Save 10-20% Memory With Compact Headers

JEP 450 proposes compact object headers as an experimental HotSpot JVM feature that reduces object header size from 128 bits to 64 bits on 64-bit systems. This is achieved by merging the compressed class pointer into the mark word, eliminating the separate class word. The change requires significant modifications to locking (replacing stack locking with a thread-local protocol), garbage collection (new self-forwarding bit, compressed forwarding pointers), and hashing (identity hash code reduced from 31 to 25 bits). Early real-world testing confirms 10–20% heap memory savings. Project Lilliput's longer-term goal is to shrink headers further to 32 bits.