sweeper.cc

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// Copyright 2020 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
 
#include "src/heap/cppgc/sweeper.h"
 
#include <algorithm>
#include <atomic>
#include <cstdint>
#include <memory>
#include <optional>
#include <vector>
 
#include "include/cppgc/platform.h"
#include "src/base/platform/mutex.h"
#include "src/base/platform/time.h"
#include "src/heap/cppgc/free-list.h"
#include "src/heap/cppgc/globals.h"
#include "src/heap/cppgc/heap-base.h"
#include "src/heap/cppgc/heap-config.h"
#include "src/heap/cppgc/heap-object-header.h"
#include "src/heap/cppgc/heap-page.h"
#include "src/heap/cppgc/heap-space.h"
#include "src/heap/cppgc/heap-visitor.h"
#include "src/heap/cppgc/memory.h"
#include "src/heap/cppgc/object-poisoner.h"
#include "src/heap/cppgc/object-start-bitmap.h"
#include "src/heap/cppgc/page-memory.h"
#include "src/heap/cppgc/raw-heap.h"
#include "src/heap/cppgc/stats-collector.h"
#include "src/heap/cppgc/task-handle.h"
 
namespace cppgc::internal {
 
namespace {
 
constexpr TaskPriority kBackgroundBoostedPriority = TaskPriority::kUserBlocking;
constexpr TaskPriority kBackgroundRegularPriority = TaskPriority::kUserVisible;
constexpr TaskPriority kForegroundRegularPriority = TaskPriority::kUserBlocking;
constexpr TaskPriority kForegroundLowPriority = TaskPriority::kUserVisible;
 
class DeadlineChecker final {
 public:
  explicit DeadlineChecker(v8::base::TimeTicks end) : end_(end) {}
 
  bool Check() {
    return V8_UNLIKELY(++count_ % kInterval == 0) &&
           (end_ < v8::base::TimeTicks::Now());
  }
 
 private:
  static constexpr size_t kInterval = 4;
 
  const v8::base::TimeTicks end_;
  size_t count_ = 0;
};
 
enum class MutatorThreadSweepingMode {
  kOnlyFinalizers,
  kAll,
};
 
constexpr const char* ToString(MutatorThreadSweepingMode sweeping_mode) {
  switch (sweeping_mode) {
    case MutatorThreadSweepingMode::kAll:
      return "all";
    case MutatorThreadSweepingMode::kOnlyFinalizers:
      return "only-finalizers";
  }
}
 
class ObjectStartBitmapVerifier final
    : private HeapVisitor<ObjectStartBitmapVerifier> {
  friend class HeapVisitor<ObjectStartBitmapVerifier>;
 
 public:
  void Verify(RawHeap& heap) {
#if DEBUG
    Traverse(heap);
#endif  // DEBUG
  }
  void Verify(NormalPage& page) {
#if DEBUG
    Traverse(page);
#endif  // DEBUG
  }
 
 private:
  bool VisitNormalPage(NormalPage& page) {
    // Remember bitmap and reset previous pointer.
    bitmap_ = &page.object_start_bitmap();
    prev_ = nullptr;
    return false;
  }
 
  bool VisitHeapObjectHeader(HeapObjectHeader& header) {
    if (header.IsLargeObject()) return true;
 
    auto* raw_header = reinterpret_cast<ConstAddress>(&header);
    CHECK(bitmap_->CheckBit<AccessMode::kAtomic>(raw_header));
    if (prev_) {
      // No other bits in the range [prev_, raw_header) should be set.
      CHECK_EQ(prev_, bitmap_->FindHeader<AccessMode::kAtomic>(raw_header - 1));
    }
    prev_ = &header;
    return true;
  }
 
  PlatformAwareObjectStartBitmap* bitmap_ = nullptr;
  HeapObjectHeader* prev_ = nullptr;
};
 
class FreeHandlerBase {
 public:
  virtual ~FreeHandlerBase() = default;
  virtual void FreeFreeList(
      std::vector<FreeList::Block>& unfinalized_free_list) = 0;
};
 
class DiscardingFreeHandler : public FreeHandlerBase {
 public:
  DiscardingFreeHandler(PageAllocator& page_allocator, FreeList& free_list,
                        BasePage& page)
      : page_allocator_(page_allocator), free_list_(free_list), page_(page) {}
 
  void Free(FreeList::Block block) {
    const auto unused_range = free_list_.AddReturningUnusedBounds(block);
    const uintptr_t aligned_begin_unused =
        RoundUp(reinterpret_cast<uintptr_t>(unused_range.first),
                page_allocator_.CommitPageSize());
    const uintptr_t aligned_end_unused =
        RoundDown(reinterpret_cast<uintptr_t>(unused_range.second),
                  page_allocator_.CommitPageSize());
    if (aligned_begin_unused < aligned_end_unused) {
      const size_t discarded_size = aligned_end_unused - aligned_begin_unused;
      page_allocator_.DiscardSystemPages(
          reinterpret_cast<void*>(aligned_begin_unused),
          aligned_end_unused - aligned_begin_unused);
      page_.IncrementDiscardedMemory(discarded_size);
      page_.space()
          .raw_heap()
          ->heap()
          ->stats_collector()
          ->IncrementDiscardedMemory(discarded_size);
    }
  }
 
  void FreeFreeList(std::vector<FreeList::Block>& unfinalized_free_list) final {
    for (auto entry : unfinalized_free_list) {
      Free(std::move(entry));
    }
  }
 
 private:
  PageAllocator& page_allocator_;
  FreeList& free_list_;
  BasePage& page_;
};
 
class RegularFreeHandler : public FreeHandlerBase {
 public:
  RegularFreeHandler(PageAllocator& page_allocator, FreeList& free_list,
                     BasePage& page)
      : free_list_(free_list) {}
 
  void Free(FreeList::Block block) { free_list_.Add(std::move(block)); }
 
  void FreeFreeList(std::vector<FreeList::Block>& unfinalized_free_list) final {
    for (auto entry : unfinalized_free_list) {
      Free(std::move(entry));
    }
  }
 
 private:
  FreeList& free_list_;
};
 
template <typename T>
class ThreadSafeStack {
 public:
  ThreadSafeStack() = default;
 
  void Push(T t) {
    v8::base::MutexGuard lock(&mutex_);
    vector_.push_back(std::move(t));
    is_empty_.store(false, std::memory_order_relaxed);
  }
 
  std::optional<T> Pop() {
    v8::base::MutexGuard lock(&mutex_);
    if (vector_.empty()) {
      is_empty_.store(true, std::memory_order_relaxed);
      return std::nullopt;
    }
    T top = std::move(vector_.back());
    vector_.pop_back();
    // std::move is redundant but is needed to avoid the bug in gcc-7.
    return std::move(top);
  }
 
  template <typename It>
  void Insert(It begin, It end) {
    v8::base::MutexGuard lock(&mutex_);
    vector_.insert(vector_.end(), begin, end);
    is_empty_.store(false, std::memory_order_relaxed);
  }
 
  bool IsEmpty() const { return is_empty_.load(std::memory_order_relaxed); }
 
 private:
  mutable v8::base::Mutex mutex_;
  std::vector<T> vector_;
  std::atomic<bool> is_empty_{true};
};
 
struct SweepingState {
  struct SweptPageState {
    BasePage* page = nullptr;
#if defined(CPPGC_CAGED_HEAP)
    // The list of unfinalized objects may be extremely big. To save on space,
    // if cage is enabled, the list of unfinalized objects is stored inlined in
    // HeapObjectHeader.
    HeapObjectHeader* unfinalized_objects_head = nullptr;
#else   // !defined(CPPGC_CAGED_HEAP)
    std::vector<HeapObjectHeader*> unfinalized_objects;
#endif  // !defined(CPPGC_CAGED_HEAP)
    FreeList cached_free_list;
    std::vector<FreeList::Block> unfinalized_free_list;
    bool is_empty = false;
    size_t largest_new_free_list_entry = 0;
  };
 
  ThreadSafeStack<BasePage*> unswept_pages;
  ThreadSafeStack<SweptPageState> swept_unfinalized_pages;
};
 
using SpaceStates = std::vector<SweepingState>;
 
void StickyUnmark(HeapObjectHeader* header, StickyBits sticky_bits) {
#if defined(CPPGC_YOUNG_GENERATION)
  // Young generation in Oilpan uses sticky mark bits.
  if (sticky_bits == StickyBits::kDisabled)
    header->Unmark<AccessMode::kAtomic>();
#else   // !defined(CPPGC_YOUNG_GENERATION)
  header->Unmark<AccessMode::kAtomic>();
#endif  // !defined(CPPGC_YOUNG_GENERATION)
}
 
class InlinedFinalizationBuilderBase {
 public:
  struct ResultType {
    bool is_empty = false;
    size_t largest_new_free_list_entry = 0;
  };
 
 protected:
  ResultType result_;
};
 
// Builder that finalizes objects and adds freelist entries right away.
template <typename FreeHandler>
class InlinedFinalizationBuilder final : public InlinedFinalizationBuilderBase,
                                         public FreeHandler {
 public:
  InlinedFinalizationBuilder(BasePage& page, PageAllocator& page_allocator)
      : FreeHandler(page_allocator,
                    NormalPageSpace::From(page.space()).free_list(), page) {}
 
  void AddFinalizer(HeapObjectHeader* header, size_t size) {
    header->Finalize();
    SetMemoryInaccessible(header, size);
  }
 
  void AddFreeListEntry(Address start, size_t size) {
    FreeHandler::Free({start, size});
    result_.largest_new_free_list_entry =
        std::max(result_.largest_new_free_list_entry, size);
  }
 
  ResultType&& GetResult(bool is_empty) {
    result_.is_empty = is_empty;
    return std::move(result_);
  }
};
 
// Builder that produces results for deferred processing.
template <typename FreeHandler>
class DeferredFinalizationBuilder final : public FreeHandler {
 public:
  using ResultType = SweepingState::SweptPageState;
 
  DeferredFinalizationBuilder(BasePage& page, PageAllocator& page_allocator)
      : FreeHandler(page_allocator, result_.cached_free_list, page) {
    result_.page = &page;
  }
 
  void AddFinalizer(HeapObjectHeader* header, size_t size) {
    if (header->IsFinalizable()) {
#if defined(CPPGC_CAGED_HEAP)
      if (!current_unfinalized_) {
        DCHECK_NULL(result_.unfinalized_objects_head);
        current_unfinalized_ = header;
        result_.unfinalized_objects_head = header;
      } else {
        current_unfinalized_->SetNextUnfinalized(header);
        current_unfinalized_ = header;
      }
#else   // !defined(CPPGC_CAGED_HEAP)
      result_.unfinalized_objects.push_back({header});
#endif  // !defined(CPPGC_CAGED_HEAP)
      found_finalizer_ = true;
    } else {
      SetMemoryInaccessible(header, size);
    }
  }
 
  void AddFreeListEntry(Address start, size_t size) {
    if (found_finalizer_) {
      result_.unfinalized_free_list.push_back({start, size});
    } else {
      FreeHandler::Free({start, size});
    }
    result_.largest_new_free_list_entry =
        std::max(result_.largest_new_free_list_entry, size);
    found_finalizer_ = false;
  }
 
  ResultType&& GetResult(bool is_empty) {
    result_.is_empty = is_empty;
    return std::move(result_);
  }
 
 private:
  ResultType result_;
  HeapObjectHeader* current_unfinalized_ = nullptr;
  bool found_finalizer_ = false;
};
 
template <typename FinalizationBuilder>
typename FinalizationBuilder::ResultType SweepNormalPage(
    NormalPage* page, PageAllocator& page_allocator, StickyBits sticky_bits) {
  constexpr auto kAtomicAccess = AccessMode::kAtomic;
  FinalizationBuilder builder(*page, page_allocator);
 
  PlatformAwareObjectStartBitmap& bitmap = page->object_start_bitmap();
 
  size_t live_bytes = 0;
 
  Address start_of_gap = page->PayloadStart();
 
  const auto clear_bit_if_coalesced_entry = [&bitmap,
                                             &start_of_gap](Address address) {
    if (address != start_of_gap) {
      // Clear only if not the first freed entry.
      bitmap.ClearBit<AccessMode::kAtomic>(address);
    } else {
      // Otherwise check that the bit is set.
      DCHECK(bitmap.CheckBit<AccessMode::kAtomic>(address));
    }
  };
 
  for (Address begin = page->PayloadStart(), end = page->PayloadEnd();
       begin != end;) {
    DCHECK(bitmap.CheckBit<AccessMode::kAtomic>(begin));
    HeapObjectHeader* header = reinterpret_cast<HeapObjectHeader*>(begin);
    const size_t size = header->AllocatedSize();
    // Check if this is a free list entry.
    if (header->IsFree<kAtomicAccess>()) {
      SetMemoryInaccessible(header, std::min(kFreeListEntrySize, size));
      // This prevents memory from being discarded in configurations where
      // `CheckMemoryIsInaccessibleIsNoop()` is false.
      CheckMemoryIsInaccessible(header, size);
      clear_bit_if_coalesced_entry(begin);
      begin += size;
      continue;
    }
    // Check if object is not marked (not reachable).
    if (!header->IsMarked<kAtomicAccess>()) {
      builder.AddFinalizer(header, size);
      clear_bit_if_coalesced_entry(begin);
      begin += size;
      continue;
    }
    // The object is alive.
    const Address header_address = reinterpret_cast<Address>(header);
    if (start_of_gap != header_address) {
      const size_t new_free_list_entry_size =
          static_cast<size_t>(header_address - start_of_gap);
      builder.AddFreeListEntry(start_of_gap, new_free_list_entry_size);
      DCHECK(bitmap.CheckBit<AccessMode::kAtomic>(start_of_gap));
    }
    StickyUnmark(header, sticky_bits);
    begin += size;
    start_of_gap = begin;
    live_bytes += size;
  }
 
  const bool is_empty = live_bytes == 0;
  CHECK_EQ(is_empty, page->marked_bytes() == 0);
  CHECK_IMPLIES(is_empty, start_of_gap == page->PayloadStart());
 
  // Empty pages are not added to the free list directly here. The free list is
  // either added later on or the page is destroyed.
  if (!is_empty && start_of_gap != page->PayloadEnd()) {
    builder.AddFreeListEntry(
        start_of_gap, static_cast<size_t>(page->PayloadEnd() - start_of_gap));
    DCHECK(bitmap.CheckBit<AccessMode::kAtomic>(start_of_gap));
  }
  page->SetAllocatedBytesAtLastGC(live_bytes);
  page->ResetMarkedBytes(sticky_bits == StickyBits::kDisabled ? 0 : live_bytes);
  return builder.GetResult(is_empty);
}
 
constexpr BaseSpace* kSweepWithoutSpaceAssignment = nullptr;
 
enum class EmptyPageHandling {
  kDestroy,
  kReturn,
};
 
// SweepFinalizer is responsible for heap/space/page finalization. Finalization
// is defined as a step following concurrent sweeping which:
// - calls finalizers;
// - returns (unmaps) empty pages;
// - merges freelists to the space's freelist.
class SweepFinalizer final {
  using FreeMemoryHandling = SweepingConfig::FreeMemoryHandling;
 
 public:
  SweepFinalizer(cppgc::Platform* platform, StatsCollector* stats_collector,
                 BaseSpace* space, size_t* unused_destroyed_normal_pages,
                 FreeMemoryHandling free_memory_handling,
                 EmptyPageHandling empty_page_handling_type)
      : platform_(platform),
        stats_collector_(stats_collector),
        space_(space),
        unused_destroyed_normal_pages_(unused_destroyed_normal_pages),
        free_memory_handling_(free_memory_handling),
        empty_page_handling_(empty_page_handling_type) {}
 
  // Finalizes all space states, irrespective of deadlines and sizes.
  void Finalize(SpaceStates& states) {
    for (SweepingState& state : states) {
      Finalize(state);
    }
  }
 
  void Finalize(SweepingState& state) {
    while (auto page_state = state.swept_unfinalized_pages.Pop()) {
      FinalizePage(&*page_state);
    }
  }
 
  // Finalizes a given SweepingState with a deadline and size. Only returns
  // true if a single memory block of at least `size` bytes was returned to the
  // free list and false otherwise.
  bool FinalizeWithDeadlineAndSize(StatsCollector::ScopeId scope_id,
                                   SweepingState& state,
                                   v8::base::TimeTicks deadline, size_t size) {
    if (state.swept_unfinalized_pages.IsEmpty()) {
      return false;
    }
    StatsCollector::DisabledScope finalize_scope(stats_collector_, scope_id);
    DeadlineChecker deadline_check(deadline);
    while (auto page_state = state.swept_unfinalized_pages.Pop()) {
      FinalizePage(&*page_state);
      if (size <= largest_consecutive_block_) {
        return true;
      }
      if (deadline_check.Check()) {
        break;
      }
    }
    return false;
  }
 
  // Finalizes a given SweepingState with a deadline. Returns false if the
  // deadline exceeded and true if all pages are finalized.
  bool FinalizeWithDeadline(StatsCollector::ScopeId scope_id,
                            SweepingState& state,
                            v8::base::TimeTicks deadline) {
    if (state.swept_unfinalized_pages.IsEmpty()) {
      return true;
    }
    StatsCollector::DisabledScope finalize_scope(stats_collector_, scope_id);
    DeadlineChecker deadline_check(deadline);
    while (auto page_state = state.swept_unfinalized_pages.Pop()) {
      FinalizePage(&*page_state);
      if (deadline_check.Check()) {
        return false;
      }
    }
    return true;
  }
 
 private:
  void FinalizePage(SweepingState::SweptPageState* page_state) {
    DCHECK(page_state);
    DCHECK(page_state->page);
    BasePage* page = page_state->page;
 
    // Call finalizers.
    const auto finalize_header = [](HeapObjectHeader* header) {
      const size_t size = header->AllocatedSize();
      header->Finalize();
      SetMemoryInaccessible(header, size);
    };
#if defined(CPPGC_CAGED_HEAP)
#if defined(CPPGC_POINTER_COMPRESSION)
    const uint64_t cage_base = CageBaseGlobal::Get();
#else
    const uint64_t cage_base = CagedHeapBase::GetBase();
#endif
    HeapObjectHeader* next_unfinalized = nullptr;
 
    for (auto* unfinalized_header = page_state->unfinalized_objects_head;
         unfinalized_header; unfinalized_header = next_unfinalized) {
      next_unfinalized = unfinalized_header->GetNextUnfinalized(cage_base);
      finalize_header(unfinalized_header);
    }
#else   // !defined(CPPGC_CAGED_HEAP)
    for (HeapObjectHeader* unfinalized_header :
         page_state->unfinalized_objects) {
      finalize_header(unfinalized_header);
    }
#endif  // !defined(CPPGC_CAGED_HEAP)
 
    // Unmap page if empty.
    if (page_state->is_empty) {
      DCHECK_IMPLIES(page->is_large(),
                     empty_page_handling_ == EmptyPageHandling::kDestroy);
      if (empty_page_handling_ == EmptyPageHandling::kDestroy) {
        if (!page->is_large()) {
          (*unused_destroyed_normal_pages_)++;
        } else {
          // Normal pages are added to the page pool when destroyed and thus
          // cannot be used for a large page allocation.
          largest_consecutive_block_ = std::max(
              LargePage::From(page)->PayloadSize(), largest_consecutive_block_);
        }
        BasePage::Destroy(page);
        return;
      }
 
      // Otherwise, we currently sweep on allocation. Reinitialize the empty
      // page and return it right away.
      auto* normal_page = NormalPage::From(page);
 
      // If a space has been assigned to the finalizer, then repurpose empty
      // pages for that space. Otherwise just retain the current space for an
      // empty page.
      if (space_) {
        normal_page->ChangeOwner(*space_);
      }
 
      page_state->cached_free_list.Clear();
      page_state->cached_free_list.Add(
          {normal_page->PayloadStart(), normal_page->PayloadSize()});
 
      page_state->unfinalized_free_list.clear();
      page_state->largest_new_free_list_entry = normal_page->PayloadSize();
    }
    // We either swept a non-empty page for which the space should already match
    // or we swept an empty page for which the owner was changed.
    DCHECK_IMPLIES(space_, space_ == &page->space());
    DCHECK(!page->is_large());
 
    // Merge freelists without finalizers.
    FreeList& space_freelist = NormalPageSpace::From(page->space()).free_list();
    space_freelist.Append(std::move(page_state->cached_free_list));
 
    // Merge freelist with finalizers.
    if (!page_state->unfinalized_free_list.empty()) {
      std::unique_ptr<FreeHandlerBase> handler =
          free_memory_handling_ == FreeMemoryHandling::kDiscardWherePossible
              ? std::unique_ptr<FreeHandlerBase>(new DiscardingFreeHandler(
                    *platform_->GetPageAllocator(), space_freelist, *page))
              : std::unique_ptr<FreeHandlerBase>(new RegularFreeHandler(
                    *platform_->GetPageAllocator(), space_freelist, *page));
      handler->FreeFreeList(page_state->unfinalized_free_list);
    }
 
    largest_consecutive_block_ = std::max(
        page_state->largest_new_free_list_entry, largest_consecutive_block_);
 
    // After the page was fully finalized and freelists have been merged, verify
    // that the bitmap is consistent.
    ObjectStartBitmapVerifier().Verify(static_cast<NormalPage&>(*page));
 
    // Add the page to the space.
    page->space().AddPage(page);
  }
 
  cppgc::Platform* platform_;
  StatsCollector* stats_collector_;
  BaseSpace* space_;
  size_t* unused_destroyed_normal_pages_;
  // Largest consecutive block of memory. This is the largest free list entry
  // for normal pages and the largest page size for large objects.
  size_t largest_consecutive_block_ = 0;
  const FreeMemoryHandling free_memory_handling_;
  const EmptyPageHandling empty_page_handling_;
};
 
class MutatorThreadSweeper final : private HeapVisitor<MutatorThreadSweeper> {
  friend class HeapVisitor<MutatorThreadSweeper>;
 
  using FreeMemoryHandling = SweepingConfig::FreeMemoryHandling;
 
 public:
  MutatorThreadSweeper(HeapBase* heap, cppgc::Platform* platform,
                       StatsCollector* stats_collector, BaseSpace* space,
                       size_t* unused_destroyed_normal_pages,
                       FreeMemoryHandling free_memory_handling,
                       EmptyPageHandling empty_page_handling)
      : platform_(platform),
        stats_collector_(stats_collector),
        space_(space),
        unused_destroyed_normal_pages_(unused_destroyed_normal_pages),
        free_memory_handling_(free_memory_handling),
        empty_page_handling_(empty_page_handling),
        sticky_bits_(heap->sticky_bits()) {}
 
  static void SweepLiveLargePage(LargePage& page, StickyBits sticky_bits) {
    HeapObjectHeader* header = page.ObjectHeader();
    CHECK(header->IsMarked());
    StickyUnmark(header, sticky_bits);
    if (sticky_bits == StickyBits::kDisabled) {
      page.ResetMarkedBytes();
    }
    page.space().AddPage(&page);
  }
 
  void Sweep(SpaceStates& states) {
    for (SweepingState& state : states) {
      Sweep(state);
    }
  }
 
  void Sweep(SweepingState& state) {
    while (auto page = state.unswept_pages.Pop()) {
      SweepPage(**page);
    }
  }
 
  void SweepPage(BasePage& page) { Traverse(page); }
 
  // Returns true if out of work. This implies that sweeping is done only if
  // `sweeping_mode` is kAll.
  bool FinalizeAndSweepWithDeadline(StatsCollector::ScopeId scope_id,
                                    SweepingState& state,
                                    v8::base::TimeTicks deadline,
                                    MutatorThreadSweepingMode sweeping_mode) {
    // First, prioritize finalization of pages that were swept concurrently.
    SweepFinalizer finalizer(
        platform_, stats_collector_, space_, unused_destroyed_normal_pages_,
        free_memory_handling_, EmptyPageHandling::kDestroy);
    if (!finalizer.FinalizeWithDeadline(scope_id, state, deadline)) {
      return false;
    }
 
    if (sweeping_mode != MutatorThreadSweepingMode::kOnlyFinalizers) {
      // Help out the concurrent sweeper.
      if (!SweepSpaceWithDeadline(&state, deadline)) {
        return false;
      }
    }
    return true;
  }
 
  bool SweepWithDeadlineAndSize(StatsCollector::ScopeId scope_id,
                                SweepingState& state,
                                v8::base::TimeTicks deadline, size_t size) {
    if (state.unswept_pages.IsEmpty()) {
      return false;
    }
    StatsCollector::DisabledScope sweep_scope(stats_collector_, scope_id);
    DeadlineChecker deadline_check(deadline);
    while (auto page = state.unswept_pages.Pop()) {
      SweepPage(**page);
      if (size <= largest_consecutive_block_) {
        return true;
      }
      if (deadline_check.Check()) {
        break;
      }
    }
    return false;
  }
 
 private:
  bool SweepSpaceWithDeadline(SweepingState* state,
                              v8::base::TimeTicks deadline) {
    DeadlineChecker deadline_check(deadline);
    while (auto page = state->unswept_pages.Pop()) {
      Traverse(**page);
      if (deadline_check.Check()) {
        return false;
      }
    }
 
    return true;
  }
 
  bool VisitNormalPage(NormalPage& page) {
    if (free_memory_handling_ == FreeMemoryHandling::kDiscardWherePossible) {
      page.ResetDiscardedMemory();
    }
    const auto result =
        free_memory_handling_ == FreeMemoryHandling::kDiscardWherePossible
            ? SweepNormalPage<
                  InlinedFinalizationBuilder<DiscardingFreeHandler>>(
                  &page, *platform_->GetPageAllocator(), sticky_bits_)
            : SweepNormalPage<InlinedFinalizationBuilder<RegularFreeHandler>>(
                  &page, *platform_->GetPageAllocator(), sticky_bits_);
    if (result.is_empty &&
        empty_page_handling_ == EmptyPageHandling::kDestroy) {
      NormalPage::Destroy(&page);
      (*unused_destroyed_normal_pages_)++;
    } else {
      if (space_) {
        DCHECK_IMPLIES(!result.is_empty, space_ == &page.space());
        page.ChangeOwner(*space_);
      }
      auto& target_space = NormalPageSpace::From(page.space());
      target_space.AddPage(&page);
      if (result.is_empty) {
        target_space.free_list().Add({page.PayloadStart(), page.PayloadSize()});
      }
      // The page was eagerly finalized and all the freelist have been merged.
      // Verify that the bitmap is consistent with headers.
      ObjectStartBitmapVerifier().Verify(page);
      largest_consecutive_block_ =
          std::max(result.is_empty ? page.PayloadSize()
                                   : result.largest_new_free_list_entry,
                   largest_consecutive_block_);
    }
    return true;
  }
 
  bool VisitLargePage(LargePage& page) {
    HeapObjectHeader* header = page.ObjectHeader();
    CHECK(!header->IsMarked());
    DCHECK_EQ(page.marked_bytes(), 0u);
    header->Finalize();
    largest_consecutive_block_ =
        std::max(page.PayloadSize(), largest_consecutive_block_);
    LargePage::Destroy(&page);
    return true;
  }
 
  cppgc::Platform* platform_;
  StatsCollector* stats_collector_;
  // Largest consecutive block of memory. This is the largest free list entry
  // for normal pages and the largest page size for large objects.
  size_t largest_consecutive_block_ = 0;
  BaseSpace* space_;
  size_t* unused_destroyed_normal_pages_;
  const FreeMemoryHandling free_memory_handling_;
  const EmptyPageHandling empty_page_handling_;
  const StickyBits sticky_bits_;
};
 
class ConcurrentSweepTask final : public cppgc::JobTask,
                                  private HeapVisitor<ConcurrentSweepTask> {
  friend class HeapVisitor<ConcurrentSweepTask>;
 
  using FreeMemoryHandling = SweepingConfig::FreeMemoryHandling;
 
 public:
  ConcurrentSweepTask(Platform* platform, HeapBase& heap,
                      SpaceStates* space_states,
                      SweepingState* empty_normal_pages,
                      SweepingState* empty_large_pages,
                      FreeMemoryHandling free_memory_handling)
      : heap_(heap),
        page_allocator_(*platform->GetPageAllocator()),
        space_states_(space_states),
        empty_normal_pages_(empty_normal_pages),
        empty_large_pages_(empty_large_pages),
        free_memory_handling_(free_memory_handling),
        sticky_bits_(heap.sticky_bits()) {}
 
  void Run(cppgc::JobDelegate* delegate) final {
    StatsCollector::EnabledConcurrentScope stats_scope(
        heap_.stats_collector(), StatsCollector::kConcurrentSweep);
 
    // Sweep empty normal pages first. These pages can be reused across all
    // regular spaces.
    if (!SweepStateOrYield(delegate, *empty_normal_pages_)) return;
    for (SweepingState& state : *space_states_) {
      if (!SweepStateOrYield(delegate, state)) return;
    }
    // Sweep empty large pages last. They generally cannot be reused.
    // TODO(mlippautz): We could split them into pages that can be split up for
    // normal pages.
    if (!SweepStateOrYield(delegate, *empty_large_pages_)) return;
 
    is_completed_.store(true, std::memory_order_relaxed);
  }
 
  size_t GetMaxConcurrency(size_t /* active_worker_count */) const final {
    return is_completed_.load(std::memory_order_relaxed) ? 0 : 1;
  }
 
 private:
  // Returns true if sweeping completed, or false if it yielded.
  bool SweepStateOrYield(cppgc::JobDelegate* delegate, SweepingState& state) {
    current_sweeping_state_ = &state;
    while (auto page = state.unswept_pages.Pop()) {
      Traverse(**page);
      if (delegate->ShouldYield()) {
        return false;
      }
    }
    current_sweeping_state_ = nullptr;
    return true;
  }
 
  bool VisitNormalPage(NormalPage& page) {
    if (free_memory_handling_ == FreeMemoryHandling::kDiscardWherePossible) {
      page.ResetDiscardedMemory();
    }
    SweepingState::SweptPageState sweep_result =
        (free_memory_handling_ == FreeMemoryHandling::kDiscardWherePossible)
            ? SweepNormalPage<
                  DeferredFinalizationBuilder<DiscardingFreeHandler>>(
                  &page, page_allocator_, sticky_bits_)
            : SweepNormalPage<DeferredFinalizationBuilder<RegularFreeHandler>>(
                  &page, page_allocator_, sticky_bits_);
    current_sweeping_state_->swept_unfinalized_pages.Push(
        std::move(sweep_result));
    return true;
  }
 
  bool VisitLargePage(LargePage& page) {
    HeapObjectHeader* header = page.ObjectHeader();
    CHECK(!header->IsMarked());
    DCHECK_EQ(page.marked_bytes(), 0u);
#if defined(CPPGC_CAGED_HEAP)
    HeapObjectHeader* const unfinalized_objects =
        header->IsFinalizable() ? page.ObjectHeader() : nullptr;
#else   // !defined(CPPGC_CAGED_HEAP)
    std::vector<HeapObjectHeader*> unfinalized_objects;
    if (header->IsFinalizable()) {
      unfinalized_objects.push_back(page.ObjectHeader());
    }
#endif  // !defined(CPPGC_CAGED_HEAP)
    // Avoid directly destroying large pages here as counter updates and
    // backend access in BasePage::Destroy() are not concurrency safe.
    current_sweeping_state_->swept_unfinalized_pages.Push(
        {&page, std::move(unfinalized_objects), {}, {}, true});
    return true;
  }
 
  HeapBase& heap_;
  PageAllocator& page_allocator_;
  SpaceStates* const space_states_;
  SweepingState* const empty_normal_pages_;
  SweepingState* const empty_large_pages_;
  SweepingState* current_sweeping_state_ = nullptr;
  std::atomic_bool is_completed_{false};
  const FreeMemoryHandling free_memory_handling_;
  const StickyBits sticky_bits_;
};
 
// This visitor starts sweeping.
//
// Normal spaces:
// - Clears free lists.
// - Moves all pages to local state (SpaceStates).
// - ASAN: Poisons all unmarked object payloads.
//
// Large spaces:
// - Directly sweeps live objects and returns pages to the space.
// - Moves dead objects to local state (SpaceStates).
// - ASAN: Poisons all unmarked object payloads.
class PrepareForSweepVisitor final
    : protected HeapVisitor<PrepareForSweepVisitor> {
  friend class HeapVisitor<PrepareForSweepVisitor>;
  using CompactableSpaceHandling = SweepingConfig::CompactableSpaceHandling;
 
 public:
  PrepareForSweepVisitor(HeapBase* heap, SpaceStates* space_states,
                         SweepingState* empty_normal_pages,
                         SweepingState* empty_large_pages,
                         CompactableSpaceHandling compactable_space_handling)
      : heap_(heap),
        space_states_(space_states),
        empty_normal_pages_(empty_normal_pages),
        empty_large_pages_(empty_large_pages),
        compactable_space_handling_(compactable_space_handling) {}
 
  void Run(RawHeap& raw_heap) {
    *space_states_ = SpaceStates(raw_heap.size());
    Traverse(raw_heap);
  }
 
 protected:
  bool VisitNormalPageSpace(NormalPageSpace& space) {
    if ((compactable_space_handling_ == CompactableSpaceHandling::kIgnore) &&
        space.is_compactable()) {
      return true;
    }
 
    CHECK(!space.linear_allocation_buffer().size());
    space.free_list().Clear();
#ifdef V8_USE_ADDRESS_SANITIZER
    UnmarkedObjectsPoisoner().Traverse(space);
#endif  // V8_USE_ADDRESS_SANITIZER
 
    BaseSpace::Pages space_pages = space.RemoveAllPages();
    std::sort(space_pages.begin(), space_pages.end(),
              [](const BasePage* a, const BasePage* b) {
                return a->marked_bytes() < b->marked_bytes();
              });
    auto first_non_empty_page = std::find_if(
        space_pages.begin(), space_pages.end(),
        [](const BasePage* page) { return page->marked_bytes() != 0; });
    empty_normal_pages_->unswept_pages.Insert(space_pages.begin(),
                                              first_non_empty_page);
    (*space_states_)[space.index()].unswept_pages.Insert(first_non_empty_page,
                                                         space_pages.end());
 
    return true;
  }
 
  bool VisitLargePageSpace(LargePageSpace& space) {
#ifdef V8_USE_ADDRESS_SANITIZER
    UnmarkedObjectsPoisoner().Traverse(space);
#endif  // V8_USE_ADDRESS_SANITIZER
 
    BaseSpace::Pages space_pages = space.RemoveAllPages();
    for (BasePage* page : space_pages) {
#ifdef DEBUG
      const auto* header = LargePage::From(page)->ObjectHeader();
      DCHECK_IMPLIES(page->marked_bytes() == 0, !header->IsMarked());
      DCHECK_IMPLIES(page->marked_bytes() != 0, header->IsMarked());
#endif  // DEBUG
      if (page->marked_bytes() != 0) {
        MutatorThreadSweeper::SweepLiveLargePage(*LargePage::From(page),
                                                 heap_->sticky_bits());
      } else {
        empty_large_pages_->unswept_pages.Push(page);
      }
    }
 
    return true;
  }
 
 private:
  HeapBase* const heap_;
  SpaceStates* const space_states_;
  SweepingState* const empty_normal_pages_;
  SweepingState* const empty_large_pages_;
  CompactableSpaceHandling compactable_space_handling_;
};
 
}  // namespace
 
class Sweeper::SweeperImpl final {
  using FreeMemoryHandling = SweepingConfig::FreeMemoryHandling;
 
 public:
  explicit SweeperImpl(HeapBase& heap)
      : heap_(heap.raw_heap()),
        page_pool_(heap.page_backend()->page_pool()),
        stats_collector_(heap.stats_collector()),
        platform_(heap.platform()) {
    CHECK_NOT_NULL(platform_);
  }
 
  ~SweeperImpl() { CancelAllSweepingTasks(); }
 
  void Start(SweepingConfig config) {
    StatsCollector::EnabledScope stats_scope(stats_collector_,
                                             StatsCollector::kAtomicSweep);
    is_in_progress_ = true;
    config_ = config;
 
    if (!foreground_task_runner_) {
      // The sweeper is already initialized when the platform may not be able to
      // return a foreground task runner. Lazily initialize the runners on first
      // sweep.
      foreground_task_runner_ =
          platform_->GetForegroundTaskRunner(kForegroundRegularPriority);
      low_priority_foreground_task_runner_ =
          platform_->GetForegroundTaskRunner(kForegroundLowPriority);
      // Having a low priority runner implies having a regular runner as well.
      CHECK_IMPLIES(low_priority_foreground_task_runner_.get(),
                    foreground_task_runner_.get());
      const auto supports_non_nestable_tasks =
          [](const std::shared_ptr<TaskRunner>& runner) {
            return runner && runner->NonNestableTasksEnabled() &&
                   runner->NonNestableDelayedTasksEnabled();
          };
      if (!supports_non_nestable_tasks(foreground_task_runner_) ||
          !supports_non_nestable_tasks(low_priority_foreground_task_runner_)) {
        foreground_task_runner_.reset();
        low_priority_foreground_task_runner_.reset();
      }
    }
 
    // Verify bitmap for all spaces regardless of |compactable_space_handling|.
    ObjectStartBitmapVerifier().Verify(heap_);
 
    // If inaccessible memory is touched to check whether it is set up
    // correctly it cannot be discarded.
    if (!CanDiscardMemory()) {
      config_.free_memory_handling = FreeMemoryHandling::kDoNotDiscard;
    }
    if (config_.free_memory_handling ==
        FreeMemoryHandling::kDiscardWherePossible) {
      // The discarded counter will be recomputed.
      heap_.heap()->stats_collector()->ResetDiscardedMemory();
    }
 
    PrepareForSweepVisitor(heap_.heap(), &space_states_, &empty_normal_pages_,
                           &empty_large_pages_,
                           config.compactable_space_handling)
        .Run(heap_);
 
    if (config.sweeping_type >= SweepingConfig::SweepingType::kIncremental) {
      ScheduleLowPriorityIncrementalSweeping();
      ScheduleIncrementalSweeping(kDelayWhileLowPrioritySweepingMakesProgress);
    }
    if (config.sweeping_type >=
        SweepingConfig::SweepingType::kIncrementalAndConcurrent) {
      ScheduleConcurrentSweeping();
    }
  }
 
  void SweepForTask(v8::base::TimeDelta max_duration) {
    // Before sweeping in a task, handle low priority sweeping cases. These are
    // no-ops if low priority sweeping is not running.
    if (low_priority_task_ran_) {
      // Low priority task made progress. Reschedule with delay.
      ScheduleIncrementalSweeping(kDelayWhileLowPrioritySweepingMakesProgress);
      return;
    }
 
    // Low priority sweeping is not running or not being invoked on time.
    switch (
        SweepInForegroundTaskImpl(max_duration, StatsCollector::kSweepInTask)) {
      case SweepResult::kFullyDone:
        return;
      case SweepResult::kInProgress:
        ScheduleIncrementalSweeping(kDelayForRegularPrioritySweeping);
        return;
      case SweepResult::kMainThreadDoneConcurrentInProgress:
        // Throttle incremental sweeping while the concurrent Job is still
        // making progress.
        ScheduleIncrementalSweeping(kDelayWhileConcurrentSweepingMakesProgress);
        return;
    }
    UNREACHABLE();
  }
 
  void SweepForLowPriorityTask(v8::base::TimeDelta max_duration) {
    low_priority_task_ran_ = true;
    switch (SweepInForegroundTaskImpl(
        max_duration, StatsCollector::kSweepInLowPriorityTask)) {
      case SweepResult::kFullyDone:
        return;
      case SweepResult::kInProgress:
        // More work to do. Continue sweeping with low priority.
        ScheduleLowPriorityIncrementalSweeping();
        return;
      case SweepResult::kMainThreadDoneConcurrentInProgress:
        ScheduleLowPriorityIncrementalSweeping(
            kDelayWhileLowPrioritySweepingMakesProgress);
        return;
    }
    UNREACHABLE();
  }
 
  bool SweepForLargeAllocation(BaseSpace* space, size_t size,
                               v8::base::TimeDelta max_duration) {
    DCHECK(space->is_large());
#ifdef DEBUG
    // SpaceState for large objects is emtpy as those objects are put directly
    // on `empty_large_pages_`.
    SweepingState& space_state = space_states_[space->index()];
    DCHECK(space_state.unswept_pages.IsEmpty());
    DCHECK(space_state.swept_unfinalized_pages.IsEmpty());
#endif  // DEBUG
    // Bail out if there's no empty large pages that could be freed and be
    // reused for a large allocation.
    if (empty_large_pages_.swept_unfinalized_pages.IsEmpty() &&
        empty_large_pages_.unswept_pages.IsEmpty()) {
      return false;
    }
 
    StatsCollector::EnabledScope incremental_sweep_scope(
        stats_collector_, StatsCollector::kIncrementalSweep);
    StatsCollector::DisabledScope sweep_on_allocation_scope(
        stats_collector_, StatsCollector::kSweepOnAllocation);
    MutatorThreadSweepingScope sweeping_in_progress(*this);
 
    const auto deadline = v8::base::TimeTicks::Now() + max_duration;
 
    SweepFinalizer finalizer(
        platform_, stats_collector_, space, &unused_destroyed_normal_pages_,
        config_.free_memory_handling, EmptyPageHandling::kDestroy);
    // Check empty pages first. Try to just finalize a page without sweeping.
    // If there's a single page in there we will use it.
    if (finalizer.FinalizeWithDeadlineAndSize(
            StatsCollector::kSweepFinalizeEmptyPages, empty_large_pages_,
            deadline, size)) {
      return true;
    }
    MutatorThreadSweeper sweeper(heap_.heap(), platform_, stats_collector_,
                                 space, &unused_destroyed_normal_pages_,
                                 config_.free_memory_handling,
                                 EmptyPageHandling::kDestroy);
    // Sweeping an empty page in case there's nothing with finalizers. If
    // there's a single page in there we will use it.
    if (sweeper.SweepWithDeadlineAndSize(StatsCollector::kSweepEmptyPages,
                                         empty_large_pages_, deadline, size)) {
      return true;
    }
 
    return false;
  }
 
  bool SweepForNormalAllocation(BaseSpace* space, size_t size,
                                v8::base::TimeDelta max_duration) {
    DCHECK(!space->is_large());
 
    if (unused_destroyed_normal_pages_ > 0 && page_pool_.pooled() > 0) {
      unused_destroyed_normal_pages_--;
      // Destroyed pages during sweeping in tasks are generally sitting in the
      // page pool and can be reused without increasing memory footprint.
      return false;
    }
 
    SweepingState& space_state = space_states_[space->index()];
 
    // Bail out if there's no empty pages and no pages to be processed for the
    // specific space at this moment.
    if (empty_normal_pages_.swept_unfinalized_pages.IsEmpty() &&
        empty_normal_pages_.unswept_pages.IsEmpty() &&
        space_state.swept_unfinalized_pages.IsEmpty() &&
        space_state.unswept_pages.IsEmpty()) {
      return false;
    }
 
    StatsCollector::EnabledScope incremental_sweep_scope(
        stats_collector_, StatsCollector::kIncrementalSweep);
    StatsCollector::DisabledScope sweep_on_allocation_scope(
        stats_collector_, StatsCollector::kSweepOnAllocation);
    MutatorThreadSweepingScope sweeping_in_progress(*this);
 
    const auto deadline = v8::base::TimeTicks::Now() + max_duration;
 
    SweepFinalizer finalizer(
        platform_, stats_collector_, space, &unused_destroyed_normal_pages_,
        config_.free_memory_handling, EmptyPageHandling::kReturn);
    // Check empty pages first. Try to just finalize a page without sweeping.
    // If there's a single page in there we will use it.
    if (finalizer.FinalizeWithDeadlineAndSize(
            StatsCollector::kSweepFinalizeEmptyPages, empty_normal_pages_,
            deadline, size)) {
      return true;
    }
    MutatorThreadSweeper sweeper(heap_.heap(), platform_, stats_collector_,
                                 space, &unused_destroyed_normal_pages_,
                                 config_.free_memory_handling,
                                 EmptyPageHandling::kReturn);
    // Sweeping an empty page in case there's nothing with finalizers. If
    // there's a single page in there we will use it.
    if (sweeper.SweepWithDeadlineAndSize(StatsCollector::kSweepEmptyPages,
                                         empty_normal_pages_, deadline, size)) {
      return true;
    }
 
    // Process unfinalized non-empty pages as finalizing a page is generally
    // faster than sweeping.
    if (finalizer.FinalizeWithDeadlineAndSize(
            StatsCollector::kSweepFinalizeSweptPages, space_state, deadline,
            size)) {
      return true;
    }
    // Then, if no matching slot is found in the unfinalized pages, search the
    // unswept page. This also helps out the concurrent sweeper.
    if (sweeper.SweepWithDeadlineAndSize(StatsCollector::kSweepPages,
                                         space_state, deadline, size)) {
      return true;
    }
    return false;
  }
 
  bool SweepForAllocationIfRunning(BaseSpace* space, size_t size,
                                   v8::base::TimeDelta max_duration) {
    if (!is_in_progress_) {
      return false;
    }
 
    // Bail out for recursive sweeping calls. This can happen when finalizers
    // allocate new memory.
    if (is_sweeping_on_mutator_thread_) {
      return false;
    }
 
    return space->is_large()
               ? SweepForLargeAllocation(space, size, max_duration)
               : SweepForNormalAllocation(space, size, max_duration);
  }
 
  bool FinishIfRunning() {
    if (!is_in_progress_) {
      return false;
    }
 
    // Bail out for recursive sweeping calls. This can happen when finalizers
    // allocate new memory.
    if (is_sweeping_on_mutator_thread_) {
      return false;
    }
 
    {
      std::optional<StatsCollector::EnabledScope> stats_scope;
      if (config_.sweeping_type != SweepingConfig::SweepingType::kAtomic) {
        stats_scope.emplace(stats_collector_,
                            StatsCollector::kIncrementalSweep);
      }
      StatsCollector::EnabledScope inner_scope(stats_collector_,
                                               StatsCollector::kSweepFinish);
      if (concurrent_sweeper_handle_ && concurrent_sweeper_handle_->IsValid() &&
          concurrent_sweeper_handle_->UpdatePriorityEnabled()) {
        concurrent_sweeper_handle_->UpdatePriority(kBackgroundBoostedPriority);
      }
      Finish();
    }
    NotifyDone();
    return true;
  }
 
  bool IsConcurrentSweepingDone() const {
    return !concurrent_sweeper_handle_ ||
           !concurrent_sweeper_handle_->IsValid() ||
           !concurrent_sweeper_handle_->IsActive();
  }
 
  void FinishIfOutOfWork() {
    if (!is_in_progress_ || is_sweeping_on_mutator_thread_) {
      return;
    }
    // We only finish through this method if concurrent sweeping is enabled but
    // not running anymore. All other paths finish sweeping through incremental
    // steps.
    if (!concurrent_sweeper_handle_ || !concurrent_sweeper_handle_->IsValid() ||
        concurrent_sweeper_handle_->IsActive()) {
      return;
    }
    // At this point we know that the concurrent sweeping task has run
    // out-of-work: all pages are swept. The main thread still needs to finalize
    // swept pages.
    DCHECK(std::all_of(space_states_.begin(), space_states_.end(),
                       [](const SweepingState& state) {
                         return state.unswept_pages.IsEmpty();
                       }));
    DCHECK(empty_normal_pages_.unswept_pages.IsEmpty());
    DCHECK(empty_large_pages_.unswept_pages.IsEmpty());
    if (std::any_of(space_states_.begin(), space_states_.end(),
                    [](const SweepingState& state) {
                      return !state.swept_unfinalized_pages.IsEmpty();
                    })) {
      return;
    }
    if (!empty_normal_pages_.swept_unfinalized_pages.IsEmpty() ||
        !empty_large_pages_.swept_unfinalized_pages.IsEmpty()) {
      return;
    }
    // All pages have also been finalized. Finalizing pages likely occured on
    // allocation, in which sweeping is not finalized even though all work is
    // done.
    {
      StatsCollector::EnabledScope stats_scope(
          stats_collector_, StatsCollector::kSweepFinishIfOutOfWork);
      FinalizeSweep();
    }
    NotifyDone();
  }
 
  void Finish() {
    DCHECK(is_in_progress_);
 
    MutatorThreadSweepingScope sweeping_in_progress(*this);
 
    // First, call finalizers on the mutator thread. This is just an
    // optimization as we need to call finalizers after sweeping as well. It
    // allows to spend the time in the concurrent sweeper for actual sweeping.
    SweepFinalizer finalizer(
        platform_, stats_collector_, kSweepWithoutSpaceAssignment,
        &unused_destroyed_normal_pages_, config_.free_memory_handling,
        EmptyPageHandling::kDestroy);
    finalizer.Finalize(space_states_);
    finalizer.Finalize(empty_normal_pages_);
    finalizer.Finalize(empty_large_pages_);
 
    // Then, help out the concurrent thread.
    MutatorThreadSweeper sweeper(
        heap_.heap(), platform_, stats_collector_, kSweepWithoutSpaceAssignment,
        &unused_destroyed_normal_pages_, config_.free_memory_handling,
        EmptyPageHandling::kDestroy);
    sweeper.Sweep(space_states_);
    sweeper.Sweep(empty_normal_pages_);
    sweeper.Sweep(empty_large_pages_);
 
    // There's nothing left to sweep here for the main thread. The concurrent
    // sweeper may still sweep pages and create pages to be finalized after
    // joining the the job.
    FinalizeSweep();
  }
 
  void FinalizeSweep() {
    // Synchronize with the concurrent sweeper and call remaining finalizers.
    SynchronizeAndFinalizeConcurrentAndIncrementalSweeping();
 
    // Clear space taken up by sweeper metadata.
    space_states_.clear();
 
    is_in_progress_ = false;
    notify_done_pending_ = true;
    unused_destroyed_normal_pages_ = 0;
  }
 
  void NotifyDone() {
    DCHECK(!is_in_progress_);
    DCHECK(notify_done_pending_);
    notify_done_pending_ = false;
    stats_collector_->NotifySweepingCompleted(config_.sweeping_type);
    if (config_.free_memory_handling ==
        FreeMemoryHandling::kDiscardWherePossible) {
      heap_.heap()->page_backend()->ReleasePooledPages();
    }
  }
 
  void WaitForConcurrentSweepingForTesting() {
    if (concurrent_sweeper_handle_) concurrent_sweeper_handle_->Join();
  }
 
  bool IsSweepingOnMutatorThread() const {
    return is_sweeping_on_mutator_thread_;
  }
 
  bool IsSweepingInProgress() const { return is_in_progress_; }
 
  bool PerformSweepOnMutatorThread(v8::base::TimeDelta max_duration,
                                   StatsCollector::ScopeId internal_scope_id,
                                   MutatorThreadSweepingMode sweeping_mode) {
    if (!is_in_progress_) return true;
 
    MutatorThreadSweepingScope sweeping_in_progress(*this);
 
    {
      StatsCollector::EnabledScope stats_scope(
          stats_collector_, StatsCollector::kIncrementalSweep);
 
      MutatorThreadSweeper sweeper(
          heap_.heap(), platform_, stats_collector_,
          kSweepWithoutSpaceAssignment, &unused_destroyed_normal_pages_,
          config_.free_memory_handling, EmptyPageHandling::kDestroy);
      {
        StatsCollector::EnabledScope inner_stats_scope(
            stats_collector_, internal_scope_id, "max_duration_ms",
            max_duration.InMillisecondsF(), "sweeping_mode",
            ToString(sweeping_mode));
        const auto deadline = v8::base::TimeTicks::Now() + max_duration;
        if (!sweeper.FinalizeAndSweepWithDeadline(
                StatsCollector::kSweepFinalizeEmptyPages, empty_normal_pages_,
                deadline, sweeping_mode)) {
          return false;
        }
        for (auto& state : space_states_) {
          if (!sweeper.FinalizeAndSweepWithDeadline(
                  StatsCollector::kSweepFinalizeSweptPages, state, deadline,
                  sweeping_mode)) {
            return false;
          }
        }
        if (!sweeper.FinalizeAndSweepWithDeadline(
                StatsCollector::kSweepFinalizeEmptyPages, empty_large_pages_,
                deadline, sweeping_mode)) {
          return false;
        }
        if (sweeping_mode != MutatorThreadSweepingMode::kAll) {
          return false;
        }
      }
      FinalizeSweep();
    }
    NotifyDone();
    return true;
  }
 
  void AddMutatorThreadSweepingObserver(
      Sweeper::SweepingOnMutatorThreadObserver* observer) {
    DCHECK_EQ(mutator_thread_sweeping_observers_.end(),
              std::find(mutator_thread_sweeping_observers_.begin(),
                        mutator_thread_sweeping_observers_.end(), observer));
    mutator_thread_sweeping_observers_.push_back(observer);
  }
 
  void RemoveMutatorThreadSweepingObserver(
      Sweeper::SweepingOnMutatorThreadObserver* observer) {
    const auto it =
        std::find(mutator_thread_sweeping_observers_.begin(),
                  mutator_thread_sweeping_observers_.end(), observer);
    DCHECK_NE(mutator_thread_sweeping_observers_.end(), it);
    mutator_thread_sweeping_observers_.erase(it);
  }
 
 private:
  class MutatorThreadSweepingScope final {
   public:
    explicit MutatorThreadSweepingScope(SweeperImpl& sweeper)
        : sweeper_(sweeper) {
      DCHECK(!sweeper_.is_sweeping_on_mutator_thread_);
      sweeper_.is_sweeping_on_mutator_thread_ = true;
      for (auto* observer : sweeper_.mutator_thread_sweeping_observers_) {
        observer->Start();
      }
    }
    ~MutatorThreadSweepingScope() {
      sweeper_.is_sweeping_on_mutator_thread_ = false;
      for (auto* observer : sweeper_.mutator_thread_sweeping_observers_) {
        observer->End();
      }
    }
 
    MutatorThreadSweepingScope(const MutatorThreadSweepingScope&) = delete;
    MutatorThreadSweepingScope& operator=(const MutatorThreadSweepingScope&) =
        delete;
 
   private:
    SweeperImpl& sweeper_;
  };
 
  class IncrementalSweepTask final : public cppgc::Task {
   public:
    using Handle = SingleThreadedHandle;
 
    static constexpr auto kMaxSweepDuration =
        v8::base::TimeDelta::FromMilliseconds(5);
 
    IncrementalSweepTask(SweeperImpl& sweeper, cppgc::TaskPriority priority)
        : sweeper_(sweeper),
          handle_(Handle::NonEmptyTag{}),
          priority_(priority) {}
 
    static Handle Post(SweeperImpl& sweeper,
                       const std::shared_ptr<cppgc::TaskRunner>& runner,
                       cppgc::TaskPriority priority,
                       std::optional<v8::base::TimeDelta> delay = {}) {
      auto task = std::make_unique<IncrementalSweepTask>(sweeper, priority);
      auto handle = task->handle_;
      if (delay.has_value()) {
        runner->PostNonNestableDelayedTask(std::move(task),
                                           delay->InSecondsF());
      } else {
        runner->PostNonNestableTask(std::move(task));
      }
      return handle;
    }
 
    void Run() override {
      if (handle_.IsCanceled()) {
        return;
      }
      switch (priority_) {
        case kForegroundRegularPriority:
          sweeper_.SweepForTask(kMaxSweepDuration);
          return;
        case kForegroundLowPriority:
          sweeper_.SweepForLowPriorityTask(kMaxSweepDuration);
          return;
        default:
          UNREACHABLE();
      }
    }
 
   private:
    SweeperImpl& sweeper_;
    // TODO(chromium:1056170): Change to CancelableTask.
    Handle handle_;
    cppgc::TaskPriority priority_;
  };
 
  enum class SweepResult {
    // Sweeping is fully done.
    kFullyDone,
    // Sweeping is still in progress.
    kInProgress,
    // Sweeping on the main thread is done but concurrent sweepers are still
    // making progress. This may be temporary.
    kMainThreadDoneConcurrentInProgress,
  };
 
  static constexpr double kMaxHeapPercentageForNoSweeping = 50;
 
  static constexpr auto kDelayWhileLowPrioritySweepingMakesProgress =
      v8::base::TimeDelta::FromMilliseconds(100);
 
  static constexpr auto kDelayWhileConcurrentSweepingMakesProgress =
      v8::base::TimeDelta::FromMilliseconds(5);
 
  // We use a small delay here to allow lower priority tasks to interrupt
  // sweeping and take over.
  static constexpr auto kDelayForRegularPrioritySweeping =
      v8::base::TimeDelta::FromMilliseconds(1);
 
  SweepResult SweepInForegroundTaskImpl(v8::base::TimeDelta max_duration,
                                        StatsCollector::ScopeId scope) {
    // First round of sweeping.
    bool concurrent_sweep_complete = IsConcurrentSweepingDone();
    const auto start = v8::base::TimeTicks::Now();
    bool main_thread_sweep_complete = PerformSweepOnMutatorThread(
        max_duration, scope,
        concurrent_sweep_complete ? MutatorThreadSweepingMode::kAll
                                  : MutatorThreadSweepingMode::kOnlyFinalizers);
    if (main_thread_sweep_complete && !concurrent_sweep_complete &&
        IsConcurrentSweepingDone()) {
      // Concurrent sweeping finished while processing the first round. Use the
      // left over time for a second round to avoid scheduling another task.
      max_duration -= (v8::base::TimeTicks::Now() - start);
      if (max_duration > v8::base::TimeDelta::FromMilliseconds(0)) {
        concurrent_sweep_complete = true;
        main_thread_sweep_complete = PerformSweepOnMutatorThread(
            max_duration, scope, MutatorThreadSweepingMode::kAll);
      }
    }
    if (main_thread_sweep_complete) {
      if (!concurrent_sweep_complete) {
        return SweepResult::kMainThreadDoneConcurrentInProgress;
      } else {
        CHECK(!is_in_progress_);
        return SweepResult::kFullyDone;
      }
    }
    return SweepResult::kInProgress;
  }
 
  void ScheduleIncrementalSweeping(
      std::optional<v8::base::TimeDelta> delay = {}) {
    DCHECK_GE(config_.sweeping_type,
              SweepingConfig::SweepingType::kIncremental);
 
    if (!foreground_task_runner_) {
      return;
    }
 
    low_priority_task_ran_ = false;
    incremental_sweeper_handle_.CancelIfNonEmpty();
    incremental_sweeper_handle_ = IncrementalSweepTask::Post(
        *this, foreground_task_runner_, kForegroundRegularPriority, delay);
  }
 
  void ScheduleLowPriorityIncrementalSweeping(
      std::optional<v8::base::TimeDelta> delay = {}) {
    DCHECK_GE(config_.sweeping_type,
              SweepingConfig::SweepingType::kIncremental);
 
    if (!low_priority_foreground_task_runner_) {
      return;
    }
 
    incremental_sweeper_low_priority_handle_.CancelIfNonEmpty();
    incremental_sweeper_low_priority_handle_ =
        IncrementalSweepTask::Post(*this, low_priority_foreground_task_runner_,
                                   kForegroundLowPriority, delay);
  }
 
  void ScheduleConcurrentSweeping() {
    DCHECK_GE(config_.sweeping_type,
              SweepingConfig::SweepingType::kIncrementalAndConcurrent);
 
    concurrent_sweeper_handle_ = platform_->PostJob(
        kBackgroundRegularPriority,
        std::make_unique<ConcurrentSweepTask>(
            platform_, *heap_.heap(), &space_states_, &empty_normal_pages_,
            &empty_large_pages_, config_.free_memory_handling));
  }
 
  void CancelAllSweepingTasks() {
    if (incremental_sweeper_handle_) {
      incremental_sweeper_handle_.Cancel();
    }
    if (incremental_sweeper_low_priority_handle_) {
      incremental_sweeper_low_priority_handle_.Cancel();
    }
    if (concurrent_sweeper_handle_ && concurrent_sweeper_handle_->IsValid()) {
      concurrent_sweeper_handle_->Cancel();
    }
  }
 
  void SynchronizeAndFinalizeConcurrentAndIncrementalSweeping() {
    // The precondition for this call is that actual sweeping is done. So all
    // that's left is potentially invoking finalizers.
 
    CancelAllSweepingTasks();
 
    DCHECK(std::all_of(space_states_.begin(), space_states_.end(),
                       [](const SweepingState& state) {
                         return state.unswept_pages.IsEmpty();
                       }));
    DCHECK(empty_normal_pages_.unswept_pages.IsEmpty());
    DCHECK(empty_large_pages_.unswept_pages.IsEmpty());
 
    SweepFinalizer finalizer(
        platform_, stats_collector_, kSweepWithoutSpaceAssignment,
        &unused_destroyed_normal_pages_, config_.free_memory_handling,
        EmptyPageHandling::kDestroy);
    finalizer.Finalize(space_states_);
    finalizer.Finalize(empty_normal_pages_);
    finalizer.Finalize(empty_large_pages_);
  }
 
  RawHeap& heap_;
  NormalPageMemoryPool& page_pool_;
  StatsCollector* const stats_collector_;
  SpaceStates space_states_;
  // States for empty normal pages. These pages do have a space as owner which
  // is updated as soon as the page is reused for a specific space.
  SweepingState empty_normal_pages_;
  // States for empty large pages.
  // TODO(372512096): This can be further split into LO pages that are less than
  // a regular page size and those that are multiple, where larger sizes can
  // contribute to `unused_destroyed_normal_pages_`.
  SweepingState empty_large_pages_;
  // Number of pages that have been destroyed and have not been reused by the
  // allocator yet. We assume that returning early on
  // SweepForAllocationIfRunning() causes such pages to be picked up.
  size_t unused_destroyed_normal_pages_ = 0;
  cppgc::Platform* platform_;
  std::shared_ptr<cppgc::TaskRunner> foreground_task_runner_;
  std::shared_ptr<cppgc::TaskRunner> low_priority_foreground_task_runner_;
  SweepingConfig config_;
  IncrementalSweepTask::Handle incremental_sweeper_handle_;
  IncrementalSweepTask::Handle incremental_sweeper_low_priority_handle_;
  std::unique_ptr<cppgc::JobHandle> concurrent_sweeper_handle_;
  std::vector<Sweeper::SweepingOnMutatorThreadObserver*>
      mutator_thread_sweeping_observers_;
  // Indicates whether a low priority task has been invoked since the last
  // scheduling of an incremental task.
  bool low_priority_task_ran_ = false;
  // Indicates whether the sweeping phase is in progress.
  bool is_in_progress_ = false;
  bool notify_done_pending_ = false;
  // Indicates whether whether the sweeper (or its finalization) is currently
  // running on the main thread.
  bool is_sweeping_on_mutator_thread_ = false;
};
 
Sweeper::Sweeper(HeapBase& heap)
    : heap_(heap), impl_(std::make_unique<SweeperImpl>(heap)) {}
 
Sweeper::~Sweeper() = default;
 
void Sweeper::Start(SweepingConfig config) { impl_->Start(config); }
 
bool Sweeper::FinishIfRunning() { return impl_->FinishIfRunning(); }
 
void Sweeper::FinishIfOutOfWork() { impl_->FinishIfOutOfWork(); }
 
void Sweeper::WaitForConcurrentSweepingForTesting() {
  impl_->WaitForConcurrentSweepingForTesting();
}
 
bool Sweeper::SweepForAllocationIfRunning(BaseSpace* space, size_t size,
                                          v8::base::TimeDelta max_duration) {
  return impl_->SweepForAllocationIfRunning(space, size, max_duration);
}
 
bool Sweeper::IsSweepingOnMutatorThread() const {
  return impl_->IsSweepingOnMutatorThread();
}
 
bool Sweeper::IsSweepingInProgress() const {
  return impl_->IsSweepingInProgress();
}
 
bool Sweeper::PerformSweepOnMutatorThread(v8::base::TimeDelta max_duration,
                                          StatsCollector::ScopeId scope_id) {
  return impl_->PerformSweepOnMutatorThread(max_duration, scope_id,
                                            MutatorThreadSweepingMode::kAll);
}
 
Sweeper::SweepingOnMutatorThreadObserver::SweepingOnMutatorThreadObserver(
    Sweeper& sweeper)
    : sweeper_(sweeper) {
  sweeper_.impl_->AddMutatorThreadSweepingObserver(this);
}
 
Sweeper::SweepingOnMutatorThreadObserver::~SweepingOnMutatorThreadObserver() {
  sweeper_.impl_->RemoveMutatorThreadSweepingObserver(this);
}
 
}  // namespace cppgc::internal