does tensorflow tf.slice incur allocation and/or memory copy?

Question

Does

b = tf.slice(a, [...], [...])

allocate a new memory buffer and then copy from a\'s buffer? or do a and b share the same buffer?

Answer 1

I was curious of this as well and went into the kernel implementation of tf.slice.

The only time memory is not allocated is when the slice is the identity slice (so there are no changes), and when the slice is dimension 0 aligned.

Identity Lines

  if (is_identity) {
    VLOG(1) << "Slice identity";
    context->set_output(0, input);
    *done = true;
    return;
  }

Dim0Aligned

  if (slice_dim0 &&
      IsDim0SliceAligned<T>(input.shape(), (*begin)[0], (*size)[0])) {
    VLOG(1) << "Slice dim 0: " << input.shape().DebugString();
    CHECK_GE(input.dims(), 1);  // Otherwise, is_identity should be true.
    context->set_output(0, input.Slice((*begin)[0], (*begin)[0] + (*size)[0]));
    *done = true;
    return;
  }

Otherwise the following will be called

  OP_REQUIRES_OK(context, context->allocate_output(0, *output_shape, result));