anchor.diff:                    (dst-index :scs (sb-vm::any-reg))
                     (src       :scs (sb-vm::sap-reg))
                     (src-index :scs (sb-vm::any-reg))
  -                  (n-words   :scs (sb-vm::unsigned-reg) :target rcx))
  +                  (n-words   :scs (sb-vm::unsigned-reg) #-x86 :target #-x86 rcx))
              (:info dst-scale dst-offset src-scale src-offset)
              (:arg-types sb-sys:system-area-pointer sb-vm::tagged-num
                          sb-sys:system-area-pointer sb-vm::tagged-num
  
positive.code:            (let ((memset-fun ',%memset-name))
               `(,memset-fun ,ptr ,index ,n-words ,fill-word ,scale ,offset)))))))
  
  (define-fast-memset :memset-name fast-memset/4 :type (unsigned-byte 32) :size :dword)
  
  #+x86-64
  
negative.code: 
  
  (defmacro define-fast-memcpy (&key memcpy-name type size)
    (declare (ignore type))
    (let ((%memcpy-name (concat-symbols '% memcpy-name)))
      `(progn
         (eval-always
  