Hello,
I've hit an interesting issue in the RTL "call" instruction. I find that I can't execute a call instruction where the return frame starts at the same place that the procedure to call is at - it must start past there. That doesn't match my expected behavior, but I'm not sure if my expectations are wrong or there's really a bug.
To see it, first define two procedures:
(define (return-three) 3)
(define (return-five) 5)
To reproduce the behavior, try running this RTL program:
((assemble-program '(begin-program foo) (assert-nargs-ee/locals 2 3) (call 1 1 ()) (values) (values))) return-three return-five)
I expected that to call return-five (which is at register 1) and then print the contents of its stack, which should contain 5. Instead, it gives an error. However, the following works:
((assemble-program '(begin-program foo) (assert-nargs-ee/locals 2 3) (call 2 1 ()) (values) (values))) return-three return-five)
The only difference is in the call instruction - it pushes the new frame on just past the procedure, instead of on top of it.
Is that expected? I first ran into this when I was cleaning up the CPS register allocator, and I could just change the register allocator, but I'm not sure if the problem is there or in the VM itself.
Also note: the example used a procedure with two arguments that tried to call the second one. If you use a procedure with just one argument and try to call that, the VM will segfault. For example,
((assemble-program '((begin-program foo) (assert-nargs-ee/locals 1 3) (call 0 0 ()) (values) (values))) return-three) => segfault
Thanks,
Noah