Re: 回复：[Patch] 8210853: C2 doesn't skip post barrier for new allocated objects
Vladimir Kozlov
vladimir.kozlov at oracle.com
Tue Oct 16 21:17:34 UTC 2018
On 10/16/18 4:56 AM, Kuai Wei wrote:
> Hi Vladimir,
>
> About the copy region, I made this fix to check if region node is copy. In my test case, the
> region is degraded to copy and
> post write barrier is removed. I haven't checked the detail of the transform. I think there's an
> ideal phase between inlining and
> parsing store oop. So the phi node is removed and region node is degraded to copy.
>
> We tested the fix in several online trade systems. We can check cpu usage difference between
> machines with same load. I will
> test with some benchmark tests and give the score.
Sounds good. Lets see if it helps in your production system.
I will submit our internal testing with your latest fix with small change. I moved req() == 2 check
from assert to condition to narrow cases (copy Region may have > 2 inputs with only 1 not-null input):
http://cr.openjdk.java.net/~kvn/8210853/webrev.00/
Vladimir
>
> Thanks,
> Kevin
>
> ------------------------------------------------------------------
> From:Vladimir Kozlov <vladimir.kozlov at oracle.com>
> Send Time:2018年10月16日(星期二) 08:45
> To:蒯微(麦庶) <kuaiwei.kw at alibaba-inc.com>; Tobias Hartmann <tobias.hartmann at oracle.com>;
> hotspot compiler <hotspot-compiler-dev at openjdk.java.net>
> Cc:李三红(三红) <sanhong.lsh at alibaba-inc.com>
> Subject:Re: 回复：[Patch] 8210853: C2 doesn't skip post barrier for new allocated objects
>
> Hi Kevin,
>
> On 10/4/18 8:18 AM, Kuai Wei wrote:
> > Hi Vladimir,
> >
> > I'm not sure about the phi node here. Is it used for merging allocation in fast path and slow path?
> > In parsing phase, the allocation node is not expanded, so there's no this phi node. If my understand
> > is wrong, please correct me. So far as I know, the region node is created while inlining initialize
> > method
> > of super type. And phi node is not necessary. But the region node is always created.
>
> It is not related to slow and fast path of allocation. It is general rule to have Phi nodes when new
>
> Region node is created. Like here:
>
> http://hg.openjdk.java.net/jdk/jdk/file/04d4f1e4aff2/src/hotspot/share/opto/parse1.cpp#l772
>
> >
> > I understand your concern. But write barrier is added in parse phase, we must check it in parse time
> > to do this optimization. If the region node is a copy, could we assume it will not have additional
> > input?
>
> Yes, I think it is true. Copy Region never goes back to normal (merge) Region.
>
> > I checked region node, a copy region node is created when only one input edge and compiler can not
> > reshape, the region node will be degraded to a copy. I think it will be dead in future phase.
>
> Yes. But does your case have a 'copy' Region? I originally thought that you have normal Region with
> just one input.
>
> >
> > In our trade system, this optimization could improve 5%+ performance. It's a big improvement and we
> > don't want to lose it. If there's better solution, we are pleasure to implement it.
>
> Yes, I agree that we should fix it.
>
> What testing did you perform?
>
> Thanks,
> Vladimir
>
> >
> > Thanks,
> > Kevin
> >
> > ------------------------------------------------------------------
> > From:Vladimir Kozlov <vladimir.kozlov at oracle.com>
> > Send Time:2018年10月4日(星期四) 06:50
> > To:蒯微(麦庶) <kuaiwei.kw at alibaba-inc.com>; Tobias Hartmann <tobias.hartmann at oracle.com>;
> > hotspot compiler <hotspot-compiler-dev at openjdk.java.net>
> > Cc:李三红(三红) <sanhong.lsh at alibaba-inc.com>
> > Subject:Re: 回复：[Patch] 8210853: C2 doesn't skip post barrier for new allocated objects
> >
> > Sorry, I mean
> >
> > "should exist Phi *associated* with allocation"
> >
> > On 10/3/18 3:48 PM, Vladimir Kozlov wrote:
> > > Hi Kevin,
> > >
> > > You correctly pointed in your analysis that if base of address is pointing to allocation there
> > > should not be other control path to this store. But it also means that before there should exist Phi
> > > not associated with allocation when Region node was added. If Phi node was removed at some time
> > > Region node should be removed too (or not add it in first place). Please, look on that.
> > >
> > > I share Tobias's concern about skipping Region node in Parse phase when IR graph is still constructed.
> > >
> > > Thanks,
> > > Vladimir
> > >
> > > On 10/3/18 8:08 AM, Kuai Wei wrote:
> > >> Hi Tobias,
> > >>
> > >> I made the change to check with RegionNode::is_copy, could you check the new patch?
> > >>
> > >> diff --git a/src/hotspot/share/opto/graphKit.cpp b/src/hotspot/share/opto/graphKit.cpp
> > >> index 068141f..884a76c 100644
> > >> --- a/src/hotspot/share/opto/graphKit.cpp
> > >> +++ b/src/hotspot/share/opto/graphKit.cpp
> > >> @@ -2126,7 +2126,15 @@ void GraphKit::uncommon_trap(int trap_request,
> > >> // We use this to determine if an object is so "fresh" that
> > >> // it does not require card marks.
> > >> Node* GraphKit::just_allocated_object(Node* current_control) {
> > >> - if (C->recent_alloc_ctl() == current_control)
> > >> + Node * ctrl = current_control;
> > >> + // Object::<init> is invoked after allocation, most of invoke nodes
> > >> + // will be reduced, but a region node is kept in parse time, we check
> > >> + // the pattern and skip the region node
> > >> + if (ctrl != NULL && ctrl->is_Region() && ctrl->as_Region()->is_copy()) {
> > >> + assert(ctrl->req() == 2, "copy region has only 2 inputs");
> > >> + ctrl = ctrl->as_Region()->is_copy();
> > >> + }
> > >> + if (C->recent_alloc_ctl() == ctrl)
> > >> return C->recent_alloc_obj();
> > >> return NULL;
> > >> }
> > >> Thanks,
> > >> Kevin
> > >>
> > >> ------------------------------------------------------------------
> > >> From:蒯微(麦庶) <kuaiwei.kw at alibaba-inc.com>
> > >> Send Time:2018年9月25日(星期二) 21:50
> > >> To:Tobias Hartmann <tobias.hartmann at oracle.com>; hotspot compiler
> > >> <hotspot-compiler-dev at openjdk.java.net>
> > >> Cc:李三红(三红) <sanhong.lsh at alibaba-inc.com>
> > >> Subject:回复：回复：
> [Patch] 8210853: C2 doesn't skip post barrier for new allocated objects
> > >>
> > >> 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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