wip: abstract stack/continuation machine

doc/abi.org

@@ -0,0 +1,218 @@
+#+title: ABI
+
+largely based on the Guile Hoot's [[https://codeberg.org/spritely/hoot/src/branch/main/design/ABI.md][ABI]].
+
+* calling convention
+
+** non-tail calls
+
+- set the global variable ~$current-closure~ to the callee's closure.
+- load arguments into globals ~$arg0~, ~$arg1~, ~$arg2~, …
+- push return continuation onto ~$cont-stack~
+
+* scratchpad
+
+#+begin_src scheme
+  ;; Scheme source
+  (define (silly f g h x)
+    (f (h x) (g x)))
+
+
+  ;; continuation-passing style
+  (define (silly f g h x ktail)
+    (h x (κ (x0)
+            (g x (κ (x1)
+                    (f x0 x1 ktail))))))
+
+  ;; with explicit stacks
+  (define (silly)
+    (define f (pop!))
+    (define g (pop!))
+    (define h (pop!))
+    (define x (pop!))
+    (define ktail (pop-cont!))
+    (push-cont! (κ (x0)
+                   (define x* (pop!))
+                   (define g* (pop!))
+                   (push-cont! (κ (x1)
+                                  (define f* (pop!))
+                                  (define x0* (pop!))
+                                  (push-cont! ktail)
+                                  (push! x0*)
+                                  (push! x1)
+                                  (call! f)))
+                   (push! x*)
+                   (call! g)))
+    (push! x)
+    (call! h))
+#+end_src
+
+** fac
+
+*** Scheme source
+
+#+begin_src scheme
+(define fac
+  (λ (n)
+    (if (zero? n)
+        1
+        (* n (fac (- n 1))))))
+
+(fac 3)
+#+end_src
+
+*** CPS
+
+#+begin_src scheme
+  (define fac
+  (λ (n ktail)
+    (zero? n (κ (x0)
+                (if x0
+                    1
+                    (- n 1
+                       (κ (x1)
+                          (fac x1
+                               (κ (x2)
+                                  (* n x2 ktail))))))))))
+
+(fac 3 halt)
+
+#+end_src
+
+*** tailified
+
+#+begin_src scheme
+(define (fac-k1)
+  (define n (pop!))
+  (define x2 (pop!))
+  (define x3 (* n x2))
+  (define ktail (pop-cont!))
+  (push! x3)
+  (call! ktail))
+
+(define (fac-k0)
+  (define x0 (pop!))
+  (define n (pop!))
+  (if x0
+      (begin (define ktail (pop-cont!))
+             (push! 1)
+             (call! ktail))
+      (begin (define x1 (- n 1))
+             (push! x1)
+             (push-cont! fac-k1)
+             (call! fac))))
+
+(define (fac)
+  (define n (pop!))
+  (push! n)
+  (push-cont! fac-k0)
+  (push! n)
+  (call! zero?))
+
+(push! 3)
+(push-cont! halt)
+(call! fac)
+#+end_src
+
+evaluation of ~(fac 0)~:
+
+#+begin_src scheme
+(push! 0)                  ; [] []
+(push-cont! halt)          ; [0] []
+(call! fac)                ; [0] [halt]
+(define n (pop!))          ; [0] [halt]
+(push! n)                  ; [] [halt]
+(push-cont! fac-k0)        ; [0] [halt]
+(push! n)                  ; [0] [halt fac-k0]
+(call! zero?)              ; [0 0] [halt fac-k0]
+#<internals of zero?>      ; [0 0] [halt fac-k0]
+(define x0 (pop!))         ; [0 #t] [halt]
+(define n (pop!))          ; [0] [halt]
+(define ktail (pop-cont!)) ; [] [halt]
+(push! 1)                  ; [] []
+(call! ktail)              ; [1] []
+#+end_src
+
+evaluation of ~(fac 3)~
+
+#+begin_src scheme
+(push! 3)                  ; [] []
+(push-cont! halt)          ; [3] []
+(call! fac)                ; [3] [halt]
+
+(define n (pop!))          ; [3] [halt]
+(push! n)                  ; [] [halt]
+(push-cont! fac-k0)        ; [3] [halt]
+(push! n)                  ; [3] [halt fac-k0]
+(call! zero?)              ; [3 3] [halt fac-k0]
+#<internals of zero?>      ; [3 3] [halt fac-k0]
+(define x0 (pop!))         ; [3 #f] [halt]
+(define n (pop!))          ; [3] [halt]
+(define x1 (- n 1))        ; [] [halt]
+(push! n)                  ; [] [halt]
+(push! x1)                 ; [3] [halt]
+(push-cont! fac-k1)        ; [3 2] [halt]
+(call! fac)                ; [3 2] [halt fac-k1]
+
+(define n (pop!))          ; [3 2] [halt fac-k1]
+(push! n)                  ; [3 ] [halt fac-k1]
+(push-cont! fac-k0)        ; [3 2] [halt fac-k1]
+(push! n)                  ; [3 2] [halt fac-k1 fac-k0]
+(call! zero?)              ; [3 2 2] [halt fac-k1 fac-k0]
+#<internals of zero?>      ; [3 2 2] [halt fac-k1 fac-k0]
+(define x0 (pop!))         ; [3 2 #f] [halt fac-k1]
+(define n (pop!))          ; [3 2] [halt fac-k1]
+(define x1 (- n 1))        ; [3] [halt fac-k1]
+(push! n)                  ; [3] [halt fac-k1]
+(push! x1)                 ; [3 2] [halt fac-k1]
+(push-cont! fac-k1)        ; [3 2 1] [halt fac-k1]
+(call! fac)                ; [3 2 1] [halt fac-k1 fac-k1]
+
+(define n (pop!))          ; [3 2 1] [halt fac-k1 fac-k1]
+(push! n)                  ; [3 2] [halt fac-k1 fac-k1]
+(push-cont! fac-k0)        ; [3 2 1] [halt fac-k1 fac-k1]
+(push! n)                  ; [3 2 1] [halt fac-k1 fac-k1 fac-k0]
+(call! zero?)              ; [3 2 1 1] [halt fac-k1 fac-k1 fac-k0]
+#<internals of zero?>      ; [3 2 1 1] [halt fac-k1 fac-k1 fac-k0]
+(define x0 (pop!))         ; [3 2 1 #f] [halt fac-k1 fac-k1]
+(define n (pop!))          ; [3 2 1] [halt fac-k1 fac-k1]
+(define x1 (- n 1))        ; [3 2] [halt fac-k1 fac-k1]
+(push! n)                  ; [3 2] [halt fac-k1]
+(push! x1)                 ; [3 2 1] [halt fac-k1 fac-k1]
+(push-cont! fac-k1)        ; [3 2 1 0] [halt fac-k1 fac-k1]
+(call! fac)                ; [3 2 1 0] [halt fac-k1 fac-k1 fac-k1]
+
+(define n (pop!))          ; [3 2 1 0] [halt fac-k1 fac-k1 fac-k1]
+(push! n)                  ; [3 2 1] [halt fac-k1 fac-k1 fac-k1]
+(push-cont! fac-k0)        ; [3 2 1 0] [halt fac-k1 fac-k1 fac-k1]
+(push! n)                  ; [3 2 1 0] [halt fac-k1 fac-k1 fac-k1 fac-k0]
+(call! zero?)              ; [3 2 1 0 0] [halt fac-k1 fac-k1 fac-k1 fac-k0]
+#<internals of zero?>      ; [3 2 1 0 0] [halt fac-k1 fac-k1 fac-k1 fac-k0]
+(define x0 (pop!))         ; [3 2 1 0 #t] [halt fac-k1 fac-k1 fac-k1]
+(define n (pop!))          ; [3 2 1 0] [halt fac-k1 fac-k1 fac-k1]
+(define ktail (pop-cont!)) ; [3 2 1] [halt fac-k1 fac-k1 fac-k1]
+(push! 1)                  ; [3 2 1 1] [halt fac-k1 fac-k1]
+(call! ktail)              ; [3 2 1 1] [halt fac-k1 fac-k1]
+
+(define n (pop!))          ; [3 2 1 1] [halt fac-k1 fac-k1]
+(define x2 (pop!))         ; [3 2 1] [halt fac-k1 fac-k1]
+(define x3 (* n x2))       ; [3 2] [halt fac-k1 fac-k1]
+(define ktail (pop-cont!)) ; [3 2] [halt fac-k1 fac-k1]
+(push! x3)                 ; [3 2] [halt fac-k1]
+(call! ktail)              ; [3 2 1] [halt fac-k1]
+
+(define n (pop!))          ; [3 2 1] [halt fac-k1]
+(define x2 (pop!))         ; [3 2] [halt fac-k1]
+(define x3 (* n x2))       ; [3] [halt fac-k1]
+(define ktail (pop-cont!)) ; [3] [halt fac-k1]
+(push! x3)                 ; [3] [halt]
+(call! ktail)              ; [3 2] [halt]
+
+(define n (pop!))          ; [3 2] [halt]
+(define x2 (pop!))         ; [3] [halt]
+(define x3 (* n x2))       ; [] [halt]
+(define ktail (pop-cont!)) ; [] [halt]
+(push! x3)                 ; [] []
+(call! ktail)              ; [6] []
+;; => (halt 6)
+#+end_src

doc/closure-conversion.org

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+#+title: closure-conversion
+
+the closure-conversion phase makes closed-over variables explicit by addition of the primitive ~make-closure~, taking a code pointer (in the CPS language, bare lambda) and the environment.
+
+* scratchpad
+
+#+begin_src scheme
+(letrec ((make-adder
+        (lambda (n)
+          (lambda (x)
+            (+ n x)))))
+((make-adder 3) 2))
+#+end_src
+
+#+begin_src scheme
+(define add-code
+  (lambda (n env)
+    (+ n (env-ref env 'x))))
+
+(define make-adder-code
+  (lambda (n)
+    (make-closure add-code ('x n))))
+
+(define make-adder (make-closure make-adder-code))
+
+(apply-closure (apply-closure make-addder 3) 2)
+#+end_src
+
+#+begin_src wat
+(module
+ (type $heap-object (sub (struct (field $hash (mut i32)))))
+ (type $closure (sub $heap-object
+                     (struct (field $hash (mut i32))
+                             (field $code (ref $cont-type)))))
+ (type $closure1 (sub $closure
+                      (struct (field $hash (mut i32))
+                              (field $code (ref $cont-type))
+                              (field $env0 (ref eq)))))
+ (global $arg0 (mut (ref null eq)) (ref.null eq))
+ (global $arg1 (mut (ref null eq)) (ref.null eq))
+ (global $arg2 (mut (ref null eq)) (ref.null eq))
+ (global $arg3 (mut (ref null eq)) (ref.null eq))
+ (global $arg4 (mut (ref null eq)) (ref.null eq))
+ (global $arg5 (mut (ref null eq)) (ref.null eq))
+ ;; ⋮
+ ;; (global $argn (mut (ref null eq)) (ref.null eq))
+
+ (global $current-closure (mut (ref null $closure)) (ref.null $closure))
+
+ (func $add-code (param $nargs i32)
+       (local $n (ref eq))
+       (local $x (ref eq))
+       (local.set $n (global.get $arg0))
+       (local.set $x (struct.get $closure1
+                                 (global.get $current-closure)
+                                 $env0))
+       (return (i32.add $n $x)))
+
+ (func $make-adder-code (param $nargs i32)
+       (local $n (ref eq))
+       (local.set $n (global.get $arg0))
+       (return (struct.new $closure1
+                           0
+                           $add-code)))
+
+ (func $main
+       (local.set $make-adder
+                  (struct.new $closure
+                              0
+                              $make-adder-code))
+       (global.set $current-closure $make-adder)
+       (global.set $arg0 (i32.const 3))
+       (local.set $f (call (struct.get $closure
+                                       $make-adder
+                                       $code)
+                           1))
+       (global.set $current-closure $f)
+       (global.set $arg0 (i32.const 2))
+       (return (call (struct.get $closure
+                                 $f
+                                 $code)
+                     1))))
+#+end_src

doc/notes.org

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+#+title: assorted notes on compilation
+
+* letrec
+
+consider:
+
+#+begin_src scheme
+  (letrec ((even? (lambda (n)
+                    (if (zero? n)
+                        #t
+                        (odd? (- n 1)))))
+           (odd? (lambda (n)
+                   (if (zero? n)
+                       #f
+                       (even? (- n 1))))))
+    (even? 12))
+#+end_src
+
+#+RESULTS:
+: #t
+
+since ~letrec~ is a primitive construct in the CPS language, the translation of mutually recursive functions is straightforward:
+
+#+begin_src scheme
+  (define (-& x y k) (k (- x y)))
+  (define (zero?& x k) (k (zero? x)))
+  (define (halt x) x)
+
+  (letrec ((even? (lambda (n ktail)
+                    (zero?& n
+                            (lambda (x1)
+                              (if x1
+                                  #t
+                                  (-& n 1
+                                      (lambda (x2)
+                                        (odd? x2 ktail))))))))
+           (odd? (lambda (n ktail)
+                   (zero?& n
+                           (lambda (x1)
+                             (if x1
+                                 #f
+                                 (-& n 1
+                                     (lambda (x2)
+                                       (even? x2 ktail)))))))))
+    (even? 12 halt))
+#+end_src
+
+#+RESULTS:
+: #t
+
+however, Scheme permits ~letrec~-expressions with non-lambda right-hand sides, while the CPS language permits only kappa and lambda forms. thus, the handling of these forms is less trivial.
+
+for now we'll just reject any ~letrec~ forms with non-lambda right-hand sides, lol. they aren't very important.