G1 requires a SATB read barrier when reading from potentially weak tables such as the string interning table. Currently the code enqueues a reference on every String.intern operation, when it is really only needed when the intern operation finds a previously interned string. Adding a new string to the table should not require a SATB read barrier.
This code may contribute to the test failure JDK-8134992 "vm/gc/compact/Compact_InternedStrings_Strings failed due to a malloc() failure"