memmove for
// trivially copyable objects (POD) otherwise, it should be a std::move if
// possible, and as a last resort it falls back to a copy. This behavior is
// similar to std::vector.
//
// No special consideration is done for noexcept move constructors since
// we compile without exceptions.
//
// The current API does not support moving overlapping ranges.
template <typename T>
class VectorBuffer {
 public:
  constexpr VectorBuffer() = default;

#if defined(__clang__) && !defined(__native_client__)
  // This constructor converts an uninitialized void* to a T* which triggers
  // clang Control Flow Integrity. Since this is as-designed, disable.
  __attribute__((no_sanitize("cfi-unrelated-cast", "vptr")))
#endif
  VectorBuffer(size_t count)
      : buffer_(reinterpret_cast<T*>(
            malloc(CheckMul(sizeof(T), count).ValueOrDie()))),
        capacity_(count) {
  }
  VectorBuffer(VectorBuffer&& other) noexcept
      : buffer_(other.buffer_), capacity_(other.capacity_) {
    other.buffer_ = nullptr;
    other.capacity_ = 0;
  }

  VectorBuffer(const VectorBuffer&) = delete;
  VectorBuffer& operator=(const VectorBuffer&) = delete;

  ~VectorBuffer() { free(buffer_); }

  VectorBuffer& operator=(VectorBuffer&& other) {
    free(buffer_);
    buffer_ = other.buffer_;
    capacity_ = other.capacity_;

    other.buffer_ = nullptr;
    other.capacity_ = 0;
    return *this;
  }

  size_t capacity() const { return capacity_; }

  T& operator[](size_t i) {
    // TODO(crbug.com/817982): Some call sites (at least circular_deque.h) are
    // calling this with `i == capacity_` as a way of getting `end()`. Therefore
    // we have to allow this for now (`i <= capacity_`), until we fix those call
    // sites to use real iterators. This comment applies here and to `const T&
    // operator[]`, below.
    CHECK_LE(i, capacity_);
    return buffer_[i];
  }

  const T& operator[](size_t i) const {
    CHECK_LE(i, capacity_);
    return buffer_[i];
  }

  T* begin() { return buffer_; }
  T* end() { return &buffer_[capacity_]; }

  // DestructRange ------------------------------------------------------------

  void DestructRange(T* begin, T* end) {
    // Trivially destructible objects need not have their destructors called.
    if constexpr (!std::is_trivially_destructible_v<T>) {
      CHECK_LE(begin, end);
      while (begin != end) {
        begin->~T();
        begin++;
      }
    }
  }

  // MoveRange ----------------------------------------------------------------
  //
  // The destructor will be called (as necessary) for all moved types. The
  // ranges must not overlap.
  //
  // The parameters and begin and end (one past the last) of the input buffer,
  // and the address of the first element to copy to. There must be sufficient
  // room in the destination for all items in the range [begin, end).

  // Trivially copyable types can use memcpy. Trivially copyable implies
  // that there is a trivial destructor as we don't have to call it.

  // Trivially relocatable types can also use memcpy. Trivially relocatable
  // imples that memcpy is equivalent to move + destroy.

  template <typename T2>
  static inline constexpr bool is_trivially_copyable_or_relocatable =
      std::is_trivially_copyable_v<T2> || IS_TRIVIALLY_RELOCATABLE(T2);

  static void MoveRange(T* from_begin, T* from_end, T* to) {
    CHECK(!RangesOverlap(from_begin, from_end, to));

    if constexpr (is_trivially_copyable_or_relocatable<T>) {
      memcpy(to, from_begin,
             CheckSub(get_uintptr(from_end), get_uintptr(from_begin))
                 .ValueOrDie());
    } else {
      while (from_begin != from_end) {
        if constexpr (std::move_constructible<T>) {
          new (to) T(std::move(*from_begin));
        } else {
          new (to) T(*from_begin);
        }
        from_begin->~T();
        from_begin++;
        to++;
      }
    }
  }

 private:
  static bool RangesOverlap(const T* from_begin,
                            const T* from_end,
                            const T* to) {
    const auto from_begin_uintptr = get_uintptr(from_begin);
    const auto from_end_uintptr = get_uintptr(from_end);
    const auto to_uintptr = get_uintptr(to);
    return !(
        to >= from_end ||
        CheckAdd(to_uintptr, CheckSub(from_end_uintptr, from_begin_uintptr))
                .ValueOrDie() <= from_begin_uintptr);
  }

  // `buffer_` is not a raw_ptr<...> for performance reasons (based on analysis
  // of sampling profiler data and tab_search:top100:2020).
  RAW_PTR_EXCLUSION T* buffer_ = nullptr;
  size_t capacity_ = 0;
};

}  // namespace base::internal

#endif  // BASE_CONTAINERS_VECTOR_BUFFER_H_