RFR(S): 8147844: new method j.l.Runtime.onSpinWait() and the corresponding x86 hotspot instrinsic

[Ivan Krylov]

Hello,

Some of you may have a seen a few e-mails on the core-libs alias about a 
proposed “spin wait hint”. The JEP is forming up nicely at 
https://bugs.openjdk.java.net/browse/JDK-8147832. There seems to be a 
consensus on the API side. It is now in a draft state and I hope this 
JEP will get targeted for java 9 shortly.  The upcoming API changes can 
be seen at the webrev:
http://cr.openjdk.java.net/~ikrylov/8147844.jdk.00/

At this time I would like to ask for a review of the hs-comp changes. 
The plan is push changes into class libraries and hotspot synchronously 
but that may happen after the JEP gets targeted.

Bug: https://bugs.openjdk.java.net/browse/JDK-8147844
Webrev: http://cr.openjdk.java.net/~ikrylov/8147844.hs.00/

The idea of the fix is pretty simple: hotspot replaces a call to 
java.lang.Runtime.onSpinWait() with an intrinsic that is effectively a 
'pause' instruction on x86.  This intrinsic is guarded by the 
-XX:±UseOnSpinWaitIntrinsic flag. For non-x86 platforms there is a 
verification code that makes sure the flag is off, VM will just execute 
at empty method java.lang.Runtime.onSpinWait() – effectively a no-op. 
According the [1] the 'pause' instruction is functional since SSE2, but 
even on CPUs prior to SSE2 the  'pause' instruction is a no-op and hence 
harmless, there seems to be no need to add guarding code for older 
generations of Intel CPUs.

The proposed patch includes a simple regression test that simply makes 
sure that method java.lang.Runtime.onSpinWait() gets intrinsified.  
There are several other producer-consumer-like performance tests ready 
that the authors of this JEP would be happy to make available under 
JEP-230 but I am uncertain about the process.

Thanks,

Ivan

[1]  - 
https://software.intel.com/en-us/articles/benefitting-power-and-performance-sleep-loops 

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