There are a few bugs in JDK-8338912 that we discovered in Leyden that uses multiple segments: a) root_index in ArchiveHeapWriter::copy_roots_to_buffer is initialized to zero for every segment, so all segments have the same set of roots; b) HeapShared::get_root and friends compute segment and internal indices modulo _byte_ size, while they should instead do it modulo _element_ size.

This does not affect current mainline directly, since we run with a single segment the overwhelming majority of the time.