RFR(S): 8243670: Unexpected test result caused by C2 MergeMemNode::Ideal

[Yangfei (Felix)]

Hi Nils,

    CCing to Tobias.

> -----Original Message-----
> From: hotspot-compiler-dev [mailto:hotspot-compiler-dev-
> bounces at openjdk.java.net] On Behalf Of Nils Eliasson
> Sent: Wednesday, June 3, 2020 9:55 PM
> To: hotspot-compiler-dev at openjdk.java.net
> Subject: Re: RFR(S): 8243670: Unexpected test result caused by C2
> MergeMemNode::Ideal
> 
> Hi,
> 
> I second Tobias. How often is this code path triggered? Would this be
> removing an optimization that is important is some cases?

I see the logic is also triggered for non-OSR compiles with the reduced test case.
But it didn't trigger a bug in that case even through the logic didn't do the right thing.

My initial patch removes the logic and no performance impact witnessed for specjbb2017. 
Since the code is there from day one, maybe it's hard to find out. 

> If this is extremely rare - I would be ok to remove it, otherwise we should
> evaluate Tobias suggestion.

Thanks,
Felix