RFR: 8356708: C2: loop strip mining expansion doesn't take sunk stores into account [v8]

[Emanuel Peter]

On Wed, 18 Jun 2025 13:11:46 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 one additional commit since the last revision:
> 
>   review

@rwestrel The patch looks good, thanks for the work @rwestrel !

That said, and as mentioned above: we should probably investigate if we can add the Phi's from the beginning, so that we do not violate the C2 IR assumptions.

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Marked as reviewed by epeter (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/25717#pullrequestreview-2945290515