[Sbcl-help] system-area-pointer: compiler note

[Jerry J. <log...@gm...>]

I'm working my way through the compiler notes on a small project,
trying to understand what each one means (and fix those that are
fixable).  I've encountered some notes relating to the use of the
system-area-pointer type, which is used to interface with some C code.
 I can distill my question down to this code:

(proclaim '(optimize (debug 0) (safety 0) (space 1) (speed 3)
	    (compilation-speed 3)))

;; Gets a compiler note about SAP to pointer coercion
(defun sap-test1 (ptr fun)
  (declare (type system-area-pointer ptr)
	   (type (function (system-area-pointer) fixnum) fun))
  (funcall fun (sb-sys:sap+ ptr 10)))

;; Doesn't get a compiler note
(defun sap-test2 (ptr fun)
  (declare (type system-area-pointer ptr)
	   (type (function (system-area-pointer) fixnum) fun))
  (funcall fun ptr))

which produces this output when compiled:

; compiling file "/tmp/test.lisp" (written 23 JUN 2010 07:48:20 PM):
; compiling (PROCLAIM (QUOTE #))
; compiling (DEFUN SAP-TEST1 ...)
; file: /tmp/test.lisp
; in: DEFUN SAP-TEST1
;     (FUNCALL FUN (SB-SYS:SAP+ PTR 10))
; ==>
;   (SB-C::%FUNCALL (SB-KERNEL:%COERCE-CALLABLE-TO-FUN FUN)
(SB-SYS:SAP+ PTR 10))
;
; note: doing SAP to pointer coercion (cost 20)

; compiling (DEFUN SAP-TEST2 ...);
; compilation unit finished
;   printed 1 note


; /tmp/test.fasl written
; compilation finished in 0:00:00.003

Why is the SAP to pointer coercion taking place in the first version?  Thanks,
-- 
Jerry James
http://www.jamezone.org/