Ndarray.h GetDataSize() question

erickim555 · November 17, 2021, 5:59am

In this implementation of GetDataSize():

apache/tvm/blob/58716081637fdf21bdae44122086f1dbc35f7c35/include/tvm/runtime/ndarray.h#L316


  friend class NDArray;
};


// implementations of inline functions
/*!
 * \brief return the size of data the DLTensor hold, in term of number of bytes
 *
 *  \param arr the input DLTensor
 *  \return number of  bytes of data in the DLTensor.
 */
inline size_t GetDataSize(const DLTensor& arr) {
  size_t size = 1;
  for (tvm_index_t i = 0; i < arr.ndim; ++i) {
    size *= static_cast<size_t>(arr.shape[i]);
  }
  size *= (arr.dtype.bits * arr.dtype.lanes + 7) / 8;
  return size;
}


/*!
 * \brief check if a DLTensor is contiguous.

there is this mysterious line:

size *= (arr.dtype.bits * arr.dtype.lanes + 7) / 8;

I was instead expecting the simpler:

size *= (arr.dtype.bits * arr.dtype.lanes);

Can somebody enlighten me as to why the former line (+7 / 8) is doing? Thanks!

erickim555 · November 17, 2021, 6:07am

Oh, I think I see. It’s an arithmetic trick to ensure that a dtype where bits<8 will still result in the nbytes-per-element to be 1 byte.