From Camm Maguire:
Our lisp spec expert tells me that routines returning floating point
values like (log10...) are not required to return exact integers in
cases like these. 

What you want is something like this:

(defun logi (x y &aux i r)
        (declare (longfloat x y r))
        (declare (fixnum i))
        (multiple-value-bind (i r) (round (log x y))
                i))

In some versions of GCL the LOG10 function returns improperly rounded values.
The symptom is:
\begin{verbatim}
(24) -> [1000]
   (24)  [100]
\end{verbatim}
The common lisp failure can be shown with:

(25) -> )lisp (log10 1000)
Value = 2.9999999999999996

This previous boot code was:

    u < MOST_-POSITIVE_-LONG_-FLOAT => 1+negative+FLOOR LOG10 u
and should be restored when the GCL bug is fixed.
    u < MOST_-POSITIVE_-LONG_-FLOAT => 1+negative+FLOOR ((LOG10 u) + 0.0000001)