[llvm] Introduce paged vector (PR #66430)

[Richard Smith via llvm-commits]

================
@@ -0,0 +1,323 @@
+//===- llvm/ADT/PagedVector.h - 'Lazyly 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 does not have iterators, because if you have iterators it
+/// probably means you are going to touch all the memory in any case, so better
+/// use a std::vector in the first place.
+template <typename T, size_t PageSize = 1024 / sizeof(T)> class PagedVector {
+  static_assert(PageSize > 1, "PageSize must be greater than 0. Most likely "
+                              "you want it to be greater than 16.");
+  /// The actual number of element in the vector which can be accessed.
+  size_t Size = 0;
+
+  /// The position of the initial element of the page in the Data vector.
+  /// Pages are allocated contiguously in the Data vector.
+  mutable SmallVector<T *, 0> PageToDataPtrs;
----------------
zygoloid wrote:

I have no particular preference here. I don't think the switch back to an integer index is a likely future change (because that'd take us back to paying for a reallocation if the slab becomes full like with the original approach of using a vector), and it wouldn't be hard to change the places in this class that use `nullptr` as the invalid page over to using a different constant, so the evolution argument doesn't really persuade me. But having an explicit name for the invalid page makes the code marginally more readable.

https://github.com/llvm/llvm-project/pull/66430