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The goal of the class is to be an (almost) drop in replacement for SmallVector and std::vector when those are presized and filled later, as it happens in SourceManager and ASTReader. By doing so, sparsely accessed PagedVector can profit from reduced memory footprint.
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//===- 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 | ||
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#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> | ||
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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; | ||
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/// 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; | ||
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public: | ||
using value_type = T; | ||
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/// 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"); | ||
} | ||
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~PagedVector() { | ||
clear(); | ||
// If we own the allocator, delete it. | ||
if (Allocator.getInt()) | ||
delete Allocator.getPointer(); | ||
} | ||
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// 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; | ||
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/// 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]; | ||
} | ||
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/// 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; | ||
} | ||
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/// Return the size of the vector. | ||
[[nodiscard]] size_t size() const { return Size; } | ||
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/// 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); | ||
} | ||
} | ||
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Size = NewSize; | ||
PageToDataPtrs.resize(NewLastPage + 1); | ||
} | ||
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[[nodiscard]] bool empty() const { return Size == 0; } | ||
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/// 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(); | ||
} | ||
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/// Iterator on all the elements of the vector | ||
/// which have actually being constructed. | ||
class MaterializedIterator { | ||
const PagedVector *PV; | ||
size_t ElementIdx; | ||
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public: | ||
using iterator_category = std::forward_iterator_tag; | ||
using value_type = T; | ||
using difference_type = std::ptrdiff_t; | ||
using pointer = T *; | ||
using reference = T &; | ||
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MaterializedIterator(PagedVector const *PV, size_t ElementIdx) | ||
: PV(PV), ElementIdx(ElementIdx) {} | ||
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/// 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]) | ||
ElementIdx += PageSize; | ||
if (ElementIdx > PV->Size) | ||
ElementIdx = PV->Size; | ||
} | ||
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return *this; | ||
} | ||
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MaterializedIterator operator++(int) { | ||
MaterializedIterator Copy = *this; | ||
++*this; | ||
return Copy; | ||
} | ||
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T const &operator*() const { | ||
assert(ElementIdx < PV->Size); | ||
assert(PV->PageToDataPtrs[ElementIdx / PageSize]); | ||
T *PagePtr = PV->PageToDataPtrs[ElementIdx / PageSize]; | ||
return PagePtr[ElementIdx % PageSize]; | ||
} | ||
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friend bool operator==(MaterializedIterator const &LHS, | ||
MaterializedIterator const &RHS); | ||
friend bool operator!=(MaterializedIterator const &LHS, | ||
MaterializedIterator const &RHS); | ||
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[[nodiscard]] size_t getIndex() const { return ElementIdx; } | ||
}; | ||
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/// 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])); | ||
assert(RHS.ElementIdx == RHS.PV->Size || | ||
(RHS.ElementIdx < RHS.PV->Size && | ||
RHS.PV->PageToDataPtrs[RHS.ElementIdx / PageSize])); | ||
return LHS.ElementIdx == RHS.ElementIdx; | ||
} | ||
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friend bool operator!=(MaterializedIterator const &LHS, | ||
MaterializedIterator const &RHS) { | ||
return !(LHS == RHS); | ||
} | ||
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/// Iterators over the materialized 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 materialize all the elements of the page. | ||
MaterializedIterator materialized_begin() const { | ||
// Look for the first valid page. | ||
for (size_t ElementIdx = 0; ElementIdx < Size; ElementIdx += PageSize) | ||
if (PageToDataPtrs[ElementIdx / PageSize]) | ||
return MaterializedIterator(this, ElementIdx); | ||
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return MaterializedIterator(this, Size); | ||
} | ||
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MaterializedIterator materialized_end() const { | ||
return MaterializedIterator(this, Size); | ||
} | ||
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[[nodiscard]] llvm::iterator_range<MaterializedIterator> | ||
materialized() const { | ||
return {materialized_begin(), materialized_end()}; | ||
} | ||
}; | ||
} // namespace llvm | ||
#endif // LLVM_ADT_PAGEDVECTOR_H |
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