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

[Christian Thalinger]

> On Dec 9, 2015, at 1:30 PM, Dean Long <dean.long at oracle.com> wrote:
> 
> The new System.identityHashCode optimization can't be turned off on the command-line,
> because InlineObjectHash only applies to Object.hashCode.  Does it matter?

All these command line flags are a pain since they are all product flags.  Doesn’t Compiler Control provide a way to disable an intrinsic?

Actually, InlineObjectHash is a develop flag so you can’t turn it off in a release build:

src/share/vm/runtime/globals.hpp
806:  develop(bool, InlineObjectHash, true,                                     \

> 
> dl
> 
> On 12/9/2015 1:12 AM, Rahul Raghavan wrote:
>> 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
>