Varjo and Multiple Value Return

Hi folks, having recently got married (yay!) and got a flat (another yay!) I have been getting back into lisp coding again.

The latest addition to the Varjo compiler is support for ‘values’ and ‘multiple-value-bind’. Now as glsl doesn’t have support for multiple value returns we have to use something else. The mechanism I have now works something like this:

  • If you have a ‘values’ form within a multiple-value-bind form the multiple-value-bind form turns into a ‘let’ and the ‘values’ becomes ‘setf’s to the variables in the let.

  • If you have a values form and it isnt within a multiple-value-bind then one of the following happens ** If the values form can propagate to a ‘return’ form then the function is given out-vars and they are set from the place where the values form was originally. ** If it cant reach a return then it collapses to a progn.

If a varjo function with out-vars is used within a multiple-value-bind then the multiple-value-bind becomes a let statement and the first var is set by the function return and the rest are passed as out-vars to the function.

If the function is used outside of a multiple-value-bind then the form is surrounded in a ‘let’ as the outvars still need to be set.

This results is something like the following.

VARJO> (defshader test ()
         (labels ((thing ((x :int)) (return (values x (* x 2)))))
           (multiple-value-bind (x y) (thing 1)
             (+ x y)))))

;; Gets compiled to

#version 330

int thing_26v(int x_23v, out int return1);

int thing_26v(int x_23v, out int return1) {
    return0 = x_23v;
    return1 = (x_23v * 2);
    return return0;

void main() {
    int a_27v0;
    int a_27v1;
    a_27v0 = thing_26v(1,a_27v1);
    (a_27v0 + a_27v1);

I still have a few bugs to iron out but it is very nearly there. One step closer to lisp!

Published: July 24 2014

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