Re: [Sbcl-help] Inference type from slots?

[Steve L. <sjl...@mi...>]

Thank you Christophe! Your explanation if clear and useful, and your
example works great.

I've been digging a little deeper and using structures as you recommend,
which has led me to a similar question. I find that when using structures,
SBCL similarly does not infer type when I use (slot-value ...), but if I
use the corresponding accessor for the slot, everything works great. This
isn't a problem for me (as I'll just use the accessors), but I was curious
why this might be.

Here is another toy example, which uses a structure A and four methods x,
y, z, and w (that all do the same thing differently):

(defstruct A
  (m-num 0 :type fixnum))

;; Optimizes great!
(defmethod x ((a A))
  (declare (optimize speed (safety 0)))
  (incf (a-m-num a)))

;; Also works great!
(defmethod y ((a A))
  (declare (optimize speed (safety 0)))
  (with-accessors ((m-num a-m-num)) a
    (incf m-num)))

;; Can't optimize the incf
(defmethod z ((a A))
  (declare (optimize speed (safety 0)))
  (incf (slot-value a 'm-num)))

;; Also can't optimize (because macroexpands to slot-value)
(defmethod w ((a A))
  (declare (optimize speed (safety 0)))
  (with-slots (m-num) a
    (incf m-num)))

This example shows that using the accessor (a-m-num ...), or the similar
(with-accessors ...) macro, optimizes great. However, using (slot-value
...) or the corresponding (with-slots ...) macro doesn't optimize the incf.
Again, not a problem, but I'm curious.

Thanks again for all your useful help!

Best regards,
Steve




On Thu, Apr 24, 2014 at 1:41 AM, Christophe Rhodes <cs...@ca...> wrote:

> Steve Levine <sjl...@mi...> writes:
>
> > I have a question about SBCL optimizations for slot types in classes.
> I've
> > noticed that when using (with-slots ...) in a certain way, I seem to get
> a
> > lot of "unable to optimize" notices. Here's a toy example:
> >
> > (defclass A ()
> >   ((m_num
> >     :type fixnum
> >     :initform 2)))
> >
> >
> > (defmethod f ((a A))
> >   (with-slots (m_num) a
> >     (incf m_num)))
>
> The problem with SBCL acting on the information that the slot is
> declared to be of type fixnum here, is that the class can be redefined
> at any time, including changing any and all slot constraints, *without*
> the method needing to be touched at all.  So if we compiled the method
> function while baking in the fixnumness of the slot, we would have to
> invalidate that function on incompatible class redefinition.
>
> This isn't impossible, but it's hard.  Generally, CLOS has enough layers
> of indirection that it's not easy to get maximum speed out of it; if
> your design has settled down enough that redefinition is not necessary,
> you could consider using structures for the things needing type
> inference and speed.  Alternatively,
>   (defmethod f ((a A))
>     (let ((m (slot-value a 'm_num)))
>       (declare (type fixnum m) (optimize speed (safety 0)))
>       (incf m)
>       (setf (slot-value a 'm_num) m)))
> is inelegant but the fixnum addition should be fast.
>
> Cheers,
>
> Christophe
>