[llvm] Introduce paged vector (PR #66430)
Giulio Eulisse via llvm-commits
llvm-commits at lists.llvm.org
Thu Sep 28 01:06:47 PDT 2023
================
@@ -0,0 +1,320 @@
+//===- 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;
+ /// 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 T *InvalidPage = nullptr;
+
+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) {}
+ PagedVector(BumpPtrAllocator *A) : Allocator(A, false) {
+ assert(A != nullptr && "Allocator cannot be null");
+ }
+
+ ~PagedVector() {
+ clear();
+ // If we own the allocator, delete it.
+ if (Allocator.getInt())
+ delete Allocator.getPointer();
+ }
+
+ /// Look up an element at position `Index`.
+ /// 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[](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 == InvalidPage) {
+ T *NewPagePtr = 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(NewPagePtr, PageSize);
+
+ PagePtr = NewPagePtr;
+ }
+ // 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. I.e. the maximum index that can be
+ /// accessed, i.e. the maximum value which was used as argument of the
+ /// resize method.
+ [[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 anymore 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 only 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 neeeded.
+ if (NewSize < Size) {
+ size_t NewLastPage = (NewSize - 1) / PageSize;
+ for (size_t I = NewLastPage + 1, N = PageToDataPtrs.size(); I < N; ++I) {
+ T *PagePtr = PageToDataPtrs[I];
+ if (PagePtr == InvalidPage)
+ continue;
+ T *Page = PagePtr;
+ // We need to invoke the destructor on all the elements of the page.
+ std::destroy_n(Page, PageSize);
+ Allocator.getPointer()->Deallocate(Page);
+ // We mark the page invalid, to avoid double deletion.
+ PageToDataPtrs[I] = InvalidPage;
+ }
+ PageToDataPtrs.resize(NewLastPage + 1);
+ }
+ Size = NewSize;
+ // If the capacity is enough, just update the size and continue
+ // with the currently allocated pages. Notice that we do not
+ // need to default construct any new element, because that was already done
+ // when the page was allocated.
+ if (Size <= capacity())
+ return;
+ // The number of pages to allocate. The Remainder is calculated
+ // for the case in which the NewSize is not a multiple of PageSize.
+ // In that case we need one more page.
+ size_t Pages = Size / PageSize;
+ size_t Remainder = Size % PageSize;
+ if (Remainder != 0)
+ Pages += 1;
+ assert(Pages > PageToDataPtrs.size());
+ // We use InvalidPage to indicate that a page has not been allocated yet.
+ // This cannot be 0, because 0 is a valid page id.
+ // We use InvalidPage instead of a separate bool to avoid wasting space.
+ PageToDataPtrs.resize(Pages, InvalidPage);
+ }
+
+ [[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 == InvalidPage)
+ 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() == true)
+ Allocator.getPointer()->Reset();
+ PageToDataPtrs.clear();
+ }
+
+ /// Iterator on all the elements of the vector
+ /// which have actually being constructed.
+ class MaterialisedIterator {
+ 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 &;
+
+ MaterialisedIterator(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 materialised yet.
+ MaterialisedIterator &operator++() {
+ ++ElementIdx;
+ if (ElementIdx % PageSize == 0) {
+ while (ElementIdx < PV->Size &&
+ PV->PageToDataPtrs[ElementIdx / PageSize] == InvalidPage)
+ ElementIdx += PageSize;
+ if (ElementIdx > PV->Size)
+ ElementIdx = PV->Size;
+ }
+
+ return *this;
+ }
+
+ MaterialisedIterator operator++(int) {
+ MaterialisedIterator Copy = *this;
+ ++*this;
+ return Copy;
+ }
+
+ /// Dereference operator.
+ ///
+ /// Notice this can materialise elements if needed so there might be
+ /// a page allocation and additional construction of the elements of
+ /// such page.
----------------
ktf wrote:
Indeed, sorry for misreading. I will drop the comment.
https://github.com/llvm/llvm-project/pull/66430
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