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

Fri Oct 29 23:50:36 UTC 2021

Hi Vladimir and Tobias,

>> Sure, here are four examples of EA and/or scalarization failing due to
>> complicated control/data flow:
>> https://cr.openjdk.java.net/~thartmann/EA_examples

>> 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
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?

>> 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?

>> There is also suggestin from Amazon Java group about "C2 Partial Escape
>> Analysis" which needs more discsussion:
>> https://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2021-May/047486.html

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.

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%7C63920cb1798f48c3487508d9996efd44%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637709524262611242%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=QrR7%2BGxXon4ToV6x3PhtQzZGl5tF7f1RUDbEi2AMTqA%3D&reserved=0
https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8059378&data=04%7C01%7CDivino.Cesar%40microsoft.com%7C63920cb1798f48c3487508d9996efd44%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637709524262611242%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=rsMHgOyTDYF%2B%2Ba38jGeown5TcZfIEDucAWI5QuAaTd4%3D&reserved=0
https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8073358&data=04%7C01%7CDivino.Cesar%40microsoft.com%7C63920cb1798f48c3487508d9996efd44%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637709524262611242%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=CypHNEd5B5EymTYMnF6jf30LspY6sBqXoz1sypE2tSg%3D&reserved=0
https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugs.openjdk.java.net%2Fbrowse%2FJDK-8155769&data=04%7C01%7CDivino.Cesar%40microsoft.com%7C63920cb1798f48c3487508d9996efd44%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637709524262611242%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=BE170%2BZrn2c2%2FDLcijZsol25q2zY5X5idHXXwjCn7ug%3D&reserved=0

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%7C63920cb1798f48c3487508d9996efd44%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637709524262611242%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=fqaQ7zhAHGdsnUcw7wjA6c4XX96Aaa3acTIzc6%2FJXmY%3D&reserved=0

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%7C63920cb1798f48c3487508d9996efd44%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637709524262621193%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=F%2Bz1CFuCK6ZgXi5%2FWOcOgBWuXKeap0oZJh4873QKRgk%3D&reserved=0

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%7C63920cb1798f48c3487508d9996efd44%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637709524262621193%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=QFszHSDnPkYLBkjqzNkmU92P6VlBFSok1mOku5sNudw%3D&reserved=0

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%7C63920cb1798f48c3487508d9996efd44%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637709524262621193%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=YbaF4T0zt9dle23nulvUWWLktuTvaWFWENQHD7Q13CE%3D&reserved=0
>
> 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
>