Turns out that I had the optimize declaration nested one level too deep.
Bringing it out fixed the problem.

--shiv--


   >Received: from turtle.franz.com
	   by balrog (SMI-8.6/ECE.UCSB-v1.5)
	   id QAA15342; Mon, 8 Mar 1999 16:37:49 -0800
   Date: Mon, 8 Mar 1999 12:38:15 -0800
   From: Shiv <balrog at shiv>
   Content-Type: text
   Content-Length: 2554


   I am having trouble getting the compiler not to box a single-float.
   The compiler of both ACL 4.3.1 and 5.0 (Solaris) give the same
   'problem'.

   I have condensed the situation into this simple function:

   USER(26): (defun foo (a)
	       (let ((C (make-array 2 :element-type 'single-float
				    :initial-element 2.0)))
		 (declare (optimize (speed 3) (safety 0) (debug 0))
			  (single-float a)
			  (type (simple-array single-float (*)) C)
			  (:explain :boxing :types :calls))
		 (incf (aref C 0) (* (the short-float a) 2.0))
		 C))
   FOO

   Here is what I get when I compile it:

   USER(27): (compile *)
   ;Examining a (possibly unboxed) call to +_2OP with arguments:
   ;  call to AREF type (SINGLE-FLOAT * *)
   ;  call to *_2OP type (SINGLE-FLOAT * *)
   ; which returns a value of type (SINGLE-FLOAT * *)
   ;Examining a (possibly unboxed) call to AREF with arguments:
   ;  symeval G2772 type (SIMPLE-ARRAY (SINGLE-FLOAT * *) (*))
   ;  constant 0 type (INTEGER 0 0)
   ; which returns a value of type (SINGLE-FLOAT * *)
   ;Examining a (possibly unboxed) call to *_2OP with arguments:
   ;  symeval A type (SINGLE-FLOAT * *)
   ;  constant 2.0 type (SINGLE-FLOAT 2.0 2.0)
   ; which returns a value of type (SINGLE-FLOAT * *)
   ;Generating a SINGLE-FLOAT box  <----- !!!!! shiv !!!!!!!!!!! <--------
   ;Examining a (possibly unboxed) call to .INV-S-AREF with arguments:
   ;  symeval G2773 type (SINGLE-FLOAT * *)
   ;  symeval G2772 type (SIMPLE-ARRAY (SINGLE-FLOAT * *) (*))
   ;  constant 0 type (INTEGER 0 0)
   ; which returns a value of type (SINGLE-FLOAT * *)
   ;Examining a call to .INV-S-AREF with arguments:
   ;  symeval G2773 type (SINGLE-FLOAT * *)
   ;  symeval G2772 type (SIMPLE-ARRAY (SINGLE-FLOAT * *) (*))
   ;  constant 0 type (INTEGER 0 0)
   ; which returns a value of type (SINGLE-FLOAT * *)
   FOO
   NIL
   NIL


   I have marked the line (with shiv) where it warns that it is
   generating a boxed single-float.  As can be seen the declarations are
   enough for the compiler to identify all intermediate quantities as
   single-float.  I guess the problem is that the compiler is not
   intelligent enough to see that the + operation in incf generates
   output that although ultimately ends up in the output of the function,
   can still be unboxed since it is inside a simple-array which contains
   unboxed single-floats (phew!).  I have tried writing to a temporary
   variable first before writing to the output array but with no luck.

   Any advice on how to work around this will be appreciated.

   Thanks,

   --shiv--

   Sanity check: this operation (not foo) occurs deep inside a nested
   loop in my code.