Tag: scheme

After trying, and failing, to write a flocking system in jellyfish bytecode I wrote a compiler using the prototype betablocker one. It reads a scheme-ish imperative language and generates bytecode (which is also invented, and implemented in C++) it only took a couple evenings and a train journey to write, and it even seems to work.

The basic idea is to walk through the code tree described by the scheme lists generating bits of bytecode that fit together. Let’s take logical “not” as an example. Like GPU processors, the only datatype is vectors of 3 floats, and we define false as 0 in the x position and anything else in x to be true (ignoring what’s in y or z). There is no single instruction for “not” so we have to build it from the other instructions. For example this bit of code:

(not (vector 0 0 0))

should return (vector 1 0 0). When we are walking the tree of lists we check the first element and dispatch to a set of functions, one for each type of (higher level) instruction which ’emit’s a list containing the bytecode required. The one for ‘not’ looks like this, where x is the expression, e.g. ‘(not (vector 0 0 0))’:

(define (emit-not x)
  (append
   (emit-expr (cadr x))
   (emit (vector jmz 3 0))
   (emit (vector ldl 0 0))
   (emit (vector jmr 2 0))
   (emit (vector ldl 1 0))))

The first thing it does is return all the instructions required for the expression we pass in the second element of ‘x’ with ’emit-expr’. With our simple example it will just push (vector 0 0 0) onto the stack, but it could be a whole load of complicated nested expressions, and it will work the same.

After that we have some bytecode:

jmz 3 0 ;; if top of stack is 0, jump forward 3 instructions (ldl 1 0)
ldl 0 0 ;; load 0 onto the stack
jmr 2 0 ;; jump forward 2 instructions (skip to next code section)
ldl 1 0 ;; load 1 onto the stack

So this just checks (and removes) the top element on the stack and pushes the opposite logical value. Pushing a single float like the ‘ldl’ (load literal) instructions above expands to a vector value internally, it’s just a convenience. Some instructions (such as those involving vector maths) are just a single instruction, others like conditionals or loops are a bit trickier as they need to count instructions to skip over variable length sections of program.

We add variables in the form of ‘let’ that map to addresses a the start of memory, read and write for accessing model memory like array lookups. The full flocking system looks like this, and animates a points primitive in OpenGL:

(let ((vertex 512) 
      (accum-vertex 512)  
      (closest 9999)
      (closest-dist 9999)
      (diff 0)
      (vel 1024))
      (loop 1 ;; infinite loop
        (loop (< vertex 532) ;; for every vertex
          ;; find the closest vertex
          (loop (< accum-vertex 532) 
            (cond 
              ;; if they're not the same vert
              ((not (eq? accum-vertex vertex))
              ;; get vector between the points
              (set! diff (- (read vertex) (read accum-vertex)))
              (cond 
                ;; if it's closer so far
                ((< (mag diff) closest-dist)
                ;; record vector and distance
                (set! closest diff)
                (set! closest-dist (mag closest))))))
              (set! accum-vertex (+ accum-vertex 1)))
              ;; reset accum-vertex for next time
              (set! accum-vertex 512)

              ;; use closest to do the flocking, add new velocity 
              ;; to old (to add some inertia)
              (write! vel (+ (* (read vel) 0.99)
                   ;; attract to centre
                   (* (+ (* (- (read vertex) (vector 0 0 0)) 0.05)
                         ;; repel from closest vertex
                         (* (normalise closest) -0.15)) 0.01)))
              ;; add velocity to vertex position
              (write! vertex (+ (read vel) (read vertex))) 
                
              ;; reset and increment stuff
              (set! closest-dist 9999)
              (set! vel (+ vel 1))
              (set! vertex (+ vertex 1)))
            ;; reset for main loop
            (set! vertex 512)
            (set! vel 1024)))

This compiles to 112 vectors of bytecode (I should call it vectorcode really) with extra debugging information added so we can see the start and the end of each higher level instruction. It all looks like this – which most importantly I didn’t need to write by hand!

10 30000 0 ;; top memory positions are for registers controlling 
512 2 1    ;; program and graphics state (primitive type and number of verts)
nop 0 0    ;; space
nop 0 0    ;; for all
nop 0 0    ;; the variables
nop 0 0    ;; we use 
nop 0 0    ;; in the program
nop 0 0
nop 0 0
nop 0 0
;; starting let  <- program starts here
ldl 512 0        ;; load all the 'let' variable data up
sta 4 0
ldl 512 0
sta 5 0
ldl 9999 0
sta 6 0
ldl 9999 0
sta 7 0
ldl 0 0
sta 8 0
ldl 1024 0
sta 9 0
;; starting loop  <- start the main loop
;; starting loop
;; starting loop
;; starting cond
;; starting not
;; starting eq?
lda 5 0
lda 4 0
sub 0 0
jmz 3 0
ldl 0 0
jmr 2 0
ldl 1 0
;; ending eq?
jmz 3 0
ldl 0 0
jmr 2 0
ldl 1 0
;; ending not
jmz 38 0
;; starting set!
;; starting -
;; starting read
ldi 4 0
;; ending read
;; starting read
ldi 5 0
;; ending read
sub 0 0
;; ending -
sta 8 0
;; ending set!
;; starting cond
;; starting <
;; starting mag
lda 8 0
len 0 0
;; ending mag
lda 7 0
jlt 3 0
ldl 1 0
jmr 2 0
ldl 0 0
;; ending <
jmz 12 0
;; starting set!
lda 8 0
sta 6 0
;; ending set!
;; starting set!
;; starting mag
lda 6 0
len 0 0
;; ending mag
sta 7 0
;; ending set!
;; ending cond
;; ending cond
;; starting set!
;; starting +
lda 5 0
ldl 1 0
add 0 0
;; ending +
sta 5 0
;; ending set!
;; starting <
lda 5 0
ldl 532 0
jlt 3 0
ldl 1 0
jmr 2 0
ldl 0 0
;; ending <
jmz 2 0
jmr -72 0
;; ending loop
;; starting set!
ldl 512 0
sta 5 0
;; ending set!
;; starting write!
;; starting +
;; starting *
;; starting read
ldi 9 0
;; ending read
ldl 0.9900000095 0
mul 0 0
;; ending *
;; starting *
;; starting +
;; starting *
;; starting -
;; starting read
ldi 4 0
;; ending read
ldlv 0 0
nop 0 0
sub 0 0
;; ending -
ldl 0.05000000075 0
mul 0 0
;; ending *
;; starting *
;; starting normalise
lda 6 0
nrm 0 0
;; ending normalise
ldl -0.150000006 0
mul 0 0
;; ending *
add 0 0
;; ending +
ldl 0.009999999776 0
mul 0 0
;; ending *
add 0 0
;; ending +
sti 9 0
;; ending write!
;; starting write!
;; starting +
;; starting read
ldi 9 0
;; ending read
;; starting read
ldi 4 0
;; ending read
add 0 0
;; ending +
sti 4 0
;; ending write!
;; starting set!
ldl 9999 0
sta 7 0
;; ending set!
;; starting set!
;; starting +
lda 9 0
ldl 1 0
add 0 0
;; ending +
sta 9 0
;; ending set!
;; starting set!
;; starting +
lda 4 0
ldl 1 0
add 0 0
;; ending +
sta 4 0
;; ending set!
;; starting <
lda 4 0
ldl 532 0
jlt 3 0
ldl 1 0
jmr 2 0
ldl 0 0
;; ending <
jmz 2 0
jmr -160 0
;; ending loop
;; starting set!
ldl 512 0
sta 4 0
;; ending set!
;; starting set!
ldl 1024 0
sta 9 0
;; ending set!
ldl 1 0
jmz 2 0
jmr -173 0
;; ending loop
;; ending let