RFR: 8287840: Dead copy region node blocks IfNode's fold-compares

[Vladimir Kozlov]

On Mon, 6 Jun 2022 01:15:35 GMT, Xin Liu <xliu at openjdk.org> wrote:

> IfNode::fold_compares() requires ctrl has a single output. I found some fold-compares case postpone to IterGVN2. The reason is that a dead region prevents IfNode::fold_compares() from transforming code.  The dead node is removed in IterGVN, but it's too late. 
> 
> This PR extends Node::has_special_unique_user() so `PhaseIterGVN::remove_globally_dead_node()` puts IfNode back to worklist. The following attempt will carry out fold-compares().

Looks good. Let me test it.

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PR: https://git.openjdk.java.net/jdk/pull/9035