RFR: 8373524: C2: no reachable node should have no use

Wed Dec 17 12:49:21 UTC 2025

On Tue, 16 Dec 2025 10:02:46 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> The failure occurs because `PhiNode::Ideal` uses `set_req` to update
> an input of a `Phi`. That causes the previous input to be disconnected
> but because of the use of `set_req`, the previous input that has no
> use is not enqueued for `igvn` to be reclaimed. The fix is to use
> `set_req_X` instead. I replaced uses of `set_req` with `set_req_X` in
> `PhiNode::Ideal` where I thought it made sense.

Thank you for fixing this, @rwestrel. The changes look good. I also kicked off some testing.

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PR Review: https://git.openjdk.org/jdk/pull/28841#pullrequestreview-3587565866