RFR(S): 8139046: Compiler Control: IVGPrintLevel directive should set PrintIdealGraph

[Liu, Xin]

Hi, Nils, 

Thank you to look into the webrev.
  
  Can you change the text for -1 to "printing is disabled" - because that
  is the practical difference between 0 and -1.

Got it. here is the revision change the description. 
http://cr.openjdk.java.net/~xliu/8139046/03/webrev/src/hotspot/share/opto/c2_globals.hpp.udiff.html

  For a future fix I would suggest creating the printer on demand. Then
  the -1 value wouldn't be needed anymore.

I don't quite understand here.  This patch does create the printer on demand. 
A compiler thread only creates the instance of printer when should_print(level) returns true. PrintIdealGraphLevel=-1 roles a global switch to disable IdealGraph dumping. 

I think in this way. JDK-8139046 can also be treated as a feature, right? Previously, we can use PrintIdealGraph as a global switch. Without it, c2 just ignore whatever you write in directives.
After c2 picks up IGVPrintLevel directive automatically, hotspot loses that feature.  -XX:PrintIdealGraphLevel=-1 and its synonym -XX:-PrintIdealGraph serve as the global switch. 

Thanks,
--lx