Immix GC straddling object optimisation

[wenyuzhao]

Add OpenJDK fast path code to log straddling objects. Do this after Immix PR is merged.

[qinsoon]

In Java MMTk, if an object is straddling lines, we set a bit for the object to know that it is a straddling object. And when we mark lines, we do this:

mark_one_line
if straddling
  mark_multiple_lines

In our current code, we always do mark_multiple_lines which always calls get_current_size() for the object.

[qinsoon]

Steve mentioned that straddling object was an important optimisation in the original implementation. @wenyuzhao Did you implement this in LXR?

[wenyuzhao]

No this is not implemented in both LXR and Immix. The only problem is that it needs a post-alloc "barrier" or hook to initialize the straddle bit for each object. We don't have the hook on openjdk.

[qinsoon]

We are generating code for post-alloc to set VO bit: https://github.com/mmtk/mmtk-openjdk/blob/71969f34d61e6662bbb7391513426d41431bb1a0/openjdk/mmtkBarrierSetC2.cpp#L279

We can set the straddling bit in the same way, right?

[wenyuzhao]

We are generating code for post-alloc to set VO bit: https://github.com/mmtk/mmtk-openjdk/blob/71969f34d61e6662bbb7391513426d41431bb1a0/openjdk/mmtkBarrierSetC2.cpp#L279

We can set the straddling bit in the same way, right?

Yes it's possible.