[aidenn0]

The conservative GC approach has worked well enough in practice that nobody is going to do the work. Also there is a performance tradeoff in non-allocating code: Sometimes you need more unboxed registers, other times you need more boxed registers so with only ~6 of each[1] you will run into register pressure.

1: 2 stacks means 2 stack pointers and 2 frame pointers leaving only 12 registers left for values; it's also possible that the SBCL ABI uses a global register for something else as well, which would leave only 11. PowerPC is a really luxurious platform in which you have 32 GPRs so even if you use 8 GPRs for various bookkeeping purposes that leaves 24 remaining, which is enough for pretty much everyone.