(defun foo(z)
   (declare (optimize (speed 3)(safety 0))
	    (type (simple-array single-float (*)) z) (inline aref))
	     (setf (aref z 0)(+ (aref z 1)(aref z 2))) 
	     t 
	     ;; can't return a float without putting it in a box...
	     ;; so we return t instead
	     )

This takes 80 bytes, and no "newfloats"...

[4] USER(75): (disassemble 'foo)
;; disassembly of #<Function FOO @ #xe6c0b6>
;; formals: 

;; code vector @ #xe6bf0c:
   0:	save	#x-88,%o6
   4:	sethi	%o7,#x203	; #x80c00, 527360
   8:	or	#x1e7,%o7
  12:	move.l	%o7,-12(%i6)
  16:	or	#x8,%o7
  20:	move.l	%o7,-16(%i6)
  24:	sethi	%o7,#x203	; #x80c00, 527360
  28:	or	#x2b7,%o7
  32:	move.l	%o7,-40(%i6)
  36:	or	#x8,%o7
  40:	move.l	%o7,-36(%i6)
  44:	move.s	2(%i0),%f28
  48:	move.s	6(%i0),%f30
  52:	ADD.S	%f30,%f28,%f30
  56:	move.s	%f30,-2(%i0)
  60:	move.l	-37(%g4),%o0	; T
  64:	move.l	#x1,%g3
  68:	move.l	%g0,-12(%i6)
  72:	jmpl	8(%i7),%g0
  76:	restore	%g0,%o0