X-Git-Url: http://git.lukelau.me/?p=scheme.git;a=blobdiff_plain;f=codegen.scm;h=7a12fc2228795b588cfbf117d3f1e7712f282be6;hp=a19b53df53fa239bc700a5d06cccfd86298d7601;hb=9d93b066cfd6505849dff12146159bedeadf96b9;hpb=25a4c23da2d76f3bb951d01b1c3175be80441a0a

diff --git a/codegen.scm b/codegen.scm
index a19b53d..7a12fc2 100644
--- a/codegen.scm
+++ b/codegen.scm
@@ -1,5 +1,8 @@
 (load "typecheck.scm")
 (load "ast.scm")
+(load "platform.scm")
+
+(define target host-os)
 
 (define (emit . s)
   (begin
@@ -9,28 +12,29 @@
 (define (codegen-add xs si env)
   (define (go ys)
     (if (null? ys)
-      (emit "movq ~a(%rsp), %rax" si)
+	(emit "movq ~a(%rbp), %rax" si)
 	(begin
 	  (let ((y (car ys)))
 	    (if (integer? y)
-		 (emit "addq $~a, ~a(%rsp)" y si)
+		(emit "addq $~a, ~a(%rbp)" y si)
 		(begin
 		  (codegen-expr y (- si wordsize) env)
-		   (emit "addq %rax, ~a(%rsp)" si))))
+		  (emit "addq %rax, ~a(%rbp)" si))))
 	  (go (cdr ys)))))
   (begin
-					; use si(%rsp) as the accumulator
-    (emit "movq $0, ~a(%rsp)" si)
+					; use si(%rbp) as the accumulator
+    (emit "movq $0, ~a(%rbp)" si)
     (go xs)))
 
 (define (codegen-binop opcode)
   (lambda (a b si env)
     (codegen-expr b si env)
-    (emit "movq %rax, ~a(%rsp)" si)
+    (emit "movq %rax, ~a(%rbp)" si)
     (codegen-expr a (- si wordsize) env)
-    (emit "~a ~a(%rsp), %rax" opcode si)))
+    (emit "~a ~a(%rbp), %rax" opcode si)))
 
 (define codegen-sub (codegen-binop "sub"))
+
 (define codegen-mul (codegen-binop "imul"))
 
 (define (codegen-not x si env)
@@ -40,11 +44,37 @@
 
 (define (codegen-eq a b si env)
   (codegen-expr a si env)
-  (emit "movq %rax, ~a(%rsp)" si)
+  (emit "movq %rax, ~a(%rbp)" si)
   (codegen-expr b (- si wordsize) env)
-  (emit "subq ~a(%rsp), %rax" si)
-  (emit "not %rax")
-  (emit "andq $1, %rax"))
+  (emit "## ~a = ~b" a b)
+  (emit "cmpq ~a(%rbp), %rax" si)
+  (emit "sete %al"))
+
+					; 'write file handle addr-string num-bytes
+
+(define (codegen-print x si env)
+  (codegen-expr x si env) ; x should be a static-string, producing a label
+
+					; make a copy of string address since %rax and %rdi are clobbered
+  (emit "mov %rax, %rbx")
+  
+					; get the length of the null terminated string
+  (emit "mov %rax, %rdi")
+  (emit "xor %al, %al")   ; set %al to 0
+  (emit "mov $-1, %rcx") ; max search length = max int = -1
+  (emit "cld")           ; clear direction flag, search up in memory
+  (emit "repne scasb")   ; scan string, %rcx = -strlen - 1 - 1
+  
+  (emit "not %rcx")      ; -%rcx = strlen + 1
+  (emit "dec %rcx")
+  
+  (emit "movq %rbx, %rsi") ; string addr
+  (emit "movq %rcx, %rdx") ; num bytes
+  (emit "movq $1, %rdi")   ; file handle (stdout)
+  (case target
+    ('darwin (emit "mov $0x2000004, %rax")) ; syscall 4 (write)
+    ('linux  (emit "mov $1, %rax"))) ; syscall 1 (write)
+  (emit "syscall"))
 
 (define (range s n)
   (if (= 0 n) '()
@@ -54,19 +84,48 @@
 (define wordsize 8)
 
 (define (codegen-let bindings body si env)
-  (let* ((stack-offsets (map (lambda (x) (- si (* x wordsize)))
+
+					; is this a closure that captures itself?
+					; e.g. (let ([x 3] [f (closure lambda0 (f x))]) (f))
+  (define (self-captive-closure? name expr)
+    (and (eqv? (ast-type expr) 'closure)
+	 (memv name (caddr expr))))
+  
+  (let* ((stack-offsets (map (lambda (name x) ; assoc map of binding name to offset
+			       (cons name (- si (* x wordsize))))
+			     (map car bindings)
 			     (range 0 (length bindings))))
 	 (inner-si (- si (* (length bindings) wordsize)))
-	 (names (map car bindings))
-	 (exprs (map cadr bindings))
-
-	 ; recursive let bindings: build environment as we go
-	 (inner-env (fold-left
-		     (lambda (env name expr offset)
-		       (codegen-expr expr inner-si env)
-		       (emit "movq %rax, ~a(%rsp)" offset)
-		       (cons (cons name offset) env))
-		     env names exprs stack-offsets)))
+
+	 (get-offset (lambda (n) (cdr (assoc n stack-offsets))))
+	 
+	 [inner-env
+	  (fold-left
+	   (lambda (env comps)
+	     (let ([scc-env
+		    (fold-left
+		     (lambda (acc name)
+		       (cons (cons name (get-offset name))
+			     acc))
+		     env
+		     comps)])
+	       (for-each 
+		(lambda (name)
+		  (let ([expr (cadr (assoc name bindings))])
+		    (emit "## generating ~a with scc-env ~a" name scc-env)
+		    (if (self-captive-closure? name expr)
+					; if self-captive, insert a flag into the environment to let
+					; codegen-closure realise this!
+			(codegen-expr expr
+				      inner-si
+				      (cons (cons name 'self-captive)
+					    scc-env))
+			(codegen-expr expr inner-si scc-env))
+		    (emit "movq %rax, ~a(%rbp)" (get-offset name))))
+		comps)
+	       scc-env))
+	   env (reverse (sccs (graph bindings))))])
+    
     (for-each (lambda (form)
 		(codegen-expr form inner-si inner-env))
 	      body)))
@@ -75,75 +134,155 @@
   (when (not (assoc name env))
     (error #f (format "Variable ~a is not bound" name)))
   (let ((offset (cdr (assoc name env))))
-    (emit "movq ~a(%rsp), %rax" offset)))
+    (emit "movq ~a(%rbp), %rax" offset)))
 
 (define cur-lambda 0)
 (define (fresh-lambda)
   (set! cur-lambda (+ 1 cur-lambda))
   (format "_lambda~a" (- cur-lambda 1)))
 
-					; for now we can only call closures
-(define (codegen-call closure args si env)
-  (when (not (eq? (ast-type closure) 'closure))
-    (error #f (format "~a is not a closure" closure)))
-  (let* ((captured (caddr closure))
-	 (label (cadr closure))
-	 (argument-start (length captured)))
-
-					; first move the captured variables into param registers
+					; a closure on the heap looks like:
+					; 0    8         16        24
+					; addr var1....  var2....  var3....
+
+(define (codegen-closure label captured si env)
+  (let* ((heap-offsets (map (lambda (i) (+ 8 (* 8 i)))
+			    (range 0 (length captured))))) ; 4, 12, 20, etc.
+
+    (emit "## creating closure")
+
+    (emit "movq heap_start@GOTPCREL(%rip), %rbx")
+    
+    (emit "movq (%rbx), %rax")          ; %rax = heap addr of closure
+
+
+    ; point heap_start to next space
+    (emit "addq $~a, (%rbx)" (+ 8 (* 8 (length captured))))
+
+    (emit "## storing address to lambda")
+					; store the address to the lambda code
+    (emit "movq ~a@GOTPCREL(%rip), %rbx" label)
+    (emit "movq %rbx, 0(%rax)")
+
+    (emit "## storing captives")
+					; store the captured vars
     (for-each
-     (lambda (e i)
-       (emit "movq ~a(%rsp), ~a"
-	     (cdr (assoc e env)) ; offset of the var
-	     (param-register i)))
-     captured (range 0 (length captured)))
+     (lambda (var-name heap-offset)
+       (let ([stack-offset (cdr (assoc var-name env))])
+	 (emit "### captive ~a" var-name)
+	 (if (eqv? stack-offset 'self-captive)
+					; captive refers to this closure:
+					; move heap addr of this closure to stack! 
+	     (emit "movq %rax, ~a(%rax)" heap-offset)
+	     (begin
+	       (emit "movq ~a(%rbp), %rbx" stack-offset)
+	       (emit "movq %rbx, ~a(%rax)" heap-offset)))))
+     captured
+     heap-offsets)))
+
+					; for now we can only call closures
+(define (codegen-call f args si env)
+  (codegen-expr f si env)
+
+  (emit "## starting call")
   
+  (emit "movq %rax, ~a(%rbp)" si) ; store address of closure first on stack
   
-					; then codegen the arguments and move them into the next param registers
+					; codegen the arguments, store them intermediately
   (for-each
    (lambda (e i)
      (begin
-	 (codegen-expr e si env)
-					; move result to correct param register
-	 (emit "movq %rax, ~a" (param-register i))))
-     args (range argument-start (length args)))
+       (emit "## arg no. ~a" (- i 1))
+       (codegen-expr e (- si (* wordsize i)) env)
+					; store intermediate result on stack
+       (emit "movq %rax, ~a(%rbp)" (- si (* wordsize i)))))
+
+   args (range 1 (length args)))
+
+					; now that we have everything we need on the stack,
+					; move them into the param registers
+
+  (emit "## moving args into place")
+  (for-each
+   (lambda (i) (emit "movq ~a(%rbp), ~a"
+		     (- si (* wordsize i))
+		     (param-register i)))
+   (range 1 (length args)))
+
+					; todo: can this be made more efficient
+  (emit "movq ~a(%rbp), %rax" si)       ; load back pointer to closure
+
+  (emit "## moving captives into place")
+  
+					; move captives into first argument
+  (emit "movq %rax, %rbx")
+  (emit "addq $8, %rbx")
+  (emit "movq %rbx, ~a" (param-register 0))
+
+  (emit "## performing call")
 
-			    		; now call
-    (emit "callq ~a" label)))
+  (emit "addq $~a, %rsp" si) ; adjust the stack pointer to account all the stuff we put in the env
+  (emit "callq *(%rax)")                ; call closure function
+  (emit "subq $~a, %rsp" si))
 
+					; LAMBDAS:
+					; 1st param: pointer to captured args
+					; 2nd param: 1st arg
+					; 3rd param: 2nd arg, etc.
 
 (define (codegen-lambda l)
   (let* ((label (car l))
-	 (args (cadr l))
-	 (captured (caddr l))
-	 (body (cadddr l))
-	 ; captured, then args
-	 (vars (append captured args))
-
-	 (param-registers (map param-register
-			     (range 0 (length vars))))
-	 (stack-offsets (map (lambda (i)
-			       (* (- wordsize) i))
-			     (range 0 (length vars))))
+	 (stuff (cdr l))
+	 (captives (car stuff))
+	 (args (cadr stuff))
+	 (body (caddr stuff))
+					; params = what actually gets passed
+	 (params (append captives args))
 
-	 (copy-insts (map (lambda (r o)
-			    (format "movq ~a, ~a(%rsp)"
-				    r o))
-			  param-registers stack-offsets))
+	 (stack-offsets (map (lambda (i)
+			       (* (- wordsize) (+ 1 i)))
+			     (range 0 (length params))))
 
-	 (env (map cons vars stack-offsets)))
+	 (env (map cons params stack-offsets)))
     (emit "~a:" label)
+
+    (display "## lambda captives: ")
+    (display captives)
+    (newline)
+    (display "## lambda args: ")
+    (display args)
+    (newline)
     (display "## lambda body: ")
     (display body)
     (newline)
-    (display "## environment: ")
-    (display env)
-    (newline)
-    (amd64-abi
-     (lambda ()
-       (for-each emit copy-insts)
-       (codegen-expr body (* (- wordsize) (length vars)) env)
-       ))))			; move args and capture vars to stack
+    
+    (emit "push %rbp") ; preserve caller's base pointer
+    
+    (emit "movq %rsp, %rbp") ; set up our own base pointer
+
+					; load the captured vars onto the stack
+    (for-each
+     (lambda (i)
+       (begin
+	 (emit "# loading captive ~a" (list-ref captives i))
+	 (emit "movq ~a(~a), %rbx" (* wordsize i) (param-register 0))
+	 (emit "movq %rbx, ~a(%rbp)" (* (- wordsize) (+ 1 i)))))
+     (range 0 (length captives)))
+
+					; load the args onto the stack
+    (for-each
+     (lambda (i)
+       (begin
+	 (emit "movq ~a, %rbx" (param-register (+ 1 i)))
+	 (emit "movq %rbx, ~a(%rbp)"
+	       (* (- wordsize)
+		  (+ 1 (length captives) i)))))
+     (range 0 (length args)))
+    
+    (codegen-expr body (* (- wordsize) (+ 1 (length params))) env)
+
+    (emit "pop %rbp") ; restore caller's base pointer
+    (emit "ret")))
 
 (define cur-label 0)
 (define (fresh-label)
@@ -163,19 +302,22 @@
     (emit "~a:" exit-label)))
 
 (define (codegen-expr e si env)
+  (emit "# ~a" e)
   (case (ast-type e)
-    ('builtin e)
-    ('closure e)
+    ('closure (codegen-closure (cadr e) (caddr e) si env))
     ('app
-     (let ((callee (codegen-expr (car e) si env)))
-       (case callee
+     (case (car e)
        ('+ (codegen-add (cdr e) si env))
        ('- (codegen-sub (cadr e) (caddr e) si env))
        ('* (codegen-mul (cadr e) (caddr e) si env))
        ('! (codegen-not (cadr e) si env))
        ('= (codegen-eq  (cadr e) (caddr e) si env))
        ('bool->int (codegen-expr (cadr e) si env))
-	 (else (codegen-call callee (cdr e) si env)))))
+       ('print (codegen-print (cadr e) si env))
+       (else (codegen-call (car e) (cdr e) si env))))
+
+					; this is a builtin being passed around as a variable
+    ('builtin (emit "movq $~a, %rax" (builtin-id e)))
 
     ('let (codegen-let (let-bindings e)
 		       (let-body e)
@@ -186,37 +328,45 @@
 
     ('if (codegen-if (cadr e) (caddr e) (cadddr e) si env))
     
-    ('string-literal (emit "movq ~a, %rax" label))
     ('bool-literal (emit "movq $~a, %rax" (if e 1 0)))
     ('int-literal (emit "movq $~a, %rax" e))
     
+    ('static-string (emit "movq ~a@GOTPCREL(%rip), %rax"
+			  (cadr e)))
+
     (else (error #f "don't know how to codegen this"))))
 
 
-(define (fold-map f x) (fold-left append '() (map f x)))
 
 (define (free-vars prog)
   (define bound '())
   (define (collect e)
     (case (ast-type e)
       ('builtin '()) ; do nothing
-      ('var (if (memq e bound) '() (list e)))
+      ('var (if (memv e bound) '() (list e)))
       ('lambda
+	  (begin
 	    (set! bound (append (lambda-args e) bound))
-	(collect (lambda-body e)))
+	    (collect (lambda-body e))))
 
       ('app (fold-map collect e))
+      ('if (fold-map collect (cdr e)))
       ('let
-	  (let ((bind-fvs (fold-map (lambda (a)
-				      ((set! bound (cons (car a) bound))
+	  (let ([bind-fvs (fold-map (lambda (a)
+				      (begin
+					(set! bound (cons (car a) bound))
 					(collect (cdr a))))
-				    (let-bindings cadr)))
-		(body-fvs (fold-map collect (let-body e))))
-	    (append bind-fvs body-fvs)))
+				    (let-bindings e))])
+	    (append bind-fvs (fold-map collect (let-body e)))))
       (else '())))
   (collect prog))
 
-					; ((lambda (x) (+ x 1)) 42) => {lambda0: (x) (+ x 1)}, (@lambda0 42)
+					; ((lambda (x) (+ x y)) 42) => ((closure lambda1 (y)) 42)
+                                        ;                              [(lambda1 . ((y), (x), (+ x y))]
+					; for builtins, this generates a closure if it is used
+					; outside of an immediate app
+					; but only one closure for each builtin
+
 (define (extract-lambdas program)
   (define lambdas '())
   (define (add-lambda e)
@@ -224,42 +374,108 @@
 	   (args (lambda-args e))
 	   (captured (free-vars e))
 	   (body (extract (lambda-body e)))
-	   (new-lambda (list label args captured body)))
+	   (new-lambda (cons label (list captured args body))))
       (set! lambdas (cons new-lambda lambdas))
       `(closure ,label ,captured))) ; todo: should we string->symbol?
+
+  (define (find-builtin-lambda e)
+    (let [(l (assq (builtin-name e) lambdas))]
+      (if l `(closure ,(car l) ,(caadr l)) #f)))
+
+  (define (builtin-name e)
+    (case e
+      ('+ "_add")
+      ('- "_sub")
+      ('* "_mul")
+      ('! "_not")
+      ('= "_eq")
+      ('bool->int "_bool2int")
+      ('print "_print")
+      (else (error #f "don't know this builtin"))))
+  (define (builtin-args e)
+    (case e
+      ('+ '(x y))
+      ('- '(x y))
+      ('* '(x y))
+      ('! '(x))
+      ('= '(x y))
+      ('bool->int '(x))
+      ('print '(x))
+      (else (error #f "don't know this builtin"))))
+
+  (define (add-builtin-lambda e)
+    (let* [(label (builtin-name e))
+	   (captured '())
+	   (args (builtin-args e))
+	   (body `(,e ,@args))
+	   (new-lambda (cons label (list captured args body)))]
+      (set! lambdas (cons new-lambda lambdas))
+      `(closure ,label ,captured)))
+  
   (define (extract e)
     (case (ast-type e)
       ('lambda (add-lambda e))
-      ('let `(let ,(map extract (let-bindings e))
+      ('let `(let ,(map (lambda (b) `(,(car b) ,@(extract (cdr b)))) (let-bindings e))
 	       ,@(map extract (let-body e))))
-      ('app (append (list (extract (car e)))
+      ('app (append
+					; if a builtin is used as a function, don't generate lambda
+	     (if (eqv? 'builtin (ast-type (car e)))
+		 (list (car e))
+		 (list (extract (car e))))
 	     (map extract (cdr e))))
+      
+      ('builtin
+       (if (find-builtin-lambda e)
+	   (find-builtin-lambda e)
+	   (add-builtin-lambda e)))
+
+      
       (else (ast-traverse extract e))))
   (let ((transformed (extract program)))
     (cons lambdas transformed)))
 
-;(define (extract-strings program))
-
-(define (amd64-abi f)
-					; preserve registers
-  (emit "push %rbp")
-  (emit "push %rbx")
-  (for-each (lambda (i)
-	      (emit (string-append
-		     "push %r"
-		     (number->string i))))
-	    '(12 13 14 15))
-
-  (f) ; call stuff
-					; restore preserved registers
-  (for-each (lambda (i)
-	      (emit (string-append
-		     "pop %r"
-		     (number->string i))))
-	    '(15 14 13 12))
-  (emit "pop %rbx")
-  (emit "pop %rbp")
-  (emit "ret"))
+(define (extract-strings program)
+  (let ((cur-string 0)
+	(strings '())) ; assoc list of labels -> string
+    (define (fresh-string)
+      (set! cur-string (+ cur-string 1))
+      (format "string~a" (- cur-string 1)))
+    (define (extract e)
+      (case (ast-type e)
+	('string-literal
+	 (let ((label (fresh-string)))
+	   (set! strings (cons (cons label e) strings))
+	   `(static-string ,label)))
+	(else (ast-traverse extract e))))
+    (let ((transformed (extract program)))
+      (cons strings transformed))))
+
+(define (emit-string-data s)
+  (emit "~a:" (car s))
+  (emit "\t.string \"~a\"" (cdr s)))
+
+;; (define (amd64-abi f)
+;; 					; preserve registers
+;;   (emit "push %rbp")
+;;   ;; (emit "push %rbx")
+;;   ;; (for-each (lambda (i)
+;;   ;; 	      (emit (string-append
+;;   ;; 		     "push %r"
+;;   ;; 		     (number->string i))))
+;;   ;; 	    '(12 13 14 15))
+
+;;   (emit "movq %rsp, %rbp")              ; set up the base pointer
+
+;;   (f) ; call stuff
+;; 					; restore preserved registers
+;;   ;; (for-each (lambda (i)
+;;   ;; 	      (emit (string-append
+;;   ;; 		     "pop %r"
+;;   ;; 		     (number->string i))))
+;;   ;; 	    '(15 14 13 12))
+;;   ;; (emit "pop %rbx")
+;;   (emit "pop %rbp")
+;;   (emit "ret"))
 
 					; 24(%rbp) mem arg 1
 					; 16(%rbp) mem arg 0          prev frame
@@ -281,26 +497,92 @@
     (else (error #f "need to test out the below"))
     (else (format "~a(%rsp)" (- n 6)))))
 
+(define (initialize-heap)
+  (let ((mmap
+	 (case target
+	   ('darwin "0x20000c5")
+	   ('linux "9"))))
+					; allocate some heap memory
+    (emit "mov $~a, %rax" mmap) ; mmap
+    (emit "xor %rdi, %rdi")  ; addr = null
+    (emit "movq $1024, %rsi")   ; length = 1kb
+    (emit "movq $0x3, %rdx") ; prot = read | write = 0x2 | 0x1
+					;    flags = anonymous | private
+    (case target
+      ('darwin (emit "movq $0x1002, %r10")) ; anon = 0x1000, priv = 0x02
+      ('linux (emit "movq $0x22, %r10")))   ; anon = 0x20,   priv = 0x02
+    (emit "movq $-1, %r8") ; fd = -1
+    (emit "xor %r9, %r9") ; offset = 0
+    (emit "syscall")
+					; %rax now contains pointer to the start of the heap
+					; keep track of it
+
+    (emit "movq heap_start@GOTPCREL(%rip), %rsi")
+    (emit "movq %rax, (%rsi)")))
+
 (define (codegen program)
-  (let* ((extract-result (extract-lambdas program))
-	 (lambdas (car extract-result))
-	 (xform-prog (cdr extract-result)))
-    (emit ".text")
-    (emit ".p2align 4,,15")
+  (set! cur-label 0)
+  (set! cur-lambda 0)
+  (let* ((extract-res-0 (extract-strings program))
+	 (strings (car extract-res-0))
+	 (extract-res-1 (extract-lambdas (cdr extract-res-0)))
+	 (lambdas (car extract-res-1))
+	 (xform-prog (cdr extract-res-1)))
+
+    (emit "\t.global _start")
+    (emit "\t.text")
+					;    (emit ".p2align 4,,15") is this needed?
 
     (for-each codegen-lambda lambdas)
 
-    (emit ".globl _scheme_entry")
-    (emit "_scheme_entry:")
+    (emit "_start:")
+
+    (initialize-heap)
+
+    (emit "movq %rsp, %rbp")            ; set up the base pointer
     
+    (codegen-expr xform-prog (- wordsize) '())
 
-    (amd64-abi
-     (lambda () (codegen-expr xform-prog 0 '())))))
+					; exit syscall
+    (emit "mov %rax, %rdi")
+    (case target
+      ('darwin (emit "movq $0x2000001, %rax"))
+      ('linux (emit "mov $60, %rax")))
+    (emit "syscall")
 
-(define (compile-to-binary program output)
+    (emit ".data")
+
+    (emit "heap_start:")
+    (emit "\t.quad 0")
+
+    (for-each emit-string-data strings)))
+
+(define (compile-to-binary program output t)
+  (set! target t)
   (when (not (eq? (typecheck program) 'int)) (error #f "not an int"))
   (let ([tmp-path "/tmp/a.s"])
     (when (file-exists? tmp-path) (delete-file tmp-path))
     (with-output-to-file tmp-path
       (lambda () (codegen program)))
-    (system (format "clang -fomit-frame-pointer /tmp/a.s rts.c -o ~a" output))))
+
+    (case target
+      ('darwin
+       (system "as /tmp/a.s -o /tmp/a.o")
+       (system (format "ld /tmp/a.o -e _start -macosx_version_min 10.14 -static -o ~a" output)))
+      ('linux
+       (system "as /tmp/a.s -o /tmp/a.o")
+       (system (format "ld /tmp/a.o -o ~a" output))))))
+
+; NOTES
+; syscalls in linux and darwin use the following arguments for syscall instruction:
+; %rax = syscall #
+; %rdi = 1st arg
+; %rsi = 2nd arg
+; %rdx = 3rd arg
+; %r10 = 4th arg
+; %r8  = 5th arg
+; %r9  = 6th arg
+
+; on darwin, the syscall is offset by 0x2000000
+; https://opensource.apple.com/source/xnu/xnu-2782.20.48/bsd/kern/syscalls.master
+; documentation for most syscalls: /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/sys