[pro] Looking for Sign-extend elegance

[David McClain]

I have a 64-bit integer obtained by the FLI from external storage. It was obtained by reading two 32-bit unsigned values and depositing them into a zero valued integer. Now I want to sign-extend the result so that if it has its MSB set, the value will be a twos-complement negative value.

(defun read-int64 (ptr)
  (let ((v (read-uint64 ptr)))
    (if (logbitp 63 v)
        (- v #.(ash 1 64))
      v)))

But this looks inelegant to me, requiring the storage of a constant #.(ash 1 64) used in a subtraction operation.

Been racking my brain on the BOOLE operations and the LOGNOT et al, looking for a more elegant solution to this. Not a high-value item, just curious.

Dr. David McClain
Chief Technical Officer
Refined Audiometrics Laboratory
4391 N. Camino Ferreo
Tucson, AZ  85750

email: dbm at refined-audiometrics.com
phone: 1.520.390.3995
web: http://www.refined-audiometrics.com



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