compactor.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/compactor.h"
 
#include <map>
#include <numeric>
#include <unordered_map>
#include <unordered_set>
 
#include "include/cppgc/macros.h"
#include "src/heap/cppgc/compaction-worklists.h"
#include "src/heap/cppgc/globals.h"
#include "src/heap/cppgc/heap-base.h"
#include "src/heap/cppgc/heap-page.h"
#include "src/heap/cppgc/heap-space.h"
#include "src/heap/cppgc/memory.h"
#include "src/heap/cppgc/object-poisoner.h"
#include "src/heap/cppgc/raw-heap.h"
#include "src/heap/cppgc/stats-collector.h"
 
namespace cppgc {
namespace internal {
 
namespace {
// Freelist size threshold that must be exceeded before compaction
// should be considered.
static constexpr size_t kFreeListSizeThreshold = 512 * kKB;
 
// The real worker behind heap compaction, recording references to movable
// objects ("slots".) When the objects end up being compacted and moved,
// relocate() will adjust the slots to point to the new location of the
// object along with handling references for interior pointers.
//
// The MovableReferences object is created and maintained for the lifetime
// of one heap compaction-enhanced GC.
class MovableReferences final {
  using MovableReference = CompactionWorklists::MovableReference;
 
 public:
  explicit MovableReferences(HeapBase& heap)
      : heap_(heap), heap_has_move_listeners_(heap.HasMoveListeners()) {}
 
  // Adds a slot for compaction. Filters slots in dead objects.
  void AddOrFilter(MovableReference*);
 
  // Relocates a backing store |from| -> |to|.
  void Relocate(Address from, Address to, size_t size_including_header);
 
  // Relocates interior slots in a backing store that is moved |from| -> |to|.
  void RelocateInteriorReferences(Address from, Address to, size_t size);
 
  // Updates the collection of callbacks from the item pushed the worklist by
  // marking visitors.
  void UpdateCallbacks();
 
 private:
  HeapBase& heap_;
 
  // Map from movable reference (value) to its slot. Upon moving an object its
  // slot pointing to it requires updating. Movable reference should currently
  // have only a single movable reference to them registered.
  std::unordered_map<MovableReference, MovableReference*> movable_references_;
 
  // Map of interior slots to their final location. Needs to be an ordered map
  // as it is used to walk through slots starting at a given memory address.
  // Requires log(n) lookup to make the early bailout reasonably fast.
  //
  // - The initial value for a given key is nullptr.
  // - Upon moving an object this value is adjusted accordingly.
  std::map<MovableReference*, Address> interior_movable_references_;
 
  const bool heap_has_move_listeners_;
 
#if DEBUG
  // The following two collections are used to allow refer back from a slot to
  // an already moved object.
  std::unordered_set<const void*> moved_objects_;
  std::unordered_map<MovableReference*, MovableReference>
      interior_slot_to_object_;
#endif  // DEBUG
};
 
void MovableReferences::AddOrFilter(MovableReference* slot) {
  const BasePage* slot_page = BasePage::FromInnerAddress(&heap_, slot);
  CHECK_NOT_NULL(slot_page);
 
  const void* value = *slot;
  if (!value) return;
 
  // All slots and values are part of Oilpan's heap.
  // - Slots may be contained within dead objects if e.g. the write barrier
  //   registered the slot while backing itself has not been marked live in
  //   time. Slots in dead objects are filtered below.
  // - Values may only be contained in or point to live objects.
 
  const HeapObjectHeader& slot_header =
      slot_page->ObjectHeaderFromInnerAddress(slot);
  // Filter the slot since the object that contains the slot is dead.
  if (!slot_header.IsMarked()) return;
 
  const BasePage* value_page = BasePage::FromInnerAddress(&heap_, value);
  CHECK_NOT_NULL(value_page);
 
  // The following cases are not compacted and do not require recording:
  // - Compactable object on large pages.
  // - Compactable object on non-compactable spaces.
  if (value_page->is_large() || !value_page->space().is_compactable()) return;
 
  // Slots must reside in and values must point to live objects at this
  // point. |value| usually points to a separate object but can also point
  // to the an interior pointer in the same object storage which is why the
  // dynamic header lookup is required.
  const HeapObjectHeader& value_header =
      value_page->ObjectHeaderFromInnerAddress(value);
  CHECK(value_header.IsMarked());
 
  // Slots may have been recorded already but must point to the same value.
  auto reference_it = movable_references_.find(value);
  if (V8_UNLIKELY(reference_it != movable_references_.end())) {
    CHECK_EQ(slot, reference_it->second);
    return;
  }
 
  // Add regular movable reference.
  movable_references_.emplace(value, slot);
 
  // Check whether the slot itself resides on a page that is compacted.
  if (V8_LIKELY(!slot_page->space().is_compactable())) return;
 
  CHECK_EQ(interior_movable_references_.end(),
           interior_movable_references_.find(slot));
  interior_movable_references_.emplace(slot, nullptr);
#if DEBUG
  interior_slot_to_object_.emplace(slot, slot_header.ObjectStart());
#endif  // DEBUG
}
 
void MovableReferences::Relocate(Address from, Address to,
                                 size_t size_including_header) {
#if DEBUG
  moved_objects_.insert(from);
#endif  // DEBUG
 
  if (V8_UNLIKELY(heap_has_move_listeners_)) {
    heap_.CallMoveListeners(from - sizeof(HeapObjectHeader),
                            to - sizeof(HeapObjectHeader),
                            size_including_header);
  }
 
  // Interior slots always need to be processed for moved objects.
  // Consider an object A with slot A.x pointing to value B where A is
  // allocated on a movable page itself. When B is finally moved, it needs to
  // find the corresponding slot A.x. Object A may be moved already and the
  // memory may have been freed, which would result in a crash.
  if (!interior_movable_references_.empty()) {
    const HeapObjectHeader& header = HeapObjectHeader::FromObject(to);
    const size_t size = header.ObjectSize();
    RelocateInteriorReferences(from, to, size);
  }
 
  auto it = movable_references_.find(from);
  // This means that there is no corresponding slot for a live object.
  // This may happen because a mutator may change the slot to point to a
  // different object because e.g. incremental marking marked an object
  // as live that was later on replaced.
  if (it == movable_references_.end()) {
    return;
  }
 
  // If the object is referenced by a slot that is contained on a compacted
  // area itself, check whether it can be updated already.
  MovableReference* slot = it->second;
  auto interior_it = interior_movable_references_.find(slot);
  if (interior_it != interior_movable_references_.end()) {
    MovableReference* slot_location =
        reinterpret_cast<MovableReference*>(interior_it->second);
    if (!slot_location) {
      interior_it->second = to;
#if DEBUG
      // Check that the containing object has not been moved yet.
      auto reverse_it = interior_slot_to_object_.find(slot);
      DCHECK_NE(interior_slot_to_object_.end(), reverse_it);
      DCHECK_EQ(moved_objects_.end(), moved_objects_.find(reverse_it->second));
#endif  // DEBUG
    } else {
      slot = slot_location;
    }
  }
 
  // Compaction is atomic so slot should not be updated during compaction.
  DCHECK_EQ(from, *slot);
 
  // Update the slots new value.
  *slot = to;
}
 
void MovableReferences::RelocateInteriorReferences(Address from, Address to,
                                                   size_t size) {
  // |from| is a valid address for a slot.
  auto interior_it = interior_movable_references_.lower_bound(
      reinterpret_cast<MovableReference*>(from));
  if (interior_it == interior_movable_references_.end()) return;
  DCHECK_GE(reinterpret_cast<Address>(interior_it->first), from);
 
  size_t offset = reinterpret_cast<Address>(interior_it->first) - from;
  while (offset < size) {
    if (!interior_it->second) {
      // Update the interior reference value, so that when the object the slot
      // is pointing to is moved, it can reuse this value.
      Address reference = to + offset;
      interior_it->second = reference;
 
      // If the |slot|'s content is pointing into the region [from, from +
      // size) we are dealing with an interior pointer that does not point to
      // a valid HeapObjectHeader. Such references need to be fixed up
      // immediately.
      Address& reference_contents = *reinterpret_cast<Address*>(reference);
      if (reference_contents > from && reference_contents < (from + size)) {
        reference_contents = reference_contents - from + to;
      }
    }
 
    interior_it++;
    if (interior_it == interior_movable_references_.end()) return;
    offset = reinterpret_cast<Address>(interior_it->first) - from;
  }
}
 
class CompactionState final {
  CPPGC_STACK_ALLOCATED();
  using Pages = std::vector<NormalPage*>;
 
 public:
  CompactionState(NormalPageSpace* space, MovableReferences& movable_references)
      : space_(space), movable_references_(movable_references) {}
 
  void AddPage(NormalPage* page) {
    DCHECK_EQ(space_, &page->space());
    // If not the first page, add |page| onto the available pages chain.
    if (!current_page_)
      current_page_ = page;
    else
      available_pages_.push_back(page);
  }
 
  void RelocateObject(const NormalPage* page, const Address header,
                      size_t size) {
    // Allocate and copy over the live object.
    Address compact_frontier =
        current_page_->PayloadStart() + used_bytes_in_current_page_;
    if (compact_frontier + size > current_page_->PayloadEnd()) {
      // Can't fit on current page. Add remaining onto the freelist and advance
      // to next available page.
      ReturnCurrentPageToSpace();
 
      current_page_ = available_pages_.back();
      available_pages_.pop_back();
      used_bytes_in_current_page_ = 0;
      compact_frontier = current_page_->PayloadStart();
    }
    if (V8_LIKELY(compact_frontier != header)) {
      // Use a non-overlapping copy, if possible.
      if (current_page_ == page)
        memmove(compact_frontier, header, size);
      else
        memcpy(compact_frontier, header, size);
      movable_references_.Relocate(header + sizeof(HeapObjectHeader),
                                   compact_frontier + sizeof(HeapObjectHeader),
                                   size);
    }
    current_page_->object_start_bitmap().SetBit(compact_frontier);
    used_bytes_in_current_page_ += size;
    DCHECK_LE(used_bytes_in_current_page_, current_page_->PayloadSize());
  }
 
  void FinishCompactingSpace() {
    // If the current page hasn't been allocated into, add it to the available
    // list, for subsequent release below.
    if (used_bytes_in_current_page_ == 0) {
      available_pages_.push_back(current_page_);
    } else {
      ReturnCurrentPageToSpace();
    }
 
    // Return remaining available pages back to the backend.
    for (NormalPage* page : available_pages_) {
      SetMemoryInaccessible(page->PayloadStart(), page->PayloadSize());
      NormalPage::Destroy(page);
    }
  }
 
  void FinishCompactingPage(NormalPage* page) {
#if DEBUG || defined(V8_USE_MEMORY_SANITIZER) || \
    defined(V8_USE_ADDRESS_SANITIZER)
    // Zap the unused portion, until it is either compacted into or freed.
    if (current_page_ != page) {
      ZapMemory(page->PayloadStart(), page->PayloadSize());
    } else {
      ZapMemory(page->PayloadStart() + used_bytes_in_current_page_,
                page->PayloadSize() - used_bytes_in_current_page_);
    }
#endif
    page->object_start_bitmap().MarkAsFullyPopulated();
  }
 
 private:
  void ReturnCurrentPageToSpace() {
    DCHECK_EQ(space_, &current_page_->space());
    space_->AddPage(current_page_);
    if (used_bytes_in_current_page_ != current_page_->PayloadSize()) {
      // Put the remainder of the page onto the free list.
      size_t freed_size =
          current_page_->PayloadSize() - used_bytes_in_current_page_;
      Address payload = current_page_->PayloadStart();
      Address free_start = payload + used_bytes_in_current_page_;
      SetMemoryInaccessible(free_start, freed_size);
      space_->free_list().Add({free_start, freed_size});
      current_page_->object_start_bitmap().SetBit(free_start);
    }
  }
 
  NormalPageSpace* space_;
  MovableReferences& movable_references_;
  // Page into which compacted object will be written to.
  NormalPage* current_page_ = nullptr;
  // Offset into |current_page_| to the next free address.
  size_t used_bytes_in_current_page_ = 0;
  // Additional pages in the current space that can be used as compaction
  // targets. Pages that remain available at the compaction can be released.
  Pages available_pages_;
};
 
void CompactPage(NormalPage* page, CompactionState& compaction_state,
                 StickyBits sticky_bits) {
  compaction_state.AddPage(page);
 
  page->object_start_bitmap().Clear();
 
  for (Address header_address = page->PayloadStart();
       header_address < page->PayloadEnd();) {
    HeapObjectHeader* header =
        reinterpret_cast<HeapObjectHeader*>(header_address);
    size_t size = header->AllocatedSize();
    DCHECK_GT(size, 0u);
    DCHECK_LT(size, kPageSize);
 
    if (header->IsFree()) {
      // Unpoison the freelist entry so that we can compact into it as wanted.
      ASAN_UNPOISON_MEMORY_REGION(header_address, size);
      header_address += size;
      continue;
    }
 
    if (!header->IsMarked()) {
      // Compaction is currently launched only from AtomicPhaseEpilogue, so it's
      // guaranteed to be on the mutator thread - no need to postpone
      // finalization.
      header->Finalize();
 
      // As compaction is under way, leave the freed memory accessible
      // while compacting the rest of the page. We just zap the payload
      // to catch out other finalizers trying to access it.
#if DEBUG || defined(V8_USE_MEMORY_SANITIZER) || \
    defined(V8_USE_ADDRESS_SANITIZER)
      ZapMemory(header, size);
#endif
      header_address += size;
      continue;
    }
 
    // Object is marked.
#if defined(CPPGC_YOUNG_GENERATION)
    if (sticky_bits == StickyBits::kDisabled) header->Unmark();
#else   // !defined(CPPGC_YOUNG_GENERATION)
    header->Unmark();
#endif  // !defined(CPPGC_YOUNG_GENERATION)
 
    // Potentially unpoison the live object as well as it is the source of
    // the copy.
    ASAN_UNPOISON_MEMORY_REGION(header->ObjectStart(), header->ObjectSize());
    compaction_state.RelocateObject(page, header_address, size);
    header_address += size;
  }
 
  compaction_state.FinishCompactingPage(page);
}
 
void CompactSpace(NormalPageSpace* space, MovableReferences& movable_references,
                  StickyBits sticky_bits) {
  using Pages = NormalPageSpace::Pages;
 
#ifdef V8_USE_ADDRESS_SANITIZER
  UnmarkedObjectsPoisoner().Traverse(*space);
#endif  // V8_USE_ADDRESS_SANITIZER
 
  DCHECK(space->is_compactable());
 
  space->free_list().Clear();
 
  // Compaction generally follows Jonker's algorithm for fast garbage
  // compaction. Compaction is performed in-place, sliding objects down over
  // unused holes for a smaller heap page footprint and improved locality. A
  // "compaction pointer" is consequently kept, pointing to the next available
  // address to move objects down to. It will belong to one of the already
  // compacted pages for this space, but as compaction proceeds, it will not
  // belong to the same page as the one being currently compacted.
  //
  // The compaction pointer is represented by the
  // |(current_page_, used_bytes_in_current_page_)| pair, with
  // |used_bytes_in_current_page_| being the offset into |current_page_|, making
  // up the next available location. When the compaction of an arena page causes
  // the compaction pointer to exhaust the current page it is compacting into,
  // page compaction will advance the current page of the compaction
  // pointer, as well as the allocation point.
  //
  // By construction, the page compaction can be performed without having
  // to allocate any new pages. So to arrange for the page compaction's
  // supply of freed, available pages, we chain them together after each
  // has been "compacted from". The page compaction will then reuse those
  // as needed, and once finished, the chained, available pages can be
  // released back to the OS.
  //
  // To ease the passing of the compaction state when iterating over an
  // arena's pages, package it up into a |CompactionState|.
 
  Pages pages = space->RemoveAllPages();
  if (pages.empty()) return;
 
  CompactionState compaction_state(space, movable_references);
  for (BasePage* page : pages) {
    page->ResetMarkedBytes();
    // Large objects do not belong to this arena.
    CompactPage(NormalPage::From(page), compaction_state, sticky_bits);
  }
 
  compaction_state.FinishCompactingSpace();
  // Sweeping will verify object start bitmap of compacted space.
}
 
size_t UpdateHeapResidency(const std::vector<NormalPageSpace*>& spaces) {
  return std::accumulate(spaces.cbegin(), spaces.cend(), 0u,
                         [](size_t acc, const NormalPageSpace* space) {
                           DCHECK(space->is_compactable());
                           if (!space->size()) return acc;
                           return acc + space->free_list().Size();
                         });
}
 
}  // namespace
 
Compactor::Compactor(RawHeap& heap) : heap_(heap) {
  for (auto& space : heap_) {
    if (!space->is_compactable()) continue;
    DCHECK_EQ(&heap, space->raw_heap());
    compactable_spaces_.push_back(static_cast<NormalPageSpace*>(space.get()));
  }
}
 
bool Compactor::ShouldCompact(GCConfig::MarkingType marking_type,
                              StackState stack_state) const {
  if (compactable_spaces_.empty() ||
      (marking_type == GCConfig::MarkingType::kAtomic &&
       stack_state == StackState::kMayContainHeapPointers)) {
    // The following check ensures that tests that want to test compaction are
    // not interrupted by garbage collections that cannot use compaction.
    DCHECK(!enable_for_next_gc_for_testing_);
    return false;
  }
 
  if (enable_for_next_gc_for_testing_) {
    return true;
  }
 
  size_t free_list_size = UpdateHeapResidency(compactable_spaces_);
 
  return free_list_size > kFreeListSizeThreshold;
}
 
void Compactor::InitializeIfShouldCompact(GCConfig::MarkingType marking_type,
                                          StackState stack_state) {
  DCHECK(!is_enabled_);
 
  if (!ShouldCompact(marking_type, stack_state)) return;
 
  compaction_worklists_ = std::make_unique<CompactionWorklists>();
 
  is_enabled_ = true;
  is_cancelled_ = false;
}
 
void Compactor::CancelIfShouldNotCompact(GCConfig::MarkingType marking_type,
                                         StackState stack_state) {
  if (!is_enabled_ || ShouldCompact(marking_type, stack_state)) return;
 
  is_cancelled_ = true;
  is_enabled_ = false;
}
 
Compactor::CompactableSpaceHandling Compactor::CompactSpacesIfEnabled() {
  if (is_cancelled_ && compaction_worklists_) {
    compaction_worklists_->movable_slots_worklist()->Clear();
    compaction_worklists_.reset();
  }
  if (!is_enabled_) return CompactableSpaceHandling::kSweep;
 
  StatsCollector::EnabledScope stats_scope(heap_.heap()->stats_collector(),
                                           StatsCollector::kAtomicCompact);
 
  MovableReferences movable_references(*heap_.heap());
 
  CompactionWorklists::MovableReferencesWorklist::Local local(
      *compaction_worklists_->movable_slots_worklist());
  CompactionWorklists::MovableReference* slot;
  while (local.Pop(&slot)) {
    movable_references.AddOrFilter(slot);
  }
  compaction_worklists_.reset();
 
  const StickyBits sticky_bits = heap_.heap()->sticky_bits();
 
  for (NormalPageSpace* space : compactable_spaces_) {
    CompactSpace(space, movable_references, sticky_bits);
  }
 
  enable_for_next_gc_for_testing_ = false;
  is_enabled_ = false;
  return CompactableSpaceHandling::kIgnore;
}
 
void Compactor::EnableForNextGCForTesting() {
  DCHECK_NULL(heap_.heap()->marker());
  enable_for_next_gc_for_testing_ = true;
}
 
}  // namespace internal
}  // namespace cppgc