數(shù)據(jù)結(jié)構(gòu)

struct PACKED(sizeof(void*)) tls_ptr_sized_values {

.......

    // Thread-local allocation pointer. Moved here to force alignment for thread_local_pos on ARM.
    uint8_t* thread_local_start;

    // thread_local_pos and thread_local_end must be consecutive for ldrd and are 8 byte aligned for
    // potentially better performance.
    uint8_t* thread_local_pos;
    uint8_t* thread_local_end;

    // Thread local limit is how much we can expand the thread local buffer to, it is greater or
    // equal to thread_local_end.
    uint8_t* thread_local_limit;
  
//表示TLAB上分配的對象數(shù)量
    size_t thread_local_objects;

.......
}

創(chuàng)建

void Thread::SetTlab(uint8_t* start, uint8_t* end, uint8_t* limit) {
  DCHECK_LE(start, end);
  DCHECK_LE(end, limit);
  tlsPtr_.thread_local_start = start;
  tlsPtr_.thread_local_pos  = tlsPtr_.thread_local_start;
  tlsPtr_.thread_local_end = end;
  tlsPtr_.thread_local_limit = limit;
  tlsPtr_.thread_local_objects = 0;
}

Thread對象分配TLAB的實現(xiàn)

可以使用BumpPointerSpace分配內(nèi)存

uint8_t* BumpPointerSpace::AllocBlock(size_t bytes) {
  bytes = RoundUp(bytes, kAlignment);
  if (!num_blocks_) {
    UpdateMainBlock();
  }
  uint8_t* storage = reinterpret_cast<uint8_t*>(
      AllocNonvirtualWithoutAccounting(bytes + sizeof(BlockHeader)));
  if (LIKELY(storage != nullptr)) {
    BlockHeader* header = reinterpret_cast<BlockHeader*>(storage);
    header->size_ = bytes;  // Write out the block header.
    storage += sizeof(BlockHeader);
    ++num_blocks_;
  }
  return storage;
}

或使用RegionSpace分配

bool RegionSpace::AllocNewTlab(Thread* self,
                               const size_t tlab_size,
                               size_t* bytes_tl_bulk_allocated) {
  MutexLock mu(self, region_lock_);
  RevokeThreadLocalBuffersLocked(self, /*reuse=*/ gc::Heap::kUsePartialTlabs);
  Region* r = nullptr;
  uint8_t* pos = nullptr;
  *bytes_tl_bulk_allocated = tlab_size;
  // First attempt to get a partially used TLAB, if available.
  if (tlab_size < kRegionSize) {
    // Fetch the largest partial TLAB. The multimap is ordered in decreasing
    // size.
    auto largest_partial_tlab = partial_tlabs_.begin();
    if (largest_partial_tlab != partial_tlabs_.end() && largest_partial_tlab->first >= tlab_size) {
      r = largest_partial_tlab->second;
      pos = r->End() - largest_partial_tlab->first;
      partial_tlabs_.erase(largest_partial_tlab);
      *bytes_tl_bulk_allocated -= r->Top() - pos;
    }
  }
  if (r == nullptr) {
    // Fallback to allocating an entire region as TLAB.
    r = AllocateRegion(/*for_evac=*/ false);
  }
  if (r != nullptr) {
    uint8_t* start = pos != nullptr ? pos : r->Begin();
    DCHECK_ALIGNED(start, kObjectAlignment);
    r->is_a_tlab_ = true;
    r->thread_ = self;
    r->SetTop(r->End());
    self->SetTlab(start, start + tlab_size, r->End());
    return true;
  }
  return false;
}

分配

inline mirror::Object* Thread::AllocTlab(size_t bytes) {
  DCHECK_GE(TlabSize(), bytes);
  ++tlsPtr_.thread_local_objects;
  mirror::Object* ret = reinterpret_cast<mirror::Object*>(tlsPtr_.thread_local_pos);
  tlsPtr_.thread_local_pos += bytes;
  return ret;
}