Just a fair warning, setcontext is horrendously slow. It seems to be doing kernel transitions or something. It's about 600 times slower than longjmp with jmpbuf

Just a fair warning, setcontext is horrendously slow. It seems to be doing kernel transitions or something. It's about 600 times slower than longjmp with jmpbuf stack pointer hacking, which is itself already about twice as slow as an ideal assembly version.