JEP draft: [Lilliput] Remove mark word from objects

Investigate downsizing average object size by removing (or downsizing) the mark word from the oopDesc layout.

DRAFT DRAFT DRAFT

The 64-bit mark word can be used for several purposes:

• GC state during a GC, such as a forwarding pointer
• lock state (not during a GC), including unlocked, thin-locked, biased, and inflated monitor
• object annotations by GC (a few "age" bits)
• storage for the System.identityHashCode
• a tag to distinguish object instances from value instances

There are well-known alternative representations for all of these mechanisms which do not use the 64-bit object header.

For example:

• GC state can be stored in a side array
• lock state can be stored in a side array and/or in a word preceding the object
• age annotations and "value tags" can be inserted into other "waste" space of the object, such as the high bits of the klass field (if it is present)
• hash codes can be stored in a word before the object, and/or be derived from the object's address in new-space

If we could remove mark words from all objects, the density of the heap would go up by a significant factor, basically (1+8/S), where S is the average object size; since S is typically something like 30-50 bytes (including arrays, which may be quite large), the overhead of mark words can often be a double-digit percentage.