protobuf/upb/mem/arena_internal.h

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#ifndef UPB_MEM_ARENA_INTERNAL_H_
#define UPB_MEM_ARENA_INTERNAL_H_

#include "upb/mem/arena.h"

// Must be last.
#include "upb/port/def.inc"

typedef struct _upb_MemBlock _upb_MemBlock;

struct upb_Arena {
  _upb_ArenaHead head;
  /* Stores cleanup metadata for this arena.
   * - a pointer to the current cleanup counter.
   * - a boolean indicating if there is an unowned initial block.  */
  uintptr_t cleanup_metadata;

  /* Allocator to allocate arena blocks.  We are responsible for freeing these
   * when we are destroyed. */
  upb_alloc* block_alloc;

  /* When multiple arenas are fused together, each arena points to a parent
   * arena (root points to itself). The root tracks how many live arenas
   * reference it.
   *
   * The low bit is tagged:
   *   0: pointer to parent
   *   1: count, left shifted by one
   */
  UPB_ATOMIC(uintptr_t) parent_or_count;

  // All nodes that are fused together are in a singly-linked list.
  UPB_ATOMIC(upb_Arena*) next;  // NULL at end of list.

  // The last element of the linked list.  This is present only as an
  // optimization, so that we do not have to iterate over all members for every
  // fuse.  Only significant for an arena root.  In other cases it is ignored.
  UPB_ATOMIC(upb_Arena*) tail;  // == self when no other list members.

  // Linked list of blocks to free/cleanup.  Atomic only for the benefit of
  // upb_Arena_SpaceAllocated().
  UPB_ATOMIC(_upb_MemBlock*) blocks;
};

UPB_INLINE bool _upb_Arena_IsTaggedRefcount(uintptr_t parent_or_count) {
  return (parent_or_count & 1) == 1;
}

UPB_INLINE bool _upb_Arena_IsTaggedPointer(uintptr_t parent_or_count) {
  return (parent_or_count & 1) == 0;
}

UPB_INLINE uint32_t _upb_Arena_RefCountFromTagged(uintptr_t parent_or_count) {
  UPB_ASSERT(_upb_Arena_IsTaggedRefcount(parent_or_count));
  return parent_or_count >> 1;
}

UPB_INLINE uintptr_t _upb_Arena_TaggedFromRefcount(uint32_t refcount) {
  uintptr_t parent_or_count = (((uintptr_t)refcount) << 1) | 1;
  UPB_ASSERT(_upb_Arena_IsTaggedRefcount(parent_or_count));
  return parent_or_count;
}

UPB_INLINE upb_Arena* _upb_Arena_PointerFromTagged(uintptr_t parent_or_count) {
  UPB_ASSERT(_upb_Arena_IsTaggedPointer(parent_or_count));
  return (upb_Arena*)parent_or_count;
}

UPB_INLINE uintptr_t _upb_Arena_TaggedFromPointer(upb_Arena* a) {
  uintptr_t parent_or_count = (uintptr_t)a;
  UPB_ASSERT(_upb_Arena_IsTaggedPointer(parent_or_count));
  return parent_or_count;
}

#include "upb/port/undef.inc"

#endif /* UPB_MEM_ARENA_INTERNAL_H_ */