I don't think it amounted to any performance issue because SM was only using the setjmp() buffer in that way as a shortcut to get all of the register values in

I don't think it amounted to any performance issue because SM was only using the setjmp() buffer in that way as a shortcut to get all of the register values in a buffer to scan linearly. Since there was the RSE backing store, there was an extra block of memory to scan regardless, but you're still talking about a reasonably bounded area only scanned on each GC run, which is not a frequent (to the processor) event.