Ah, I was not clear enough: native SMP threads, as in each thread accessing the same shared memory heap. Gauche allocates a new VM for each thread ... ah, but I see it has data synchronization primitives. I believe it has moved forward since the last time I checked it, years ago.

Thanks for the tip!

Repeated from the other reply on Guile: Hmmm, now I'll have to narrow the statement to "The only native code compiling Scheme that I know of" (which is harder to do).