src/gc/ArenaList.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 * vim: set ts=8 sts=2 et sw=2 tw=80:
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

/*
 * GC-internal definitions of ArenaList and associated heap data structures.
 */

#ifndef gc_ArenaList_h
#define gc_ArenaList_h

#include "gc/AllocKind.h"
#include "js/GCAPI.h"
#include "js/HeapAPI.h"
#include "js/SliceBudget.h"
#include "js/TypeDecls.h"
#include "threading/ProtectedData.h"

namespace js {

class Nursery;
class TenuringTracer;

namespace gcstats {
struct Statistics;
}

namespace gc {

class Arena;
class BackgroundUnmarkTask;
struct FinalizePhase;
class FreeSpan;
class TenuredCell;

/*
 * A single segment of a SortedArenaList. Each segment has a head and a tail,
 * which track the start and end of a segment for O(1) append and concatenation.
 */
struct SortedArenaListSegment {
  Arena* head;
  Arena** tailp;

  void clear() {
    head = nullptr;
    tailp = &head;
  }

  bool isEmpty() const { return tailp == &head; }

  // Appends |arena| to this segment.
  inline void append(Arena* arena);

  // Points the tail of this segment at |arena|, which may be null. Note
  // that this does not change the tail itself, but merely which arena
  // follows it. This essentially turns the tail into a cursor (see also the
  // description of ArenaList), but from the perspective of a SortedArenaList
  // this makes no difference.
  void linkTo(Arena* arena) { *tailp = arena; }
};

/*
 * Arena lists contain a singly linked lists of arenas starting from a head
 * pointer.
 *
 * They also have a cursor, which conceptually lies on arena boundaries,
 * i.e. before the first arena, between two arenas, or after the last arena.
 *
 * Arenas are usually sorted in order of increasing free space, with the cursor
 * following the Arena currently being allocated from. This ordering should not
 * be treated as an invariant, however, as the free lists may be cleared,
 * leaving arenas previously used for allocation partially full. Sorting order
 * is restored during sweeping.
 *
 * Arenas following the cursor should not be full.
 */
class ArenaList {
  // The cursor is implemented via an indirect pointer, |cursorp_|, to allow
  // for efficient list insertion at the cursor point and other list
  // manipulations.
  //
  // - If the list is empty: |head| is null, |cursorp_| points to |head|, and
  //   therefore |*cursorp_| is null.
  //
  // - If the list is not empty: |head| is non-null, and...
  //
  //   - If the cursor is at the start of the list: |cursorp_| points to
  //     |head|, and therefore |*cursorp_| points to the first arena.
  //
  //   - If cursor is at the end of the list: |cursorp_| points to the |next|
  //     field of the last arena, and therefore |*cursorp_| is null.
  //
  //   - If the cursor is at neither the start nor the end of the list:
  //     |cursorp_| points to the |next| field of the arena preceding the
  //     cursor, and therefore |*cursorp_| points to the arena following the
  //     cursor.
  //
  // |cursorp_| is never null.
  //
  Arena* head_;
  Arena** cursorp_;

  // Transfers the contents of |other| to this list and clears |other|.
  inline void moveFrom(ArenaList& other);

 public:
  inline ArenaList();
  inline ArenaList(ArenaList&& other);
  inline ~ArenaList();

  inline ArenaList& operator=(ArenaList&& other);

  // It doesn't make sense for arenas to be present in more than one list, so
  // list copy operations are not provided.
  ArenaList(const ArenaList& other) = delete;
  ArenaList& operator=(const ArenaList& other) = delete;

  inline explicit ArenaList(const SortedArenaListSegment& segment);

  inline void check() const;

  inline void clear();
  inline bool isEmpty() const;

  // This returns nullptr if the list is empty.
  inline Arena* head() const;

  inline bool isCursorAtHead() const;
  inline bool isCursorAtEnd() const;

  // This can return nullptr.
  inline Arena* arenaAfterCursor() const;

  // This returns the arena after the cursor and moves the cursor past it.
  inline Arena* takeNextArena();

  // This does two things.
  // - Inserts |a| at the cursor.
  // - Leaves the cursor sitting just before |a|, if |a| is not full, or just
  //   after |a|, if |a| is full.
  inline void insertAtCursor(Arena* a);

  // Inserts |a| at the cursor, then moves the cursor past it.
  inline void insertBeforeCursor(Arena* a);

  // This inserts the contents of |other|, which must be full, at the cursor of
  // |this| and clears |other|.
  inline ArenaList& insertListWithCursorAtEnd(ArenaList& other);

  inline Arena* takeFirstArena();

  Arena* removeRemainingArenas(Arena** arenap);
  Arena** pickArenasToRelocate(size_t& arenaTotalOut, size_t& relocTotalOut);
  Arena* relocateArenas(Arena* toRelocate, Arena* relocated,
                        js::SliceBudget& sliceBudget,
                        gcstats::Statistics& stats);

#ifdef DEBUG
  void dump();
#endif
};

/*
 * A class that holds arenas in sorted order by appending arenas to specific
 * segments. Each segment has a head and a tail, which can be linked up to
 * other segments to create a contiguous ArenaList.
 */
class SortedArenaList {
 public:
  // The minimum size, in bytes, of a GC thing.
  static const size_t MinThingSize = 16;

  static_assert(ArenaSize <= 4096,
                "When increasing the Arena size, please consider how"
                " this will affect the size of a SortedArenaList.");

  static_assert(MinThingSize >= 16,
                "When decreasing the minimum thing size, please consider"
                " how this will affect the size of a SortedArenaList.");

 private:
  // The maximum number of GC things that an arena can hold.
  static const size_t MaxThingsPerArena =
      (ArenaSize - ArenaHeaderSize) / MinThingSize;

  size_t thingsPerArena_;
  SortedArenaListSegment segments[MaxThingsPerArena + 1];

  // Convenience functions to get the nth head and tail.
  Arena* headAt(size_t n) { return segments[n].head; }
  Arena** tailAt(size_t n) { return segments[n].tailp; }

 public:
  inline explicit SortedArenaList(size_t thingsPerArena = MaxThingsPerArena);

  inline void setThingsPerArena(size_t thingsPerArena);

  // Resets the first |thingsPerArena| segments of this list for further use.
  inline void reset(size_t thingsPerArena = MaxThingsPerArena);

  // Inserts an arena, which has room for |nfree| more things, in its segment.
  inline void insertAt(Arena* arena, size_t nfree);

  // Remove all empty arenas, inserting them as a linked list.
  inline void extractEmpty(Arena** empty);

  // Links up the tail of each non-empty segment to the head of the next
  // non-empty segment, creating a contiguous list that is returned as an
  // ArenaList. This is not a destructive operation: neither the head nor tail
  // of any segment is modified. However, note that the Arenas in the
  // resulting ArenaList should be treated as read-only unless the
  // SortedArenaList is no longer needed: inserting or removing arenas would
  // invalidate the SortedArenaList.
  inline ArenaList toArenaList();
};

enum class ShouldCheckThresholds {
  DontCheckThresholds = 0,
  CheckThresholds = 1
};

// For each arena kind its free list is represented as the first span with free
// things. Initially all the spans are initialized as empty. After we find a new
// arena with available things we move its first free span into the list and set
// the arena as fully allocated. That way we do not need to update the arena
// after the initial allocation. When starting the GC we only move the head of
// the of the list of spans back to the arena only for the arena that was not
// fully allocated.
class FreeLists {
  AllAllocKindArray<FreeSpan*> freeLists_;

 public:
  // Because the JITs can allocate from the free lists, they cannot be null.
  // We use a placeholder FreeSpan that is empty (and wihout an associated
  // Arena) so the JITs can fall back gracefully.
  static FreeSpan emptySentinel;

  FreeLists();

#ifdef DEBUG
  inline bool allEmpty() const;
  inline bool isEmpty(AllocKind kind) const;
#endif

  inline void clear();

  MOZ_ALWAYS_INLINE TenuredCell* allocate(AllocKind kind);

  inline TenuredCell* setArenaAndAllocate(Arena* arena, AllocKind kind);

  inline void unmarkPreMarkedFreeCells(AllocKind kind);

  FreeSpan** addressOfFreeList(AllocKind thingKind) {
    return &freeLists_[thingKind];
  }
};

class ArenaLists {
  enum class ConcurrentUse : uint32_t { None, BackgroundFinalize };

  using ConcurrentUseState =
      mozilla::Atomic<ConcurrentUse, mozilla::SequentiallyConsistent>;

  JS::Zone* zone_;

  // Whether this structure can be accessed by other threads.
  UnprotectedData<AllAllocKindArray<ConcurrentUseState>> concurrentUseState_;

  ZoneData<FreeLists> freeLists_;

  /* The main list of arenas for each alloc kind. */
  ArenaListData<AllAllocKindArray<ArenaList>> arenaLists_;

  /*
   * Arenas which are currently being collected. The collector can move arenas
   * from arenaLists_ here and back again at various points in collection.
   */
  ZoneOrGCTaskData<AllAllocKindArray<ArenaList>> collectingArenaLists_;

  /* During incremental sweeping, a list of the arenas already swept. */
  ZoneOrGCTaskData<AllocKind> incrementalSweptArenaKind;
  ZoneOrGCTaskData<ArenaList> incrementalSweptArenas;

  // Arena lists which have yet to be swept, but need additional foreground
  // processing before they are swept.
  ZoneData<Arena*> gcCompactPropMapArenasToUpdate;
  ZoneData<Arena*> gcNormalPropMapArenasToUpdate;

  // The list of empty arenas which are collected during the sweep phase and
  // released at the end of sweeping every sweep group.
  ZoneOrGCTaskData<Arena*> savedEmptyArenas;

 public:
  explicit ArenaLists(JS::Zone* zone);
  ~ArenaLists();

  FreeLists& freeLists() { return freeLists_.ref(); }
  const FreeLists& freeLists() const { return freeLists_.ref(); }

  FreeSpan** addressOfFreeList(AllocKind thingKind) {
    return freeLists_.refNoCheck().addressOfFreeList(thingKind);
  }

  inline Arena* getFirstArena(AllocKind thingKind) const;
  inline Arena* getFirstCollectingArena(AllocKind thingKind) const;
  inline Arena* getFirstSweptArena(AllocKind thingKind) const;
  inline Arena* getArenaAfterCursor(AllocKind thingKind) const;

  inline bool arenaListsAreEmpty() const;

  inline bool doneBackgroundFinalize(AllocKind kind) const;
  inline bool needBackgroundFinalizeWait(AllocKind kind) const;

  /* Clear the free lists so we won't try to allocate from swept arenas. */
  inline void clearFreeLists();

  inline void unmarkPreMarkedFreeCells();

  MOZ_ALWAYS_INLINE TenuredCell* allocateFromFreeList(AllocKind thingKind);

  inline void checkEmptyFreeLists();
  inline void checkEmptyArenaLists();
  inline void checkEmptyFreeList(AllocKind kind);

  void checkEmptyArenaList(AllocKind kind);

  bool relocateArenas(Arena*& relocatedListOut, JS::GCReason reason,
                      js::SliceBudget& sliceBudget, gcstats::Statistics& stats);

  void queueForegroundObjectsForSweep(JSFreeOp* fop);
  void queueForegroundThingsForSweep();

  Arena* takeSweptEmptyArenas();

  void setIncrementalSweptArenas(AllocKind kind, SortedArenaList& arenas);
  void clearIncrementalSweptArenas();

  void mergeFinalizedArenas(AllocKind thingKind,
                            SortedArenaList& finalizedArenas);

  void moveArenasToCollectingLists();
  void mergeArenasFromCollectingLists();

  void checkGCStateNotInUse();
  void checkSweepStateNotInUse();
  void checkNoArenasToUpdate();
  void checkNoArenasToUpdateForKind(AllocKind kind);

 private:
  ArenaList& arenaList(AllocKind i) { return arenaLists_.ref()[i]; }
  const ArenaList& arenaList(AllocKind i) const { return arenaLists_.ref()[i]; }

  ArenaList& collectingArenaList(AllocKind i) {
    return collectingArenaLists_.ref()[i];
  }
  const ArenaList& collectingArenaList(AllocKind i) const {
    return collectingArenaLists_.ref()[i];
  }

  ConcurrentUseState& concurrentUse(AllocKind i) {
    return concurrentUseState_.ref()[i];
  }
  ConcurrentUse concurrentUse(AllocKind i) const {
    return concurrentUseState_.ref()[i];
  }

  inline JSRuntime* runtime();
  inline JSRuntime* runtimeFromAnyThread();

  void initBackgroundSweep(AllocKind thingKind);

  TenuredCell* refillFreeListAndAllocate(FreeLists& freeLists,
                                         AllocKind thingKind,
                                         ShouldCheckThresholds checkThresholds);

  friend class BackgroundUnmarkTask;
  friend class GCRuntime;
  friend class js::Nursery;
  friend class js::TenuringTracer;
};

} /* namespace gc */
} /* namespace js */

#endif /* gc_ArenaList_h */