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

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