Integrated: 8356708: C2: loop strip mining expansion doesn't take sunk stores into account

[Roland Westrelin]

On Tue, 10 Jun 2025 10:17:11 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.

This pull request has now been integrated.

Changeset: c11f36e6
Author:    Roland Westrelin <roland at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/c11f36e6200b6c39fd59530f28e9318c4153db49
Stats:     282 lines in 3 files changed: 255 ins; 1 del; 26 mod

8356708: C2: loop strip mining expansion doesn't take sunk stores into account

Reviewed-by: rcastanedalo, epeter

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PR: https://git.openjdk.org/jdk/pull/25717