Document why %unary-ftruncate multiplies the result by x when result =

;; if x is known to be of the right type.  Also, if the result is

;; known to fit in the same range as a (signed-byte 32), convert this

;; to %unary-truncate, which might be a single instruction, and float

;; the result.  However, for sparc, we have a vop to do this so call

;; that, and for Sparc V9, we can actually handle a 64-bit integer

;; range.

;; the result.  However, for sparc and x86, we have a vop to do this

;; so call that, and for Sparc V9, we can actually handle a 64-bit

;; integer range.

(macrolet ((frob (ftype func)

	     `(deftransform %unary-ftruncate ((x) (,ftype))

	       (let* ((x-type (continuation-type x))

			  (< hi limit-hi))

		     #-(or sparc (and x86 sse2))

		     '(let ((result (coerce (%unary-truncate x) ',ftype)))

		       ;; Multiply by x when result is 0 so that we

		       ;; get the correct signed zero to match what

		       ;; ftruncate in float.lisp would return.

		       (if (zerop result)

			   (* result x)

			   result))