Why is the size of alignment for char3 is 1 in CUDA Programming Guide?

Question

I found that alignment for char1 is 1, for char2 is 2, for char3 is 1 and for char4 is 4. Why is the alignment size for char3 is 1 instead of 3?

I also need some help to understand the concept of "Alignment Requirements".

Please refer to this CUDA Programming Guide (Table B-1 of Section B.3.1 in Appendix B)

Many thanks,

Yik

Answer 1

Think about what would happen if you had an array of char3 - an alignment of > 1 would mean a non-contiguous array.

For char4 however you do not have any such padding problems, and you probably want to ensure that a char4 can be treated as if it were a 32 bit int, so a 4 byte alignment is helpful.