get-setf-expansion of CxxR functions is nonconformant

Bug #1450968 reported by Douglas Katzman on 2015-05-02
This bug affects 1 person
Affects Status Importance Assigned to Milestone
Douglas Katzman

Bug Description


"This place ... Is equivalent to this place ...
(caar x) (car (car x))
(cadr x) (car (cdr x))
..." and so on.

A simple counterexample to the stipulated "equivalence" of places:

* (defun g (x) (rplaca x (cons 111 222)) 100)
* (defun h (x) (rplaca x nil) 10)

* (defun bar (selector)
  (let* ((inner (cons 5 6))
         (cell (list inner)))
    (format t "Cell=~S~%" cell)
    (ecase selector
      (:car-car (incf (car (car cell)) (g cell)))
      (:caar (incf (caar cell) (g cell)))
      (:cdr-car (incf (cdr (car cell)) (h cell)))
      (:cdar (incf (cdar cell) (h cell))))
    (format t "Cell=~S, inner=~S~%" cell inner)

* (bar :car-car)
Cell=((5 . 6))
Cell=((111 . 222)), inner=(105 . 6)
* (bar :caar)
Cell=((5 . 6))
Cell=((211 . 222)), inner=(5 . 6)
* (bar :cdr-car)
Cell=((5 . 6))
Cell=(NIL), inner=(5 . 16)
* (bar :cdar)
Cell=((5 . 6))
debugger invoked on a SIMPLE-TYPE-ERROR in thread
#<THREAD "main thread" RUNNING {1002D5ECB3}>:
  Argument X is not a NUMBER: NIL

The problem is that the setf "getter" for CAAR returns CAAR itself instead of CAR of a temp as required by equivalence to the exploded form of CAR/CDR compositions.
* (macroexpand-1 '(incf (caar x)))
(LET* ((#:G536 X) (#:G537 (+ (CAAR #:G536) 1)))
  (SB-KERNEL:%RPLACA (CAR #:G536) #:G537))

Same is true of FIRST,SECOND,etc and of all R/M/W macros that use get-setf-expansion.

Also I suspect that while our expansion of (PUSHNEW X (NTH I L)) is not non-conformant, it would be a lot nicer if if expanded to something like:
  (let ((cell (nthcdr index list))) (pushnew item (car cell))))
instead of something like:
  (%setnth ... (adjoin ... (nth )))

Douglas Katzman (dougk) on 2015-05-02
Changed in sbcl:
assignee: nobody → Douglas Katzman (dougk)
status: New → In Progress
Douglas Katzman (dougk) on 2015-05-02
Changed in sbcl:
status: In Progress → Fix Committed
Changed in sbcl:
status: Fix Committed → Fix Released
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