RFR: 8333393: PhaseCFG::insert_anti_dependences can fail to raise LCAs and to add necessary anti-dependence edges [v2]

[Emanuel Peter]

On Wed, 8 Jan 2025 14:00:24 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Daniel Lundén has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Updates after comments
>
> src/hotspot/share/opto/gcm.cpp line 757:
> 
>> 755:   // In some cases, there are other relevant initial memory states besides
>> 756:   // initial_mem. In such cases, we are rather dealing with multiple trees and
>> 757:   // their fringes.
> 
> If I look at these comments here (I reviewed a change by Roland a few months back, so my memory is coming back)...
> I see that the load is supposed to be scheduled before any `Memory state modifying nodes include Store and Phi` that is (transitively via any MergeMem) below the `initial_mem`.
> 
> In you example1, why do we therefore not put an anti-dependency edge betweeen the `183 load`, and the `106 Phi`? Would that not be enough to ensure the load is scheduled before the other memory affecting nodes further below `106 Phi`?
> 
> Or is the issue that this traversal is somehow restricted to blocks - I don't remember that from last time...
> I'll keep reading the changes now.

And in example 2, we should schedule before the Phi as well:
![image](https://github.com/user-attachments/assets/3d035602-fe4b-4c34-98fe-d2935fed92e0)

Why don't we do that?

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PR Review Comment: https://git.openjdk.org/jdk/pull/22852#discussion_r1907242566