RFR(s): 8152160: SIGFPE in CompactibleFreeListSpaceLAB::compute_desired_plab_size

[sangheon]

Hi all,

Could I have a couple of reviews for this change?

With large value of CMSOldPLABNumRefills, SIGFPE would happen at 
CompactibleFreeListSpaceLAB::compute_desired_plab_size() because related 
routine would be zero by an overflow.

Type-casting to 'double' before the problematic division will resolve 
the problem.

CR: https://bugs.openjdk.java.net/browse/JDK-8152160
Webrev: http://cr.openjdk.java.net/~sangheki/8152160/webrev.00
Testing: JPRT, TestOptionsWithRanges.java via RBT and manual test as CR 
description.

Thanks,
Sangheon