The GCM mode of operation is essentially CTR mode with an additional operation that provides authenticity. That means that we can reuse parts of the CTR mode code in GCM. The benefit of this arrangement is that there is an intrinsic for AES/CTR that significantly improves the performance of this operation on systems with AES instructions. The current GCM code has a loop that invoked the AES intrinsic one block at a time. It would be much more efficient to invoke the AES/CTR intrinsic on the entire buffer.