RFR: 8257575: C2: "failed: only phis" assert failure in loop strip mining verification

[Tobias Hartmann]

On Fri, 4 Dec 2020 12:00:25 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> Relaxing the assert seems reasonable but shouldn't we also fix the problem that the load is not eliminated?
>
>> Relaxing the assert seems reasonable but shouldn't we also fix the problem that the load is not eliminated?
> 
> I tried that but that didn't prove easy to make robust. For instance, I added code that enqueues the dominated load in the inner loop for IGVN but by the time it is processed, the dominated load's inputs have not yet all been processed so the dominated load is not found as dominating (mismatching address input). Maybe we could revert the logic in LoadNode::Ideal() so it looks for a dominated load rather than a dominating load. I gave it a try but I saw some loads were still not eliminated so I gave up given it seems like such a corner case.

Okay, thanks for the background information.

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PR: https://git.openjdk.java.net/jdk/pull/1555