RFR(M): 8136445: Performance issue with Nashorn and C2's global code motion

[Doerr, Martin]

Hi,

I have implemented a change which makes Node_Backward_Iterator more efficient for large graphs. The purpose is to fix the performance problem we observe in Octane benchmarks.
It lowers compile time dramatically in case JvmtiExport::_can_access_local_variables is on.

The webrev is here:
http://cr.openjdk.java.net/~mdoerr/8136445_c2_gcm/webrev.00/

Please review.

The previous version uses an initial node stack size of (C->unique() >> 1) + 16 which can become pretty large.
My webrev changes it to (C->unique() >> 2) + 16 which is still large. I didn't observe resizing because it was too small.
I guess the stack depth typically stays far below this value, but it may be ok to spend e.g. 0.5 MB in extreme cases.

How was that previous value determined? Should I implement it differently?

Best regards,
  Martin

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