Method enqueue

include/dmlc/concurrentqueue.h:1799–1892

    template<AllocationMode allocMode, typename U>
    inline bool enqueue(U &&element) {
      index_t currentTailIndex = this->tailIndex.load(std::memory_order_relaxed);
      index_t newTailIndex = 1 + currentTailIndex;
      if ((currentTailIndex & static_cast<index_t>(BLOCK_SIZE - 1)) == 0) {
        // We reached the end of a block, start a new one
        auto startBlock = this->tailBlock;
        auto originalBlockIndexSlotsUsed = pr_blockIndexSlotsUsed;
        if (this->tailBlock != nullptr &&
            this->tailBlock->next->template is_empty<explicit_context>()) {
          // We can re-use the block ahead of us, it's empty!
          this->tailBlock = this->tailBlock->next;
          this->tailBlock->template reset_empty<explicit_context>();

          // We'll put the block on the block index (guaranteed to be room since we're conceptually removing the
          // last block from it first -- except instead of removing then adding, we can just overwrite).
          // Note that there must be a valid block index here, since even if allocation failed in the ctor,
          // it would have been re-attempted when adding the first block to the queue; since there is such
          // a block, a block index must have been successfully allocated.
        } else {
          // Whatever head value we see here is >= the last value we saw here (relatively),
          // and <= its current value. Since we have the most recent tail, the head must be
          // <= to it.
          auto head = this->headIndex.load(std::memory_order_relaxed);
          assert(!details::circular_less_than<index_t>(currentTailIndex, head));
          if (!details::circular_less_than<index_t>(head, currentTailIndex + BLOCK_SIZE)
              || (MAX_SUBQUEUE_SIZE != details::const_numeric_max<size_t>::value &&
                  (MAX_SUBQUEUE_SIZE == 0 ||
                   MAX_SUBQUEUE_SIZE - BLOCK_SIZE < currentTailIndex - head))) {
            // We can't enqueue in another block because there's not enough leeway -- the
            // tail could surpass the head by the time the block fills up! (Or we'll exceed
            // the size limit, if the second part of the condition was true.)
            return false;
          }
          // We're going to need a new block; check that the block index has room
          if (pr_blockIndexRaw == nullptr || pr_blockIndexSlotsUsed == pr_blockIndexSize) {
            // Hmm, the circular block index is already full -- we'll need
            // to allocate a new index. Note pr_blockIndexRaw can only be nullptr if
            // the initial allocation failed in the constructor.

            if (allocMode == CannotAlloc || !new_block_index(pr_blockIndexSlotsUsed)) {
              return false;
            }
          }

          // Insert a new block in the circular linked list
          auto newBlock = this->parent->ConcurrentQueue::template requisition_block<allocMode>();
          if (newBlock == nullptr) {
            return false;
          }
#if MCDBGQ_TRACKMEM
          newBlock->owner = this;
#endif
          newBlock->template reset_empty<explicit_context>();
          if (this->tailBlock == nullptr) {
            newBlock->next = newBlock;
          } else {
            newBlock->next = this->tailBlock->next;
            this->tailBlock->next = newBlock;