Comment 1 for bug 1894057

It's almost there:

(let* ((addr (cffi:foreign-alloc :int32 :initial-element 1234))
                (starting-value (cffi:mem-ref addr :int32)))
           (print starting-value)
           (sb-int::%primitive sb-vm::signed-sap-cas-32 (cffi:mem-aptr addr :int32) 0
                               starting-value (random 32))
           (print (cffi:mem-ref addr :int32)))