+ (round (* exponent
+ ;; this is the closest double float
+ ;; to (log 2 10), but expressed so
+ ;; that we're not vulnerable to the
+ ;; host lisp's interpretation of
+ ;; arithmetic. (FIXME: it turns
+ ;; out that sbcl itself is off by 1
+ ;; ulp in this value, which is a
+ ;; little unfortunate.)
+ (load-time-value
+ #!-long-float
+ (sb!kernel:make-double-float 1070810131 1352628735)
+ #!+long-float
+ (error "(log 2 10) not computed")))))))