Remove redundant assignments and locations in Cleavir IR

remove-redundant-assignments.lisp

;;; Remove any assignment instructions and locations which appear to
;;; be redundant.  We define a "redundant" assignment to be one which
;;; assigns from a lexical location to another, either with only one
;;; defining instruction; i.e. the assignment does not cause any
;;; visible effect.

(defun remove-redundant-assignments (enter-instruction)
  (cleavir-ir:map-instructions-arbitrary-order
   (lambda (instruction)
     (when (typep instruction 'cleavir-ir:assignment-instruction)
       (destructuring-bind (input)
           (cleavir-ir:inputs instruction)
         (destructuring-bind (output)
           (cleavir-ir:outputs instruction)
           (when (and
                  (typep input 'cleavir-ir:lexical-location)
                  (alexandria:length=
                   1
                   (cleavir-ir:defining-instructions input)
                   (cleavir-ir:defining-instructions output)))
             (cleavir-ir:replace-datum input output)
             (cleavir-ir:delete-instruction instruction))))))
   enter-instruction))