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

[Vladimir Ivanov]

Ivan,

There's no need in yet another flag (-XX:±UseOnSpinWaitIntrinsic). 
-XX:DisableIntrinsic=_onSpinWait should do the same.

Best regards,
Vladimir Ivanov

On 1/26/16 1:59 PM, Ivan Krylov wrote:
> 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
>