src/compiler/6-optimization.lisp @ fd638982ad83

(in-package :temperance)

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;;; Optimization of the WAM instructions happens between the precompilation
;;; phase and the rendering phase.  We perform a number of passes over the
;;; circle of instructions, doing one optimization each time.


(defun optimize-get-constant (node constant register)
  ;; 1. get_structure c/0, Ai -> get_constant c, Ai
  (circle-replace node `(:get-constant ,constant ,register)))

(defun optimize-put-constant (node constant register)
  ;; 2. put_structure c/0, Ai -> put_constant c, Ai
  (circle-replace node `(:put-constant ,constant ,register)))

(defun optimize-subterm-constant-query (node constant register)
  ;; 3. put_structure c/0, Xi                     *** WE ARE HERE
  ;;    ...
  ;;    subterm_value Xi          -> subterm_constant c
  (loop
    :with previous = (circle-prev node)
    ;; Search for the corresponding set-value instruction
    :for n = (circle-forward-remove node) :then (circle-forward n)
    :while n
    :for (opcode . arguments) = (circle-value n)
    :when (and (eql opcode :subterm-value-local)
               (register= register (first arguments)))
    :do
    (circle-replace n `(:subterm-constant ,constant))
    (return previous)))

(defun optimize-subterm-constant-program (node constant register)
  ;; 4. subterm_variable Xi       -> subterm_constant c
  ;;    ...
  ;;    get_structure c/0, Xi                     *** WE ARE HERE
  (loop
    ;; Search backward for the corresponding subterm-variable instruction
    :for n = (circle-backward node) :then (circle-backward n)
    :while n
    :for (opcode . arguments) = (circle-value n)
    :when (and (eql opcode :subterm-variable-local)
               (register= register (first arguments)))
    :do
    (circle-replace n `(:subterm-constant ,constant))
    (return (circle-backward-remove node))))


(defun optimize-constants (instructions)
  ;; From the book and the erratum, there are four optimizations we can do for
  ;; constants (0-arity structures).

  (flet ((optimize-put (node functor register)
           (if (register-argument-p register)
             (optimize-put-constant node functor register)
             (optimize-subterm-constant-query node functor register)))
         (optimize-get (node functor register)
           (if (register-argument-p register)
             (optimize-get-constant node functor register)
             (optimize-subterm-constant-program node functor register))))
    (loop
      :for node = (circle-forward instructions) :then (circle-forward node)
      :while node :do
      (destructuring-bind (opcode . arguments) (circle-value node)
        (when (member opcode '(:put-structure :get-structure))
          (destructuring-bind (functor arity register) arguments
            (when (zerop arity)
              (setf node
                    (case opcode
                      (:put-structure (optimize-put node functor register))
                      (:get-structure (optimize-get node functor register))))))))))
  instructions)


(defun optimize-void-runs (instructions)
  ;; We can optimize runs of N (:unify-void 1) instructions into a single one
  ;; that does all N at once.
  (loop
    :for node = (circle-forward instructions) :then (circle-forward node)
    :while node
    :for opcode = (car (circle-value node))
    :when (eq opcode :subterm-void)
    :do
    (loop
      :with beginning = (circle-backward node)
      :for run-node = node :then (circle-forward run-node)
      :for run-opcode = (car (circle-value run-node))
      :while (eq opcode run-opcode)
      :do (circle-remove run-node)
      :sum 1 :into run-length fixnum ; lol
      :finally
      (progn
        (setf node (circle-forward beginning))
        (circle-insert-after beginning
                             `(,opcode ,run-length)))))
  instructions)


(defun optimize-instructions (instructions)
  (->> instructions
    (optimize-constants)
    (optimize-void-runs)))