I'm not an expert in this stuff, but I think they just get thrown away. It's one of several reasons why smaller chips are more area-efficient to make, and one

I'm not an expert in this stuff, but I think they just get thrown away. It's one of several reasons why smaller chips are more area-efficient to make, and one of several reasons why the major semiconductor manufacturers have been so interested lately in building chips out of smaller pieces manufactured separately rather than one big monolithic chip.