101 lines
3.1 KiB
Org Mode
101 lines
3.1 KiB
Org Mode
* rationale?
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previously, the VM's stack was used for storing local variables across blocks; a Scheme procedure was split into several low-level routines (one for the procedure itself and one for each continuation), and the stack was used as a communication channel for these separate routines. in contrast, registers were local to each routine. this aligns with Wasm's model of functions pretty well, with Wasm /locals/ acting as the VM's /registers/, and a global mutable stack serving as fallback.
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this worked quite well until it became time to implement ~call/cc~.
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we are considering making the following alterations to the VM:
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- explicitly segment the stack into frames.
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- passing procedures and return addresses on the stack.
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- new instructions:
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+ ~(tail-call /n/)~
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+ ~(call /n/)~
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+ ~(load /r/ /n/)~
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+ ~(return /n/)~
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* scratchpad
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#+begin_src scheme
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(letrec ((fac (λ (n)
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(if (zero? n)
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1
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(* n (fac (- n 1)))))))
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(fac 3))
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#+end_src
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#+begin_src scheme
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(λ (ktail0)
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(letrec ((fac
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(λ (n ktail1)
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(zero?
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n
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(κ (x0)
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(if x0
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(continue ktail1 1)
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(- n 1
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(κ (x1)
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(fac x1
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(κ (x2)
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(* n x2 ktail1)))))))))))
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(fac 3)))
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#+end_src
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#+begin_example
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n ktail1
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| | x0
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| | |
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| | ^
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| | x1
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| | |
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| | ^
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| | x2
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^ ^ ^
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#+end_example
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#+begin_src scheme
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(define $fac-c0
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(pop! %x0 0) ; [ x0 $fac-c0 n $fac ktail1 ]
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(if %x0 ; [ $fac-c0 n $fac ktail1 ]
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;; every variable but `ktail1' is dead so we pop them all.
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;; this probably means that `if' should take two continuations
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;; rather than two blocks.
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(then (push! 1) ; [ $fac-c0 n $fac ktail1 ]
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(return 1)) ; [ 1 $fac-c0 n $fac ktail1 ]
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(else (load %n 1) ; [ $fac-c0 n $fac ktail1 ]
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(prim %x1 (- %n 1)) ; [ $fac-c0 n $fac ktail1 ]
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(push! $fac-c1) ; [ $fac-c0 n $fac ktail1 ]
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(push! $fac) ; [ $fac-c1 $fac-c0 n $fac ktail1 ]
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(push! %x1) ; [ $fac $fac-c1 $fac-c0 n $fac ktail1 ]
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(call 1) ; [ x1 $fac $fac-c1 $fac-c0 n $fac ktail1 ]
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)))
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(define $fac-c1
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(pop! %x2) ; [ x2 $fac-c1 $fac-c0 n $fac ktail1 ]
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(load %n 3) ; [ $fac-c1 $fac-c0 n $fac ktail1 ]
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(prim %x3 (* %n %x2))
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(push! %x3) ; [ $fac-c1 $fac-c0 n $fac ktail1 ]
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(return 1) ; [ x3 $fac-c1 $fac-c0 n $fac ktail1 ]
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)
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(define $fac
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(load %ktail1 2) ; [ n $fac ktail1 ]
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(load %n 0) ; [ n $fac ktail1 ]
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(push! $fac-c0) ; [ n $fac ktail1 ]
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(push! $zero?) ; [ $fac-c0 n $fac ktail1 ]
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(push! %n) ; [ $zero? $fac-c0 n $fac ktail1 ]
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(call 1) ; [ n $zero? $fac-c0 n $fac ktail1 ]
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)
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(define $start
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(push! $fac) ; [ $start ktail0 ]
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(push! 3) ; [ $fac $start ktail0 ]
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(tail-call 1) ; [ 3 $fac $start ktail0 ]
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;; ↑ `tail-call' knows how to dispose of the caller's stack frame.
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)
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#+end_src
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