RFR: 8289943: Simplify some object allocation merges [v6]

[Cesar Soares]

On Mon, 11 Jul 2022 19:34:47 GMT, Cesar Soares <duke at openjdk.org> wrote:

>I currently found one issue we need discuss - merge allocation of different subclasses (of the same parent class) which may have different number of fields. Current implementation assume objects are the same but I don't see the check for it during RAM node creation. May be we should have it at this initial implementation.

Hi again @vnkozlov - The current implementation requires the Klass of all inputs to the Phi to be the same. I'm trying to lift that restriction as it would increase the number of cases where the merge can be removed. I've been trying to create an example where I'd need to enforce the allocations to be of the same type but I couldn't figure one out yet. Do you have an example where that must be enforced? Note that I only need fields accessed through the merge Phi node, which means only fields present in the superclass AFAIU.

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PR: https://git.openjdk.org/jdk/pull/9073