fastdebug builds performance is important to minimize testing time. In GC tests, we have a big hotspot at this line in oop::size_given_klass():
  assert(s % MinObjAlignment == 0, "Oop size is not properly aligned: %d", s);

The hotspot is because of the integer division. Alas, the divisor is not known at compile time, because it can be set from the JVM command line, so compiler cannot optimize it. There is an easy fix, use the MinObjectAlignmentMask to assert:
 http://cr.openjdk.java.net/~shade/8184753/webrev.01