[External] : Re: RFC - Improving C2 Escape Analysis

Thu Nov 11 19:24:30 UTC 2021

Hi Vladimir,

Thank you for the feedback and sorry for the delay in getting back to you!

> Yes, finding solution for allocation merges (or NULL) is a pain. I spent some
> time investigating possible solutions for it but "no cigar". May be we do
> indead need control flow analysis to resolve this.

Can you elaborate a bit on the approaches you tried and why you didn't like
them? By allocation merges do you mean nested objects like "obj1.obj2.x",
right? Did you try solving both control-flow merge issues and also allocation
merges?

> There are 2 test files with small methods for different EA cases I used to
> see how EA works:

These examples are being very helpful, thank you again!

> Yes, I think it would be good to have a prototype if you are comfortable to
> work with C2 code already.  I proposed small RFEs just for warmup ;)

I talked with my colleagues and we decided to start the work by trying to fix
the control/data-flow merge issues - *perhaps not for all cases, but at least
for some of them*. Then, based on our experience with this and some
benchmarking we'll decide if we really need flow-sensitive analysis and how to
best approach that.

We'll definitely take a look at the RFEs as we move along! Implementing Stadler
algorithm was just something that crossed my mind initially, it's very likely
the last approach we'd try ... I don't want to bite more than I can chew..

Regards,
Cesar
________________________________
From: Vladimir Kozlov <vladimir.kozlov at oracle.com>
Sent: October 29, 2021 5:27 PM
To: Cesar Soares Lucas <Divino.Cesar at microsoft.com>; Tobias Hartmann <tobias.hartmann at oracle.com>; Ron Pressler <ron.pressler at oracle.com>
Cc: John Rose <john.r.rose at oracle.com>; Mark Reinhold <mark.reinhold at oracle.com>; hotspot-dev at openjdk.java.net <hotspot-dev at openjdk.java.net>; Brian Stafford <Brian.Stafford at microsoft.com>; Martijn Verburg <Martijn.Verburg at microsoft.com>; Hohensee, Paul <hohensee at amazon.com>
Subject: Re: [External] : Re: RFC - Improving C2 Escape Analysis

On 10/29/21 4:50 PM, Cesar Soares Lucas wrote:
> Hi Vladimir and Tobias,
>
>  >> Sure, here are four examples of EA and/or scalarization failing due to
>  >> complicated control/data flow:
>  >> https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcr.openjdk.java.net%2F~thartmann%2FEA_examples&data=04%7C01%7CDivino.Cesar%40microsoft.com%7C027f2da3f2e14f914c2608d99b3c195e%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637711504731032568%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=hz4ti9lgmQeGLX%2BZ3vmSngXHHUAX%2FAvtObgeu%2Fqz1DI%3D&reserved=0
>
>  >> There are 2 test files with small methods for different EA cases I used to
>  >> see how EA works:
>  >>
>  >> test/hotspot/jtreg/compiler/escapeAnalysis/Test6726999.java
>  >> test/hotspot/jtreg/compiler/escapeAnalysis/Test6689060.java
>
> Thank you for the examples, Tobias/Vladimir. This is being very helpful.
>
>  >> Yes, finding solution for allocation merges (or NULL) is a pain. I spent
>  >> some time investigating possible solutions for it but "no cigar". May be we
>  >> do indead need control flow analysis to resolve this.
>
> By "need control flow analysis" you mean the flow-sensitive EA algorithm? My

Yes.

To clarify. I investigated solutions in current flow-insensitive EA.

> first idea to handle these control/data-merge issues was to implement in C2 the
> same algorithm used by GRAAL - i.e., the algorithm described in Stadler et. al
> PEA paper. Do you think this is reasonable?

Yes, I think it would be good to have a prototype if you are comfortable to work with C2 code already.
I proposed small RFEs just for warmup ;)

>
>  >> I am currently looking on iterative EA. Do more EA rounds if we can
>  >> eliminate more connected allocations. It was proposed by Vladimir Ivanov and
>  >> I have working prototype.
>
> Cool! I'm curious, when do you plan to submit a Pull Request for this?

I am investigating regressions in some benchmarks.

>
>  >> There is also suggestion from Amazon Java group about "C2 Partial Escape
>  >> Analysis" which needs more discussion:
>  >> https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fmail.openjdk.java.net%2Fpipermail%2Fhotspot-compiler-dev%2F2021-May%2F047486.html&data=04%7C01%7CDivino.Cesar%40microsoft.com%7C027f2da3f2e14f914c2608d99b3c195e%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637711504731032568%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=S65Ko1Jss1PRksoLs9w1Ci7lH7Cprikus4goFUXDdL4%3D&reserved=0
>
> I'd love to hear from them about their experience with these issues and if they
> have any plans to work on this moving forward! I'll ping them on the thread
> that you linked above.

Yes, I would like them to participate too (CCing to Paul). They sent proposal almost 6 months ago and we did not hear
any additional information after Vladimir Ivanov replied.

Regards,
Vladimir K

>
>
> Regards,
> Cesar
> ------------------------------------------------------------------------------------------------------------------------
> *From:* Vladimir Kozlov <vladimir.kozlov at oracle.com>
> *Sent:* October 27, 2021 10:26 AM
> *To:* Tobias Hartmann <tobias.hartmann at oracle.com>; Cesar Soares Lucas <Divino.Cesar at microsoft.com>; Ron Pressler
> <ron.pressler at oracle.com>
> *Cc:* John Rose <john.r.rose at oracle.com>; Mark Reinhold <mark.reinhold at oracle.com>; hotspot-dev at openjdk.java.net
> <hotspot-dev at openjdk.java.net>; Brian Stafford <Brian.Stafford at microsoft.com>; Martijn Verburg
> <Martijn.Verburg at microsoft.com>
> *Subject:* Re: [External] : Re: RFC - Improving C2 Escape Analysis
> First. Thank you, Cesar, for collecting data about C2 EA shortcomings.
>
> I agree with cases Tobias pointed as possible starting points to improve EA.
>
> Yes, finding solution for allocation merges (or NULL) is a pain. I spent some time investigating possible solutions for
> it but "no cigar". May be we do indead need control flow analysis to resolve this.
>
> I looked through JBS and found few issues which are not required to write new EA:
>
> https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-7149991&data=04%7C01%7CDivino.Cesar%40microsoft.com%7C027f2da3f2e14f914c2608d99b3c195e%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637711504731032568%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=w1OPBcpSVInagqRbMJ9%2BB0XYxxm84DWKGltPT5Btjss%3D&reserved=0
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>
> Tobias also has fix prototype for next bug which was not fixed yet:
> https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8236493&data=04%7C01%7CDivino.Cesar%40microsoft.com%7C027f2da3f2e14f914c2608d99b3c195e%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637711504731042513%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=KCLrH3%2FnNhLANzyGrbCLILwuDUfql5h3Lx0REVsol%2F0%3D&reserved=0
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>
> Ther are 2 test files with small methods for different EA cases I used to see how EA works:
>
> test/hotspot/jtreg/compiler/escapeAnalysis/Test6726999.java
> test/hotspot/jtreg/compiler/escapeAnalysis/Test6689060.java
>
> You can start looking on above RFE/bug or run these tests and see why scalarization failed for some cases. Except for
> known merge issue:
>
> https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-6853701&data=04%7C01%7CDivino.Cesar%40microsoft.com%7C027f2da3f2e14f914c2608d99b3c195e%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637711504731042513%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=vYIhnXEGGw%2FLx83NKcCAu0Vdt382TngtfpQ%2BCDBq7cU%3D&reserved=0
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>
> I am currently looking on iterative EA. Do more EA rounds if we can eliminate more connected allocations. It was
> proposed by Vladimir Ivanov and I have working prototype.
>
> There is also suggestin from Amazon Java group about "C2 Partial Escape Analysis" which needs more discsussion:
> https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fmail.openjdk.java.net%2Fpipermail%2Fhotspot-compiler-dev%2F2021-May%2F047486.html&data=04%7C01%7CDivino.Cesar%40microsoft.com%7C027f2da3f2e14f914c2608d99b3c195e%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637711504731052481%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=VUDTiLcJzwQkcUHQzLk7vcOIjmqSKQt8glKSrTHRX6w%3D&reserved=0
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>
> Thanks,
> Vladimir K
>
> On 10/27/21 3:04 AM, Tobias Hartmann wrote:
>> Hi Cesar,
>>
>> On 27.10.21 08:20, Cesar Soares Lucas wrote:
>>> Right. I was suspecting this to be the most critical issue indeed. However, I
>>> didn't know there was a case where "... the object does not escape on any paths
>>> but control flow is too complicated for EA to prove that." Is this an issue
>>> tracked in JBS or perhaps you can show me an example where this happens?
>>
>> Sure, here are four examples of EA and/or scalarization failing due to complicated control/data
>> flow: https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fcr.openjdk.java.net%2F~thartmann%2FEA_examples&data=04%7C01%7CDivino.Cesar%40microsoft.com%7C027f2da3f2e14f914c2608d99b3c195e%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637711504731052481%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=jv5lNO928mVyKHHlZIKyQ2eZGfu4W9ADV%2BlyX2IAvlk%3D&reserved=0
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>>
>> All examples would completely fold with inline types (Valhalla).
>>
>> I'm not sure if these issues are tracked by JBS issues but there's most likely an overlap with some
>> of the issues you already described.
>>
>> Best regards,
>> Tobias
>>