[clang] Introduce paged vector (PR #66430)
Giulio Eulisse via cfe-commits
cfe-commits at lists.llvm.org
Tue Sep 19 15:18:29 PDT 2023
================
@@ -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) {
+ uintptr_t PagePtr = PageToDataIdx[I];
+ if (PagePtr == InvalidPage)
+ continue;
+ T *Page = reinterpret_cast<T *>(PagePtr);
+ // We need to invoke the destructor on all the elements of the page.
+ for (std::size_t J = 0; J < PAGE_SIZE; ++J)
+ Page[J].~T();
+ Allocator.getPointer()->Deallocate(Page);
+ }
+ // Delete the extra ones in the new last page.
+ uintptr_t PagePtr = PageToDataIdx[NewLastPage];
+ if (PagePtr != InvalidPage) {
+ T *Page = reinterpret_cast<T *>(PagePtr);
+ // If the new size and the old size are on the same page, we need to
+ // delete only the elements between the new size and the old size.
+ // Otherwise we need to delete all the remaining elements in the page.
+ std::size_t OldPage = (Size - 1) / PAGE_SIZE;
+ std::size_t NewPage = (NewSize - 1) / PAGE_SIZE;
+ std::size_t LastPageElements =
+ OldPage == NewPage ? Size % PAGE_SIZE : PAGE_SIZE;
+ for (std::size_t J = NewSize % PAGE_SIZE; J < LastPageElements; ++J)
+ Page[J].~T();
+ }
+ PageToDataIdx.resize(NewLastPage + 1);
+ }
+ Size = NewSize;
+ // If the capacity is enough, just update the size and continue
+ // with the currently allocated pages.
+ 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 PAGE_SIZE.
+ // In that case we need one more page.
+ std::size_t Pages = Size / PAGE_SIZE;
+ std::size_t Remainder = Size % PAGE_SIZE;
+ if (Remainder != 0)
+ 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);
+ }
+
+ // Return true if the vector is empty
+ [[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;
+ // If we own the allocator, simply reset it, otherwise we
+ // deallocate the pages one by one.
+ if (Allocator.getInt() == true)
+ Allocator.getPointer()->Reset();
+ else
+ for (uintptr_t Page : PageToDataIdx)
+ Allocator.getPointer()->Deallocate(reinterpret_cast<T *>(Page));
+
+ PageToDataIdx.clear();
+ }
+
+ // Iterator on all the elements of the vector
+ // which have actually being constructed.
+ class MaterialisedIterator {
+ PagedVector const *PV;
+ std::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, std::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)
+ if (PV->PageToDataIdx[ElementIdx++ / PAGE_SIZE] != InvalidPage)
+ break;
+
+ return *this;
+ }
+ // Post increment operator.
+ MaterialisedIterator operator++(int) {
+ MaterialisedIterator 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;
+
+ std::size_t ElementMin = std::min(ElementIdx, Other.ElementIdx);
+ std::size_t ElementMax = std::max(ElementIdx, Other.ElementIdx);
+ std::size_t PageMin = ElementMin / PAGE_SIZE;
+ std::size_t PageMax = ElementMax / PAGE_SIZE;
+
+ std::size_t Count = 0ULL;
+ for (std::size_t 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);
+ T *PagePtr =
+ reinterpret_cast<T *>(PV->PageToDataIdx[ElementIdx / PAGE_SIZE]);
+ return *((ElementIdx % PAGE_SIZE) + PagePtr);
+ }
+
+ // 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.
+ std::size_t PageMin = std::min(ElementIdx, Other.ElementIdx) / PAGE_SIZE;
+ std::size_t 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 (std::size_t PageIdx = PageMin; PageIdx < PV->PageToDataIdx.size();
+ ++PageIdx)
+ if (PV->PageToDataIdx[PageIdx] != InvalidPage)
+ return false;
+ return true;
+ }
+
+ uintptr_t Page1 = PV->PageToDataIdx[PageMin];
+ uintptr_t 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 (std::size_t PageIdx = PageMin; PageIdx < PageMax; ++PageIdx)
+ if (PV->PageToDataIdx[PageIdx] != InvalidPage)
+ return false;
+
+ return true;
----------------
ktf wrote:
If I drop this bit of code, I get one extra count when measuring the distance between begin and end. I need to better understand why is that...
https://github.com/llvm/llvm-project/pull/66430
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