Electrical Fire Design Runtime Monitors and Hash Codes 64-Bit Implementation

On a 64-bit address machine we put an object's monitor and its hash code into a single doubleword -- the object's subheader at offset 8 in the object. Figure 1 shows the placement of the subheader in an object. Figure 2 shows the layout of the bits in the subheader.

Figure 1

(a) Subheader bits
(b) Hash code states
(c) Monitor states
Figure 2

Unlike the 32-bit case, the hash code bits and monitor bits are separate and independent in a 64-bit subheader.

A subheader s's hash code bits can be in one of the two states below. There is no extended hash code pool.

Computed Hash	The object's hash value is the given signed 32-bit integer: s>>32.
Uncomputed Hash	The object's hash value has not been computed yet.

A subheader s's monitor bits can be in one of the three states below. A thread ID is some unique 30-bit identifier of a thread.

Unlocked	The object is unlocked.
Uncontended Locked	The object is locked. Bits 2 through 31 of s contain the ID of the thread that locked it. No other thread is waiting for the object's lock.
Contended Locked	The object is locked. Bits 2 through 31 of s contain the ID of the thread that locked it. Other threads may be waiting for the object's lock.

The procedures for entering and exiting a monitor and reading and writing hash codes are similar to their 32-bit brethren but simpler because the hash code bits and monitor bits are separate and independent. Instead of saving the subheader in a function's stack frame, MonitorEnter needs only save a flag that states whether the monitor was recursively entered.