回复：回复：[Patch] 8210853: C2 doesn't skip post barrier for new allocated objects

[Kuai Wei]

Hi Tobias,

  Thanks for your comments. I will check RegionNode::is_copy to see if it can be used to detect 
unnecessary region node. I will send new review after testing.

Best Regards,
Kevin


------------------------------------------------------------------
发件人：Tobias Hartmann <tobias.hartmann at oracle.com>
发送时间：2018年9月24日(星期一) 21:34
收件人：蒯微(麦庶) <kuaiwei.kw at alibaba-inc.com>; hotspot compiler <hotspot-compiler-dev at openjdk.java.net>
抄　送：李三红(三红) <sanhong.lsh at alibaba-inc.com>
主　题：Re: 回复：[Patch] 8210853: C2 doesn't skip post barrier for new allocated objects

Hi Kevin,

On 24.09.2018 08:06, Kuai Wei wrote:
>   Thanks for your suggestion. I think your point is the region node may have new path in later parse
> phase, so we can not make sure the region node will be optimized.

Yes, my point is that a new path to the region might be added after your optimization and that path
might contain stores to the newly allocated object.

>   It's a good question and I checked it. Now I think it may not cause trouble. In post barrier
> reduce, the oop store use allocation node as base pointer. The data graph guarantee control of
> allocation node should dominate control of store. If allocation node is in pred of region node and
> there's a new path into region, the graph is bad because we can reach store without allocation.

Yes but the new path might be a backedge from a loop that is dominated by the allocation.

> If allocation node is in a domination ancestor, the graph shape is a little complicated, so we can not
> reach control of allocation by skipping one region.

Right, that's basically the implicit assumption of your patch. I'm not sure if it always holds. But
I think you should at least use RegionNode::is_copy().

Let's see what other reviewers think.

>   The better solution is we can know the region node is created in exit_map and we will not change
> it in later. Is there any way to know it in compile time?

The region node is created in Parse::build_exits(). I don't think there is a way to keep track of this.

Thanks,
Tobias
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