8370332: C2 SuperWord: SIGSEGV because PhaseIdealLoop::split_thru_phi left dead nodes in loop _body

[eme64]

Analysis:
split_thru_phi can split a node out of the loop, through some loop phi. As a consequence, that node and the phi we split through can become dead. But split_thru_phi did not have any logic to yank the dead node and phi from the _body. If this happens in the same loop-opts-phase as a later SuperWord, and that SuperWord pass somehow accesses that loop _body, then we may find dead nodes, which is not expected.

It is not ok that split_thru_phi leaves dead nodes in the _body, so they have to be yanked.

What I did additionally: I went through all uses of split_thru_phi, and moved the replace_node from the call-site to the method itself. Removing the node and yanking from _body conceptually belongs together, so they should be together in code.

I suspect that split_thru_phi was broken for a long time already. But JDK26 changes in SuperWord started to check inputs of all nodes in _body, and that fails with dead nodes.

Future Work:

• Continue work on making VerifyLoopOptimizations work again, we should assert that there are no dead nodes in the _body. We may do that with the following task, or a subsequent one.
  • JDK-8370332 Fix VerifyLoopOptimizations - step 3 - fix ctrl/loop

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Issue

• JDK-8370332: C2 SuperWord: SIGSEGV because PhaseIdealLoop::split_thru_phi left dead nodes in loop _body (Bug - P3)

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[mlbridge]

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[rwestrel]

Does that work if, say, we're splitting:

(Add (Phi ..) (Phi ..)

With a single Phi as input twice? Doesn't the Phi have 2 uses then (the Add, twice)?

[eme64]

Hmm, good point. I'll have to see if I can find a reproducer for that...