RFR: 8356708: C2: loop strip mining expansion doesn't take sunk stores into account [v7]

[Emanuel Peter]

On Wed, 18 Jun 2025 09:24:47 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> `test1()` has a counted loop with a `Store` to `field`. That `Store`
>> is sunk out of loop. When the `OuterStripMinedLoop` is expanded, only
>> `Phi`s that exist at the inner loop are added to the outer
>> loop. There's no `Phi` for the slice of the sunk `Store` (because
>> there's no `Store` left in the inner loop) so no `Phi` is added for
>> that slice to the outer loop. As a result, there's a missing anti
>> dependency for `Load` of `field` that's before the loop and it can be
>> scheduled inside the outer strip mined loop which is incorrect.
>> 
>> `test2()` is the same as `test1()` but with a chain of 2 `Store`s.
>> 
>> `test3()` is another variant where a `Store` is left in the inner loop
>> after one is sunk out of it so the inner loop still has a `Phi`. As a
>> result, the outer loop also gets a `Phi` but it's incorrectly wired as
>> the sunk `Store` should be the input along the backedge but is
>> not. That one doesn't cause any failure AFAICT.
>> 
>> The fix I propose is some extra logic at expansion of the
>> `OuterStripMinedLoop` to handle these corner cases.
>
> Roland Westrelin has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Update src/hotspot/share/opto/loopnode.cpp
>    
>    Co-authored-by: Emanuel Peter <emanuel.peter at oracle.com>
>  - Update src/hotspot/share/opto/loopnode.cpp
>    
>    Co-authored-by: Emanuel Peter <emanuel.peter at oracle.com>

@TobiHartmann Had 2 questions:
- Is this really something we want to put into JDK 25? Feels high-risk and it's and old issue after all. Maybe we can push this to JDK26 first, and backport a little later?
- Is this really a regression from [JDK-8281322](https://bugs.openjdk.org/browse/JDK-8281322)? If not, the affects version in JBS should be updated such that we'll consider this for backporting.

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PR Comment: https://git.openjdk.org/jdk/pull/25717#issuecomment-2983777833