Basic Expression Algorithm

The basic algorithm for expressions is easy: postorder.

• Operand (leaf node): get a register; generate a load; return the register.

• Operator (interior node): generate operand subtrees; generate op; free operand register; return result register.

; given expr x, return register with value
(defun genarith (x)
  (if (atom x)              ; if leaf, id/num
      (genload x (getreg))  ;   load: mem -> reg
      (genop (op x)         ; else op
             (genarith (lhs x))  ; on subtrees
             (genarith (rhs x))) ) )

>(genarith '(* (+ a b) 3))
   MOVL   A,EAX
   MOVL   B,EBX
   ADDL   EBX,EAX
   MOVL   3,ECX
   IMULL  ECX,EAX
EAX