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  1. JDK
  2. JDK-8051805

C2: MachTemp nodes not always in same block as their "use" node

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    • Subcomponent:
    • CPU:
      generic
    • OS:
      generic

      Description

      In PhaseCFG::schedule_late(), there is code to keep MachTemp nodes in the same
      block as their use, and in PhaseCFG::select() there is more code to keep MachTemp
      nodes near their use. However, the MachTemp can still end up in a different block
      from it use (call_catch_cleanup() is one function that can cause this.)

      Since a MachTemp node doesn't actually emit any code, the register allocation for
      it hasn't actually been initialized until the use node. Normally this is harmless,
      but if the MachTemp is RegN, then any safepoint between the MachTemp and use
      will add that RegN to the safepoint oopmap, which is fatal because the value is
      uninitialized. Even if the MachTemp is not a RegN, having the MachTemp far away
      from its use unnecessarily extends the live range of the register.

      A possible fix for the general case would be to make sure that MachTemp
      nodes are always placed just above their use in the same block.

      A workaround for just the RegN case could be to treat RegN as RegI for MachTemp nodes.

      To reproduce this problem, add a RegN TEMP to a rule like LoadN, CompareAndSwapN,
      or GetAndSetN. If the safepoint is a call, then it needs to be a platform that has
      "never-save" (NS) registers that can stay live across a call. But if the safepoint is a poll,
      then probably any platform would be affected.

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              Assignee:
              rbackman Rickard Backman
              Reporter:
              dlong Dean Long
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                Created:
                Updated: