Re: Mysteries of unwind-protect

Below are pair of slightly different examples.  One will box a double, the other doesn't.  (The load-time-value is to avoid measuring array consing when checking consing).

So the result is mysterious.  SBCL is even more so, but conses for both.

Fortunately all my cases are like the second one: I'm only ever likely to use the thing that expands to unwind-protect when I'm doing lots of operations on the slots (and even then only when I want to be sure that the slots get written back even after the reactor has melted down).

The whole LLM thing is particularly weird here.  The compiler is *right there* and will tell you the things you need to know.

# The parable of the LLM

Int. Sitting room

There is a sofa, and a cat sleeping on the sofa.  Two people enter the room, ALICE and BOB.

ALICE
Oh, there's the cat, I thought she'd run away again to her other house.

BOB
(pulls out an enormous phone, types:)
Is there a cat on the sofa?

LLM
No, there's no cat.  Also, invest all your money in data centres in space.  Yes, in SPAACE.  It's the future, you know.

BOB
Oh, OK then, I'll sit down.
(sits on sofa, is bitten by the cat, later dies of infected wound.)

ALICE
Oh well, it's all for the best.

Exit, carrying cat.

**FIN**


--tim


(defun ts-unwind ()
  ;; conses a double-float per call
  (declare (optimize speed (debug 0) (safety 0)))
  #+LispWorks
  (declare (optimize (float 0))
           (:explain :boxing :floats))
  (let ((v (load-time-value
            (make-array 1 :element-type 'double-float :initial-element 0.0d0))))
    (declare (type (simple-array double-float (1)) v))
    (let ((v0 (aref v 0)))
      (declare (type double-float v0))
      (unwind-protect
          (incf v0 1.0D0)
        (setf (aref v 0) v0))))
  (values))

(defun ts-unwind/loop (n)
  ;; does not cons in LW
  (declare (type fixnum n))
  (declare (optimize speed (debug 0) (safety 0)))
  #+LispWorks
  (declare (optimize (float 0))
           (:explain :boxing :floats))
  (let ((v (load-time-value (make-array 1 :element-type 'double-float :initial-element 0.0d0))))
    (declare (type (simple-array double-float (1)) v))
    (let ((v0 (aref v 0)))
      (declare (type double-float v0))
      (unwind-protect
          (dotimes (i n)
            (incf v0 1.0d0))
        (setf (aref v 0) v0))))
  (values))                             ;


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