spills

Occupancy is how many warps an SM holds against its maximum, and it is a means, not an end. The arithmetic from registers per thread to resident warps, the launch bound that caps registers and the spills that follow, the profiler's occupancy view, and the kernel where halving occupancy doubled speed.

Registers are allocated in fixed steps out of a fixed budget per multiprocessor, so a small increase in live values can cost a whole resident block. The compiler output that shows it in two lines, the occupancy cliff arithmetic, and the two different failure modes that produce the same symptom.