Back-Branch Polling is for yield points insertion at back branches of certain kinds of loops to enable safe thread suspension. Several approaches are implemented there:

• 0 - the feature is turned off
• 1 - insert interruptable subgraph at all backedges except those going from initiallyInterruptable nodes
  (initialInterruptability is not being propagated to the successors. This is the feature of version 5)

• 2 - path analysis based on searching of pairs \[isOnThreadInterruptablePath\]\->\[!isOnThreadInterruptablePath\]

• 3 - recursive version of "2"

• 4 - "1" + suspension flag addr \[TLS base + offset\] is calculated before the loop header

• 5 - like "1" but some backedges are not patched (if all paths through it are uninteruptable)
• 6 - "4" + "5"
  
  

They can be switched using the following: jit.arg.bbp=[0-6]

Possible improvement

It may have sence to try BBPolling at earlier stages - in HLO. To clarify one need to implemet HLO pass for this and find the best position (from the performance POV) for it in the HLO path.

implementation file: http://svn.apache.org/viewvc/harmony/enhanced/drlvm/trunk/vm/jitrino/src/codegenerator/ia32/Ia32BBPolling.cpp