[llvm] Introduce paged vector (PR #66430)

[Jakub Kuderski via llvm-commits]

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@@ -0,0 +1,266 @@
+//===- 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 {
+  static_assert(PageSize > 1, "PageSize must be greater than 0. Most likely "
+                              "you want it to be greater than 16.");
+  /// The actual number of elements 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;
+  /// Actual page data. All the page elements are allocated on the
+  /// first access of any of the elements of the page. Elements are default
+  /// constructed and elements of the page are stored contiguously.
+  PointerIntPair<BumpPtrAllocator *, 1, bool> Allocator;
+
+public:
+  using value_type = T;
+
+  /// Default constructor. We build our own allocator and mark it as such with
+  /// `true` in the second pair element.
+  PagedVector() : Allocator(new BumpPtrAllocator, true) {}
+  explicit PagedVector(BumpPtrAllocator *A) : Allocator(A, false) {
+    assert(A && "Allocator cannot be nullptr");
+  }
+
+  ~PagedVector() {
+    clear();
+    // If we own the allocator, delete it.
+    if (Allocator.getInt())
+      delete Allocator.getPointer();
+  }
+
+  // Forbid copy and move as we do not need them for the current use case.
+  PagedVector(const PagedVector &) = delete;
+  PagedVector(PagedVector &&) = delete;
+  PagedVector &operator=(const PagedVector &) = delete;
+  PagedVector &operator=(PagedVector &&) = delete;
+
+  /// Look up an element at position `Index`.
+  /// If the associated page is not filled, it will be filled with default
+  /// constructed elements.
+  T &operator[](size_t Index) const {
+    assert(Index < Size);
+    assert(Index / PageSize < PageToDataPtrs.size());
+    T *&PagePtr = PageToDataPtrs[Index / PageSize];
+    // If the page was not yet allocated, allocate it.
+    if (!PagePtr) {
+      PagePtr = Allocator.getPointer()->template Allocate<T>(PageSize);
+      // We need to invoke the default constructor on all the elements of the
+      // page.
+      std::uninitialized_value_construct_n(PagePtr, PageSize);
+    }
+    // Dereference the element in the page.
+    return PagePtr[Index % PageSize];
+  }
+
+  /// 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]] size_t capacity() const {
+    return PageToDataPtrs.size() * PageSize;
+  }
+
+  /// Return the size of the vector.
+  [[nodiscard]] size_t size() const { return Size; }
+
+  /// Resize the vector. Notice that the constructor of the elements will not
+  /// be invoked until an element of a given page is accessed, at which point
+  /// all the elements of the page will be constructed.
+  ///
+  /// If the new size is smaller than the current size, the elements of the
+  /// pages that are not needed anymore will be destroyed, however, elements of
+  /// the last page will not be destroyed.
+  ///
+  /// For these reason the usage of this vector is discouraged if you rely
+  /// on the construction / destructor of the elements to be invoked.
+  void resize(size_t NewSize) {
+    if (NewSize == 0) {
+      clear();
+      return;
+    }
+    // Handle shrink case: destroy the elements in the pages that are not
+    // needed any more and deallocate the pages.
+    //
+    // On the other hand, we do not destroy the extra elements in the last page,
+    // because we might need them later and the logic is simpler if we do not
+    // destroy them. This means that elements are only destroyed when the
+    // page they belong to is destroyed. This is similar to what happens on
+    // access of the elements of a page, where all the elements of the page are
+    // constructed not only the one effectively needed.
+    size_t NewLastPage = (NewSize - 1) / PageSize;
+    if (NewSize < Size) {
+      for (size_t I = NewLastPage + 1, N = PageToDataPtrs.size(); I < N; ++I) {
+        T *Page = PageToDataPtrs[I];
+        if (!Page)
+          continue;
+        // We need to invoke the destructor on all the elements of the page.
+        std::destroy_n(Page, PageSize);
+        Allocator.getPointer()->Deallocate(Page);
+      }
+    }
+
+    Size = NewSize;
+    PageToDataPtrs.resize(NewLastPage + 1);
+  }
+
+  [[nodiscard]] bool empty() const { return Size == 0; }
+
+  /// Clear the vector, i.e. clear the allocated pages, the whole page
+  /// lookup index and reset the size.
+  void clear() {
+    Size = 0;
+    for (T *Page : PageToDataPtrs) {
+      if (Page == nullptr)
+        continue;
+      std::destroy_n(Page, PageSize);
+      // If we do not own the allocator, deallocate the pages one by one.
+      if (!Allocator.getInt())
+        Allocator.getPointer()->Deallocate(Page);
+    }
+    // If we own the allocator, simply reset it.
+    if (Allocator.getInt())
+      Allocator.getPointer()->Reset();
+    PageToDataPtrs.clear();
+  }
+
+  /// Iterator on all the elements of the vector
+  /// which have actually being constructed.
+  class MaterializedIterator {
+    const PagedVector *PV;
+    size_t ElementIdx;
+
+  public:
+    using iterator_category = std::forward_iterator_tag;
+    using value_type = T;
+    using difference_type = std::ptrdiff_t;
+    using pointer = T *;
+    using reference = T &;
+
+    MaterializedIterator(PagedVector const *PV, size_t ElementIdx)
+        : PV(PV), ElementIdx(ElementIdx) {}
+
+    /// Pre-increment operator.
+    ///
+    /// When incrementing the iterator, we skip the elements which have not
+    /// been materialized yet.
+    MaterializedIterator &operator++() {
+      ++ElementIdx;
+      if (ElementIdx % PageSize == 0) {
+        while (ElementIdx < PV->Size &&
+               PV->PageToDataPtrs[ElementIdx / PageSize] == nullptr)
+          ElementIdx += PageSize;
+        if (ElementIdx > PV->Size)
+          ElementIdx = PV->Size;
+      }
+
+      return *this;
+    }
+
+    MaterializedIterator operator++(int) {
+      MaterializedIterator Copy = *this;
+      ++*this;
+      return Copy;
+    }
+
+    T const &operator*() const {
+      assert(ElementIdx < PV->Size);
+      assert(PV->PageToDataPtrs[ElementIdx / PageSize] != nullptr);
+      T *PagePtr = PV->PageToDataPtrs[ElementIdx / PageSize];
+      return PagePtr[ElementIdx % PageSize];
+    }
+
+    friend bool operator==(MaterializedIterator const &LHS,
+                           MaterializedIterator const &RHS);
+    friend bool operator!=(MaterializedIterator const &LHS,
+                           MaterializedIterator const &RHS);
+
+    [[nodiscard]] size_t getIndex() const { return ElementIdx; }
+  };
+
+  /// Equality operator.
+  friend bool operator==(MaterializedIterator const &LHS,
+                         MaterializedIterator const &RHS) {
+    assert(LHS.PV == RHS.PV);
+    // Make sure we are comparing either end iterators or iterators pointing
+    // to materialized elements.
+    // It should not be possible to build two iterators pointing to non
+    // materialized elements.
+    assert(LHS.ElementIdx == LHS.PV->Size ||
+           (LHS.ElementIdx < LHS.PV->Size &&
+            LHS.PV->PageToDataPtrs[LHS.ElementIdx / PageSize] != nullptr));
----------------
kuhar wrote:

```suggestion
            LHS.PV->PageToDataPtrs[LHS.ElementIdx / PageSize]));
```

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