This bug can potentially crash an AArch64 JVM when a method has a suitably large enough i/vtable.
The fix corrects address generation so that it copes with offsets that overflow teh available immediate offset range.
The risk is low because this only changes code generated for i/vtable lookups and only makes a difference in the case where when i/vtables have large indices (a case whihc is already broken). This circumstance has not actually been known to affect production code but it is a potential source of error.
It has been reviewed by Andrew Haley.
webrev (for jdk10 and jdkdev) is here: