Re: Mysteries of unwind-protect

The difference is because ts-unwind passes a double float (the value of incf)
through the unwind-protect, but ts-unwind/loop passes nil through.  Using
(progn (incf v0 1.0D0) nil) instead of (incf v0 1.0D0) eliminates the boxing.

LW doesn't have an implementation of unwind-protect for unboxed values, so the
value is boxed even though it is never used.  The variable v0 is still stored
unboxed.

-- 
Martin Simmons
LispWorks Ltd
http://www.lispworks.com/



>>>>> On Fri, 16 Jan 2026 12:20:17 +0000, Tim Bradshaw (as tfb at tfeb dot org) said:
> 
> 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))                             ;
> 
> 
> _______________________________________________
> Lisp Hug - the mailing list for LispWorks users
> [email protected]
> http://www.lispworks.com/support/lisp-hug.html
> 

_______________________________________________
Lisp Hug - the mailing list for LispWorks users
[email protected]
http://www.lispworks.com/support/lisp-hug.html