llvm/llvm/include/llvm/ADT/PagedVector.h

//===- llvm/ADT/PagedVector.h - 'Lazily allocated' vectors --*- 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
//
//===----------------------------------------------------------------------===//
//
// This file defines the PagedVector class.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_ADT_PAGEDVECTOR_H
#define LLVM_ADT_PAGEDVECTOR_H

#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/Support/Allocator.h"
#include <cassert>
#include <vector>

namespace llvm {
/// A vector that allocates memory in pages.
///
/// Order is kept, but memory is allocated only when one element of the page is
/// accessed. This introduces a level of indirection, but it is useful when you
/// have a sparsely initialised vector where the full size is allocated upfront.
///
/// As a side effect the elements are initialised later than in a normal vector.
/// On the first access to one of the elements of a given page, all the elements
/// of the page are initialised. This also means that the elements of the page
/// are initialised beyond the size of the vector.
///
/// Similarly on destruction the elements are destroyed only when the page is
/// not needed anymore, delaying invoking the destructor of the elements.
///
/// Notice that this has iterators only on materialized elements. This
/// is deliberately done under the assumption you would dereference the elements
/// while iterating, therefore materialising them and losing the gains in terms
/// of memory usage this container provides. If you have such a use case, you
/// probably want to use a normal std::vector or a llvm::SmallVector.
template <typename T, size_t PageSize = 1024 / sizeof(T)> class PagedVector { … };
} // namespace llvm
#endif // LLVM_ADT_PAGEDVECTOR_H