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

[Igor Veresov]

Actually, I’d rather use Matcher::match_rule_supported() to test if it’s supported on the platform, rather than fixing all vm_version_*.* to check for the flag validity, that’s tedious (you forgot x86-32 and there’s going to be more platforms to fix for you sponsor). Something like UseOnSpinWaitIntrinsic && Matcher::match_rule_supported(Op_OnSpinWait) to decide whether or not to inline the intrinsic. Also, why are you not turning it on by default?

igor

> On Jan 27, 2016, at 4:48 AM, Ivan Krylov <ivan at azulsystems.com> wrote:
> 
> Looks like there was some good discussion while I was peacefully sleeping.
> I don't have much to add. This patch was somewhat inspired by JEP-171 changes. 
> Perhaps,there are other ways to achieve the same semantics.
> 
> So, if we can consider this reviewed - I will wait for the actual JEP to become targeted to 9 and then seek a sponsor to do the push.
> 
> Thanks,
> 
> Ivan
> 
> On 27/01/2016 09:12, Igor Veresov wrote:
>> I realize it’s not a big deal. I was just wondering if there was any specific reason control alone is not enough.
>> Anyways, looks ok for the first cut.
>> 
>> igor
>> 
>>> On Jan 26, 2016, at 9:24 PM, Gil Tene <gil at azul.com> wrote:
>>> 
>>> Since a sensical loop that calls onSpinWait() would include at least a volatile load on every iteration (and possibly a volatile store), the new node does not create significant extra move restrictions that are not already there. Modeling this with a memory effect is one simple way to prevent it from being re-ordered out of the loop. There are probably other ways to achieve this, but this one doesn't really have a performance downside…
>>> 
>>> — Gil.
>>> 
>>>> On Jan 26, 2016, at 4:44 PM, Igor Veresov <igor.veresov at oracle.com> wrote:
>>>> 
>>>> So, why does the new node have a memory effect? That would seem to prevent any movement of the subsequent loads in your loop, right? If that’s intentional I wonder why is that?
>>>> 
>>>> igor
>>>> 
>>>>> On Jan 26, 2016, at 2:59 AM, Ivan Krylov <ivan at azulsystems.com> 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
>>>> 
>>> 
>> 
>