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clear option for RLET

   Sub:    a :clear option for rlet
   From: engber@aristotle.ils.nwu.edu (Mike Engber)
   Message-Id: <9107052052.AA05946@aristotle.ils.nwu.edu>
   To: info-mcl@cambridge.apple.com
   It would be nice if rlet allowed a :clear option like make-record
   does. This would be especially helpful for allocating parameter
   blocks which are notorious for causing problems unless 'unused'
   fields are zeroed out.
   I realize there is probably not as nice(efficient) a way to do
   this as there is for make-record, but it would still be helpful.
   I much prefer rlet to make-record.
   If someone can tell me a slick way to zero the contents of a
   macptr I'd be happy to implement it myself. Off the top of
   my head all I can think of is a loop and %put-xxx. There has
   to be a better way.
We didn't include the clear option as we couldn't do it much better than
you can.  I suppose we could have included a FILL-MEMORY function written
in assembler.  If you're really adverse to using %put-xxx, try the
following assembler code.  It could be fancier by doing longword
moves for most of its work, but I doubt you'll notice the difference.
; Here's what it does
(defun fill-memory (macptr length &optional (byte 0))
  (dotimes (i length)
    (declare (fixnum i))
    (%put-byte macptr byte i)))
(in-package :ccl)
(export 'fill-memory)
(eval-when (compile eval)
  (require 'lapmacros))
(defun fill-memory (macptr length &optional (byte 0))
  (setq length (require-type length 'fixnum))
  (setq byte (require-type byte 'fixnum))
  (lap-inline (macptr length byte)
    (move.l arg_x atemp0)               ; macptr
    (jsr_subprim $sp-macptrptr)         ; address in atemp0
    (getint arg_y)                      ; unbox length
    (getint arg_z)                      ; unbox byte
    (dbfloop.l arg_y
      (move.b arg_z atemp0@+)))