[llvm] Introduce paged vector (PR #66430)

[Richard Smith via llvm-commits]

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@@ -0,0 +1,322 @@
+//===- 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/Support/Allocator.h"
+#include <cassert>
+#include <iostream>
+#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
+// with the default constructor and elements are initialised later, on first
+// access.
+//
+// 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.
+//
+// Pages are allocated in SLAB_SIZE chunks, using the BumpPtrAllocator.
+template <typename T, std::size_t PAGE_SIZE = 1024 / sizeof(T)>
+class PagedVector {
+  static_assert(PAGE_SIZE > 0, "PAGE_SIZE 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.
+  std::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 std::vector<uintptr_t> PageToDataIdx;
+  // Actual page data. All the page elements are added to this vector on the
+  // first access of any of the elements of the page. Elements default
+  // constructed and elements of the page are stored contiguously. The order of
+  // the elements however depends on the order of access of the pages.
+  PointerIntPair<BumpPtrAllocator *, 1, bool> Allocator;
+
+  constexpr static uintptr_t InvalidPage = SIZE_MAX;
+
+public:
+  using value_type = T;
+
+  // Default constructor. We build our own allocator.
+  PagedVector() : Allocator(new BumpPtrAllocator, true) {}
+  PagedVector(BumpPtrAllocator *A) : Allocator(A, false) {}
+
+  ~PagedVector() {
+    // If we own the allocator, delete it.
+    if (Allocator.getInt() == true)
+      delete Allocator.getPointer();
+  }
+
+  // Lookup an element at position i.
+  // If the associated page is not filled, it will be filled with default
+  // constructed elements. If the associated page is filled, return the element.
+  T &operator[](std::size_t Index) const {
+    assert(Index < Size);
+    assert(Index / PAGE_SIZE < PageToDataIdx.size());
+    uintptr_t &PagePtr = PageToDataIdx[Index / PAGE_SIZE];
+    // If the page was not yet allocated, allocate it.
+    if (PagePtr == InvalidPage) {
+      T *NewPagePtr = Allocator.getPointer()->template Allocate<T>(PAGE_SIZE);
+      // We need to invoke the default constructor on all the elements of the
+      // page.
+      for (std::size_t I = 0; I < PAGE_SIZE; ++I)
+        new (NewPagePtr + I) T();
+
+      PagePtr = reinterpret_cast<uintptr_t>(NewPagePtr);
+    }
+    // Dereference the element in the page.
+    return *((Index % PAGE_SIZE) + reinterpret_cast<T *>(PagePtr));
+  }
+
+  // Return the capacity of the vector. I.e. the maximum size it can be expanded
+  // to with the resize method without allocating more pages.
+  [[nodiscard]] std::size_t capacity() const {
+    return PageToDataIdx.size() * PAGE_SIZE;
+  }
+
+  // Return the size of the vector. I.e. the maximum index that can be
+  // accessed, i.e. the maximum value which was used as argument of the
+  // resize method.
+  [[nodiscard]] std::size_t size() const { return Size; }
+
+  // Expands the vector to the given NewSize number of elements.
+  // If the vector was smaller, allocates new pages as needed.
+  // It should be called only with NewSize >= Size.
+  void resize(std::size_t NewSize) {
+    // Handle shrink case: delete the pages and update the size.
+    if (NewSize < Size) {
+      std::size_t NewLastPage = (NewSize - 1) / PAGE_SIZE;
+      for (std::size_t I = NewLastPage + 1; I < PageToDataIdx.size(); ++I) {
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zygoloid wrote:

`N` rather than `E`, because this is an integer not an iterator.

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