[Sbcl-help] dynamic-extent for results from bit-vector operations

[Alexander K. <as...@pv...>]

I looked at the BITVECTOR benchmark in cl-bench in order to see if it 
could easily be optimized.  Unfortunately I saw that 90% of the time 
seems to be spent on cache-misses(?) due to the result-array of the 
BIT-AND operation not having dynamic extend.

Introducing the dummy variable in the code below makes it roughly 10x 
faster.

The code from cl-bench:
(locally
     (declare (optimize (speed 3) (safety 0) (debug 0)))
   (defun bench-bitvectors (&optional (size 1000000) (runs 700))
     (declare (fixnum size))
     (let ((zeros (make-array size :element-type 'bit :initial-element 0))
	  (ones (make-array size :element-type 'bit :initial-element 1))
	  (xors (make-array size :element-type 'bit)))
       (dotimes (runs runs)
	(bit-xor zeros ones xors)
	(bit-nand zeros ones xors)
	(let ((dummy (make-array size :element-type 'bit)))
	  (declare (dynamic-extent dummy))
	  (bit-and zeros xors dummy))
	#+nil (bit-and zeros xors)))
     (values)))

Now, since BIT-AND is in the COMMON-LISP package and thus can not be 
redefined, would it be possible to detect that the result of BIT-AND is 
not used and to allocate the result array using dynamic extent?

astor