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

[Rahul Raghavan]

Hello,

Please review the following revised patch for JDK-6378256 -
http://cr.openjdk.java.net/~thartmann/6378256/webrev.01/

This revised webrev got following changes -

 1) A minor, better optimized code with return 0 at initial stage (instead of continuing to 'slowCase' path), for special/rare null reference input!
   (as per documentation, test results confirmed it is safe to 'return 0' for null reference input, for System.identityHashCode)
 
 2) Added similar Object.hashCode, System.identityHashCode optimization support in sharedRuntime_x86_64.cpp.

Confirmed no issues with jprt testing (-testset hotspot) and expected results for unit tests.

Thanks,
Rahul


> -----Original Message-----
> From: Roland Westrelin > Sent: Wednesday, December 09, 2015 8:03 PM > To: Rahul Raghavan> Cc: hotspot-compiler-dev at openjdk.java.net
> 
> > webrev: http://cr.openjdk.java.net/~thartmann/6378256/webrev.00/ .
> 
> Justifying the comment lines 2019-2022 in sharedRuntime_sparc.cpp (lines 1743-1746 in sharedRuntime_x86_32.cpp) again would be
> nice.
> Shouldn't we use this as an opportunity to add the same optimization to sharedRuntime_x86_64.cpp?
> 
> Roland.


> -----Original Message-----
> From: Rahul Raghavan > Sent: Wednesday, December 09, 2015 2:43 PM > To: hotspot-compiler-dev at openjdk.java.net
> 
> 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