Careful, that's not what I'm claiming. First, allocation is only so cheap if it's temporary. If objects survive minor collections, then there's additional cost

Careful, that's not what I'm claiming. First, allocation is only so cheap if it's temporary. If objects survive minor collections, then there's additional cost, as they get promoted to the major heap. The key idea that I'm getting at is that with temporary objects being cheap, you have more flexibility in creating temporary data structures and do not have to fit them in the constraint of a stack frame and (unlike with alloca()) do not have to worry about stack overflow and they can be returned from a function without copying (unlike stack frame contents). Temporary data structures will still be small and generally fit in the L1 cache of any reasonably modern processor. And using pointer does not mean that everything is a pointer, and that you're necessarily sacrificing ILP.