RFR(S): 6378256: Performance problem with System.identityHashCode in client compiler

[Rahul Raghavan]

Hello, 

Please review the following patch for JDK-6378256.

webrev: http://cr.openjdk.java.net/~thartmann/6378256/webrev.00/ .

Bug: https://bugs.openjdk.java.net/browse/JDK-6378256  .
Performance problem with System.identityHashCode, compared to Object.hashCode, with client compiler (at least seven times slower).
Issue reproducible for x86_32, SPARC (with -client / -XX:TieredStopAtLevel=1 , 2, 3 options).

sample unit test:
   public class Jdk6378256Test
   {
      public static void main(String[] args) 
      {
         Object obj = new Object();
         long time = System.nanoTime();
         for(int i = 0 ; i < 1000000 ; i++)
            System.identityHashCode(obj);  //compare to obj.hashCode();
         System.out.println ("Result = " + (System.nanoTime() - time));
      }
   }

Fix: Enabled the C1 optimization which was done only for Object.hashCode, now for System.identityHashCode() also.
(looks in the header for the hashCode before calling into the VM).
Unlike for Object.hashCode, System.identityHashCode is static method and gets object as argument instead of the receiver.
So also added required additional null check for System.identityHashCode case.

Testing:
   - successful JPRT run (-testset hotspot).
   - JTREG testing (hotspot/test, jdk/test - java/util, java/io, java/lang/System).
       (with -client / -XX:TieredStopAtLevel=1 etc. options).
   - Added 'noreg-perf' label for this performance bug.
      Manual testing done and confirmed expected performance values for unit tests with fix.

Thanks,
Rahul