[clang] Introduce paged vector (PR #66430)
Jakub Kuderski via cfe-commits
cfe-commits at lists.llvm.org
Mon Sep 18 09:39:05 PDT 2023
================
@@ -0,0 +1,301 @@
+//===- 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/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<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.
+ uintptr_t Allocator = 0;
+
+ constexpr static T *invalidPage() { return reinterpret_cast<T *>(SIZE_MAX); }
+
+public:
+ // Default constructor. We build our own allocator.
+ PagedVector()
+ : Allocator(reinterpret_cast<uintptr_t>(new BumpPtrAllocator) | 0x1) {}
+ PagedVector(BumpPtrAllocator *A)
+ : Allocator(reinterpret_cast<uintptr_t>(A)) {}
+
+ ~PagedVector() {
+ // If we own the allocator, delete it.
+ if (Allocator & 0x1) {
+ delete getAllocator();
+ }
+ }
+
+ // Get the allocator.
+ BumpPtrAllocator *getAllocator() const {
+ return reinterpret_cast<BumpPtrAllocator *>(Allocator & ~0x1);
+ }
+ // Lookup an element at position Index.
+ T &operator[](std::size_t Index) const { return at(Index); }
+
+ // 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 &at(std::size_t Index) const {
+ assert(Index < Size);
+ assert(Index / PAGE_SIZE < PageToDataIdx.size());
+ auto *&PagePtr = PageToDataIdx[Index / PAGE_SIZE];
+ // If the page was not yet allocated, allocate it.
+ if (PagePtr == invalidPage()) {
+ PagePtr = getAllocator()->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 (PagePtr + I) T();
+ }
+ }
+ // Dereference the element in the page.
+ return *((Index % PAGE_SIZE) + PagePtr);
+ }
+
+ // Return the capacity of the vector. I.e. the maximum size it can be expanded
+ // to with the expand method without allocating more pages.
+ 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
+ // expand method.
+ 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 expand(std::size_t NewSize) {
+ // You cannot shrink the vector, otherwise
+ // one would have to invalidate contents which is expensive and
+ // while giving the false hope that the resize is cheap.
+ if (NewSize <= Size) {
+ return;
+ }
+ // If the capacity is enough, just update the size and continue
+ // with the currently allocated pages.
+ if (NewSize <= capacity()) {
+ Size = NewSize;
+ return;
+ }
+ // The number of pages to allocate. The Remainder is calculated
+ // for the case in which the NewSize is not a multiple of PAGE_SIZE.
+ // In that case we need one more page.
+ auto Pages = NewSize / PAGE_SIZE;
+ auto Remainder = NewSize % PAGE_SIZE;
+ if (Remainder) {
+ Pages += 1;
+ }
+ assert(Pages > PageToDataIdx.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.
+ PageToDataIdx.resize(Pages, invalidPage());
+ Size = NewSize;
+ }
+
+ // Return true if the vector is empty
+ 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;
+ // If we own the allocator, simply reset it, otherwise we
+ // deallocate the pages one by one.
+ if (Allocator & 0x1) {
+ getAllocator()->Reset();
+ } else {
+ for (auto *Page : PageToDataIdx) {
+ getAllocator()->Deallocate(Page);
+ }
+ }
+ PageToDataIdx.clear();
+ }
+
+ // Iterator on all the elements of the vector
+ // which have actually being constructed.
+ class MaterialisedIterator {
+ PagedVector const *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) {}
+
+ // When incrementing the iterator, we skip the elements which have not
+ // been materialised yet.
+ MaterialisedIterator &operator++() {
+ while (ElementIdx < PV->Size) {
+ ++ElementIdx;
+ if (PV->PageToDataIdx[ElementIdx / PAGE_SIZE] != invalidPage()) {
+ return *this;
+ }
+ }
+ return *this;
+ }
+ // Post increment operator.
+ MaterialisedIterator operator++(int) {
+ auto Copy = *this;
+ ++*this;
+ return Copy;
+ }
+
+ std::ptrdiff_t operator-(MaterialisedIterator const &Other) const {
+ assert(PV == Other.PV);
+ // If they are on the same table we can just subtract the indices.
+ // Otherwise we have to iterate over the pages to find the difference.
+ // If a page is invalid, we skip it.
+ if (PV == Other.PV) {
+ return ElementIdx - Other.ElementIdx;
+ }
+
+ auto ElementMin = std::min(ElementIdx, Other.ElementIdx);
+ auto ElementMax = std::max(ElementIdx, Other.ElementIdx);
+ auto PageMin = ElementMin / PAGE_SIZE;
+ auto PageMax = ElementMax / PAGE_SIZE;
+
+ auto Count = 0ULL;
+ for (auto PageIdx = PageMin; PageIdx < PageMax; ++PageIdx) {
+ if (PV->PageToDataIdx[PageIdx] == invalidPage()) {
+ continue;
+ }
+ Count += PAGE_SIZE;
+ }
+ Count += ElementMax % PAGE_SIZE;
+ Count += PAGE_SIZE - ElementMin % PAGE_SIZE;
+
+ return Count;
+ }
+
+ // When dereferencing the iterator, we materialise the page if needed.
+ T const &operator*() const {
+ assert(ElementIdx < PV->Size);
+ assert(PV->PageToDataIdx[ElementIdx / PAGE_SIZE] != invalidPage());
+ return *((ElementIdx % PAGE_SIZE) +
+ PV->PageToDataIdx[ElementIdx / PAGE_SIZE]);
+ }
+
+ // Equality operator.
+ bool operator==(MaterialisedIterator const &Other) const {
+ // Iterators of two different vectors are never equal.
+ if (PV != Other.PV) {
+ return false;
+ }
+ // Any iterator for an empty vector is equal to any other iterator.
+ if (PV->empty()) {
+ return true;
+ }
+ // Get the pages of the two iterators. If between the two pages there
+ // are no valid pages, we can condider the iterators equal.
+ auto PageMin = std::min(ElementIdx, Other.ElementIdx) / PAGE_SIZE;
+ auto PageMax = std::max(ElementIdx, Other.ElementIdx) / PAGE_SIZE;
+ // If the two pages are past the end, the iterators are equal.
+ if (PageMin >= PV->PageToDataIdx.size()) {
+ return true;
+ }
+ // If only the last page is past the end, the iterators are equal if
+ // all the pages up to the end are invalid.
+ if (PageMax >= PV->PageToDataIdx.size()) {
+ for (auto PageIdx = PageMin; PageIdx < PV->PageToDataIdx.size();
+ ++PageIdx) {
+ if (PV->PageToDataIdx[PageIdx] != invalidPage()) {
+ return false;
+ }
+ }
+ return true;
+ }
+
+ auto *Page1 = PV->PageToDataIdx[PageMin];
+ auto *Page2 = PV->PageToDataIdx[PageMax];
+ if (Page1 == invalidPage() && Page2 == invalidPage()) {
+ return true;
+ }
+ // If the two pages are the same, the iterators are equal if they point
+ // to the same element.
+ if (PageMin == PageMax) {
+ return ElementIdx == Other.ElementIdx;
+ }
+ // If the two pages are different, the iterators are equal if all the
+ // pages between them are invalid.
+ for (auto PageIdx = PageMin; PageIdx < PageMax; ++PageIdx) {
+ if (PV->PageToDataIdx[PageIdx] != invalidPage()) {
+ return false;
+ }
+ }
+ return true;
+ }
+
+ bool operator!=(MaterialisedIterator const &Other) const {
+ return (*this == Other) == false;
+ }
+
+ [[nodiscard]] size_t getIndex() const { return ElementIdx; }
+ };
+
+ // Iterators over the materialised elements of the vector.
+ // This includes all the elements belonging to allocated pages,
+ // even if they have not been accessed yet. It's enough to access
+ // one element of a page to materialise all the elements of the page.
+ MaterialisedIterator materialisedBegin() const {
+ // Look for the first valid page
+ auto ElementIdx = 0ULL;
+ while (ElementIdx < Size) {
+ if (PageToDataIdx[ElementIdx / PAGE_SIZE] != invalidPage()) {
+ break;
+ }
+ ++ElementIdx;
----------------
kuhar wrote:
nit: use a `for` loop?
https://github.com/llvm/llvm-project/pull/66430
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