GC: overflow in gen1 while promoting

I've the flowwing question regarding the GC in Rotor.

When promoting gen0 survivors to gen1 (copy_phase) the
function "copy_object_simple" calls "allocate_in_older_generation" to
allocate space in gen1. Is it possible to gen1 to be full at this moment,
which would require a GC in gen1 (mark-and-sweep)?

I followed the "allocate_in_older_generation" and didn't found any traces
of a GC. If it is not possible, how is this guaranteed.

I thank in advance

P. Felix