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

Vladimir Kozlov vladimir.kozlov at oracle.com
Fri Jan 29 00:34:28 UTC 2016

First, it was Igor's suggestion about match_rule_supported().

Why you check intrinsic in inline_native_Class_query() ?

I don't see the match_rule_supported() check for this intrinsic (c2compiler.cpp).

I think you should consider to implement this for C1 and Interpreter since Tiered Compilation is on by default. So that 
Client VM can benefit too.

Change test:

1. Don't use /othervm since you fork separate process and don't use flags.

2. Don't use "-server" flag - Client VM could be tested which does not have server.

3. Don't use -Xcomp - it will timeout on slow machines. Create separate test() method to be compiled:

          public static void main(final String[] args) throws Exception {
              int end = 20_000;

              for (int i=0; i < end; i++) {

          static void test() {


On 1/28/16 6:51 AM, Ivan Krylov wrote:
> Hi Igor,
> Following Vladimir's suggestion I eliminated the UseOnSpinWaitIntrinsic flag altogether. I have adopted the
> Matcher::match_rule_supported() logic - seems to work on intel, but I don't have any non-intel box to test.
> Anyway, the new webrev:
> http://cr.openjdk.java.net/~ikrylov/8147844.hs.01/
> Igor, Vladimir, thanks,
> Ivan
> On 27/01/2016 22:03, Igor Veresov wrote:
>> 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

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