Register spilling
Beginner
When a worker runs out of desk space and has to put things on a far-away shelf, which slows them down.
Novice
When a program needs more registers than it’s allowed, the compiler stores some values in slower memory instead and fetches them back when needed.
Expert
On NVIDIA GPUs spills go to ‘local memory’, which lives in device DRAM behind the caches, so a spill costs global-memory latency and bandwidth. Capping registers raises occupancy but risks spills.