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4055d681e6
The next patches remove elements from a segmented vector that is being iterated. This patch adds assertions to ensure that we don't attempt to use an iterator that points to a removed element. The assertions are added to Done() because all the other methods call that. Differential Revision: https://phabricator.services.mozilla.com/D125426
353 lines
11 KiB
C++
353 lines
11 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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// A simple segmented vector class.
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//
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// This class should be used in preference to mozilla::Vector or nsTArray when
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// you are simply gathering items in order to later iterate over them.
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//
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// - In the case where you don't know the final size in advance, using
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// SegmentedVector avoids the need to repeatedly allocate increasingly large
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// buffers and copy the data into them.
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//
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// - In the case where you know the final size in advance and so can set the
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// capacity appropriately, using SegmentedVector still avoids the need for
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// large allocations (which can trigger OOMs).
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#ifndef mozilla_SegmentedVector_h
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#define mozilla_SegmentedVector_h
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#include <new> // for placement new
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#include <utility>
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#include "mozilla/AllocPolicy.h"
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#include "mozilla/Array.h"
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#include "mozilla/Attributes.h"
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#include "mozilla/LinkedList.h"
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#include "mozilla/MemoryReporting.h"
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#include "mozilla/OperatorNewExtensions.h"
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#ifdef IMPL_LIBXUL
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# include "mozilla/Likely.h"
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# include "mozilla/mozalloc_oom.h"
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#endif // IMPL_LIBXUL
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namespace mozilla {
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// |IdealSegmentSize| specifies how big each segment will be in bytes (or as
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// close as is possible). Use the following guidelines to choose a size.
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//
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// - It should be a power-of-two, to avoid slop.
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//
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// - It should not be too small, so that segment allocations are infrequent,
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// and so that per-segment bookkeeping overhead is low. Typically each
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// segment should be able to hold hundreds of elements, at least.
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//
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// - It should not be too large, so that OOMs are unlikely when allocating
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// segments, and so that not too much space is wasted when the final segment
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// is not full.
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//
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// The ideal size depends on how the SegmentedVector is used and the size of
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// |T|, but reasonable sizes include 1024, 4096 (the default), 8192, and 16384.
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//
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template <typename T, size_t IdealSegmentSize = 4096,
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typename AllocPolicy = MallocAllocPolicy>
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class SegmentedVector : private AllocPolicy {
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template <size_t SegmentCapacity>
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struct SegmentImpl
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: public mozilla::LinkedListElement<SegmentImpl<SegmentCapacity>> {
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private:
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uint32_t mLength;
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alignas(T) MOZ_INIT_OUTSIDE_CTOR
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unsigned char mData[sizeof(T) * SegmentCapacity];
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// Some versions of GCC treat it as a -Wstrict-aliasing violation (ergo a
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// -Werror compile error) to reinterpret_cast<> |mData| to |T*|, even
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// through |void*|. Placing the latter cast in these separate functions
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// breaks the chain such that affected GCC versions no longer warn/error.
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void* RawData() { return mData; }
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public:
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SegmentImpl() : mLength(0) {}
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~SegmentImpl() {
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for (uint32_t i = 0; i < mLength; i++) {
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(*this)[i].~T();
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}
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}
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uint32_t Length() const { return mLength; }
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T* Elems() { return reinterpret_cast<T*>(RawData()); }
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T& operator[](size_t aIndex) {
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MOZ_ASSERT(aIndex < mLength);
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return Elems()[aIndex];
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}
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const T& operator[](size_t aIndex) const {
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MOZ_ASSERT(aIndex < mLength);
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return Elems()[aIndex];
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}
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template <typename U>
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void Append(U&& aU) {
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MOZ_ASSERT(mLength < SegmentCapacity);
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// Pre-increment mLength so that the bounds-check in operator[] passes.
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mLength++;
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T* elem = &(*this)[mLength - 1];
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new (KnownNotNull, elem) T(std::forward<U>(aU));
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}
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void PopLast() {
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MOZ_ASSERT(mLength > 0);
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(*this)[mLength - 1].~T();
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mLength--;
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}
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};
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// See how many we elements we can fit in a segment of IdealSegmentSize. If
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// IdealSegmentSize is too small, it'll be just one. The +1 is because
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// kSingleElementSegmentSize already accounts for one element.
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static const size_t kSingleElementSegmentSize = sizeof(SegmentImpl<1>);
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static const size_t kSegmentCapacity =
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kSingleElementSegmentSize <= IdealSegmentSize
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? (IdealSegmentSize - kSingleElementSegmentSize) / sizeof(T) + 1
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: 1;
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public:
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typedef SegmentImpl<kSegmentCapacity> Segment;
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// The |aIdealSegmentSize| is only for sanity checking. If it's specified, we
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// check that the actual segment size is as close as possible to it. This
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// serves as a sanity check for SegmentedVectorCapacity's capacity
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// computation.
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explicit SegmentedVector(size_t aIdealSegmentSize = 0) {
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// The difference between the actual segment size and the ideal segment
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// size should be less than the size of a single element... unless the
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// ideal size was too small, in which case the capacity should be one.
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MOZ_ASSERT_IF(
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aIdealSegmentSize != 0,
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(sizeof(Segment) > aIdealSegmentSize && kSegmentCapacity == 1) ||
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aIdealSegmentSize - sizeof(Segment) < sizeof(T));
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}
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SegmentedVector(SegmentedVector&& aOther)
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: mSegments(std::move(aOther.mSegments)) {}
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~SegmentedVector() { Clear(); }
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bool IsEmpty() const { return !mSegments.getFirst(); }
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// Note that this is O(n) rather than O(1), but the constant factor is very
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// small because it only has to do one addition per segment.
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size_t Length() const {
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size_t n = 0;
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for (auto segment = mSegments.getFirst(); segment;
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segment = segment->getNext()) {
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n += segment->Length();
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}
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return n;
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}
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// Returns false if the allocation failed. (If you are using an infallible
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// allocation policy, use InfallibleAppend() instead.)
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template <typename U>
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[[nodiscard]] bool Append(U&& aU) {
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Segment* last = mSegments.getLast();
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if (!last || last->Length() == kSegmentCapacity) {
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last = this->template pod_malloc<Segment>(1);
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if (!last) {
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return false;
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}
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new (KnownNotNull, last) Segment();
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mSegments.insertBack(last);
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}
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last->Append(std::forward<U>(aU));
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return true;
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}
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// You should probably only use this instead of Append() if you are using an
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// infallible allocation policy. It will crash if the allocation fails.
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template <typename U>
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void InfallibleAppend(U&& aU) {
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bool ok = Append(std::forward<U>(aU));
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#ifdef IMPL_LIBXUL
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if (MOZ_UNLIKELY(!ok)) {
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mozalloc_handle_oom(sizeof(Segment));
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}
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#else
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MOZ_RELEASE_ASSERT(ok);
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#endif // MOZ_INTERNAL_API
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}
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void Clear() {
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Segment* segment;
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while ((segment = mSegments.popFirst())) {
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segment->~Segment();
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this->free_(segment, 1);
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}
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}
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T& GetLast() {
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MOZ_ASSERT(!IsEmpty());
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Segment* last = mSegments.getLast();
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return (*last)[last->Length() - 1];
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}
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const T& GetLast() const {
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MOZ_ASSERT(!IsEmpty());
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Segment* last = mSegments.getLast();
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return (*last)[last->Length() - 1];
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}
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void PopLast() {
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MOZ_ASSERT(!IsEmpty());
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Segment* last = mSegments.getLast();
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last->PopLast();
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if (!last->Length()) {
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mSegments.popLast();
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last->~Segment();
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this->free_(last, 1);
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}
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}
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// Equivalent to calling |PopLast| |aNumElements| times, but potentially
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// more efficient.
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void PopLastN(uint32_t aNumElements) {
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MOZ_ASSERT(aNumElements <= Length());
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Segment* last;
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// Pop full segments for as long as we can. Note that this loop
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// cleanly handles the case when the initial last segment is not
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// full and we are popping more elements than said segment contains.
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do {
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last = mSegments.getLast();
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// The list is empty. We're all done.
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if (!last) {
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return;
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}
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// Check to see if the list contains too many elements. Handle
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// that in the epilogue.
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uint32_t segmentLen = last->Length();
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if (segmentLen > aNumElements) {
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break;
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}
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// Destroying the segment destroys all elements contained therein.
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mSegments.popLast();
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last->~Segment();
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this->free_(last, 1);
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MOZ_ASSERT(aNumElements >= segmentLen);
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aNumElements -= segmentLen;
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if (aNumElements == 0) {
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return;
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}
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} while (true);
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// Handle the case where the last segment contains more elements
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// than we want to pop.
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MOZ_ASSERT(last);
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MOZ_ASSERT(last == mSegments.getLast());
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MOZ_ASSERT(aNumElements < last->Length());
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for (uint32_t i = 0; i < aNumElements; ++i) {
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last->PopLast();
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}
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MOZ_ASSERT(last->Length() != 0);
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}
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// Use this class to iterate over a SegmentedVector, like so:
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//
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// for (auto iter = v.Iter(); !iter.Done(); iter.Next()) {
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// MyElem& elem = iter.Get();
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// f(elem);
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// }
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//
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// Note, adding new entries to the SegmentedVector while using iterators
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// is supported, but removing is not!
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// If an iterator has entered Done() state, adding more entries to the
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// vector doesn't affect it.
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class IterImpl {
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friend class SegmentedVector;
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Segment* mSegment;
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size_t mIndex;
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explicit IterImpl(SegmentedVector* aVector, bool aFromFirst)
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: mSegment(aFromFirst ? aVector->mSegments.getFirst()
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: aVector->mSegments.getLast()),
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mIndex(aFromFirst ? 0 : (mSegment ? mSegment->Length() - 1 : 0)) {
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MOZ_ASSERT_IF(mSegment, mSegment->Length() > 0);
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}
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public:
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bool Done() const {
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MOZ_ASSERT_IF(mSegment, mSegment->isInList());
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MOZ_ASSERT_IF(mSegment, mIndex < mSegment->Length());
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return !mSegment;
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}
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T& Get() {
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MOZ_ASSERT(!Done());
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return (*mSegment)[mIndex];
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}
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const T& Get() const {
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MOZ_ASSERT(!Done());
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return (*mSegment)[mIndex];
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}
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void Next() {
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MOZ_ASSERT(!Done());
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mIndex++;
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if (mIndex == mSegment->Length()) {
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mSegment = mSegment->getNext();
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mIndex = 0;
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}
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}
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void Prev() {
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MOZ_ASSERT(!Done());
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if (mIndex == 0) {
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mSegment = mSegment->getPrevious();
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if (mSegment) {
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mIndex = mSegment->Length() - 1;
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}
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} else {
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--mIndex;
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}
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}
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};
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IterImpl Iter() { return IterImpl(this, true); }
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IterImpl IterFromLast() { return IterImpl(this, false); }
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// Measure the memory consumption of the vector excluding |this|. Note that
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// it only measures the vector itself. If the vector elements contain
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// pointers to other memory blocks, those blocks must be measured separately
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// during a subsequent iteration over the vector.
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size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
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return mSegments.sizeOfExcludingThis(aMallocSizeOf);
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}
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// Like sizeOfExcludingThis(), but measures |this| as well.
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size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
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return aMallocSizeOf(this) + SizeOfExcludingThis(aMallocSizeOf);
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}
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private:
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mozilla::LinkedList<Segment> mSegments;
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};
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} // namespace mozilla
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#endif /* mozilla_SegmentedVector_h */
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