04.28.06

NCONC woo!

Posted in Uncategorized at 5:11 pm by UnwashedMeme

slme-profile-package had shown that practically all of the running time was spent in one function that did a lot of list building, and that this function was consing a LOT of memory. I tried to change the appends to nconcs, and after a bit of fiddling the profiling reported that the function was only consing about half as much memory as before. The program also runs a nice bit faster.

The problem I had to fix was that I had some list literals in code, that nconc–true to its nature– was destructively modifying. When recreating a minimal case here SBCL is nice enough to warn me about it, but I don’t recall seeing this when originally solving the problem. There was a lot of other code inbetween that may have been breaking its inferencing though. Another thing to notice is sbcl decided that the length of a constant list was constant=3, except in this case the list isn’t as constant as it appears to be. :-)

CL-USER> (defun foo (a)
	   (let ((b '(1 2 3)))
	     (values (length b)
		     (nconc  b a))))

; in: LAMBDA NIL
;     (NCONC B A)
;
; caught WARNING:
;   Destructive function NCONC called on constant data.
;   See also:
;     The ANSI Standard, Special Operator QUOTE
;     The ANSI Standard, Section 3.2.2.3
;
; compilation unit finished
;   caught 1 WARNING condition
STYLE-WARNING: redefining FOO in DEFUN
FOO
CL-USER> (foo '(bam baz))
3
(1 2 3 BAM BAZ)
CL-USER> (foo '(bam baz))
3
(1 2 3 BAM BAZ BAM BAZ)
CL-USER> (foo '(bam baz bad))
3
(1 2 3 BAM BAZ BAM BAZ BAM BAZ BAD)
CL-USER> (defun foo (a)
  (let ((b (list 1 2 3)))
    (values (length b)
	    (nconc  b a))))

STYLE-WARNING: redefining FOO in DEFUN
FOO
CL-USER> (foo '(bam baz bad))
3
(1 2 3 BAM BAZ BAD)
CL-USER> (foo '(bam baz bad))
3
(1 2 3 BAM BAZ BAD)

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