🚀 go-pugleaf

MCL's shiftf macro produces unneccesary complicated form in the simple
cases.  The most frequent use of shift, is probably for simple pointer
switching in list structures: Replacing a value while saving the old
value.  (The two-argument (place, newvalue) special case of shiftf used
to have its own name, swapf)

Here's what MCL's shiftf produces:
? (macroexpand '(shiftf (cddr liste) nil))
(LET* ((#:G91 LISTE) 
       (#:G89 (MULTIPLE-VALUE-LIST (CDDR #:G91)))) 
  (DECLARE (DYNAMIC-EXTENT #:G89)) 
  (MULTIPLE-VALUE-BIND (#:G90) NIL 
    (PROGN (CCL::SET-CDDR #:G91 #:G90))) 
  (VALUES-LIST #:G89))

Here's a simpler expansion suggestion (Allegro 4.1 produces something
similar to this):
(LET* ((#:G91 LISTE) 
       (#:G89 (CDDR #:G91))
       (#:G90 NIL))
  (CCL::SET-CDDR #:G91 #:G90)
  #:G89)

The second form has an execution time of appr. 1/3 of the first form,
which may be quite significant for pointer-manipulating programs.

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Espen J. Vestre,                          espen@coli.uni-sb.de
Universitaet des Saarlandes,        
Computerlinguistik, Gebaeude 17.2 
Im Stadtwald,                          tel. +49 (681) 302 4501
D-6600 SAARBRUECKEN, Germany           fax. +49 (681) 302 4351
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