No register usage shown with -arch option

I have noticed that compiling with

nvcc -res-usage -arch compute_90

or any other compute_xx argument, does not show the expected register information provided by the -res-usage option. On the other hand

nvcc -res-usage -arch sm_90

does show output. Is there anything I’m missing out her?

compute_90 generates PTX code (only). PTX is a virtual architecture with a virtual, approximately unlimited register space.

The register usage that is actually available is only available when you compile to SASS. sm_90 compiles to SASS, so the actual register usage can be identified.