RFR: 8255670: Improve C2's detection of modified nodes

[Roland Westrelin]

On Sat, 9 Jul 2022 08:37:24 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> Added `record_modified_node` to:
> 
> Node::clone
> Node::add_req
> Node::add_req_batch
> Node::ins_req
> Node::add_prec
> Node::rm_prec
> Node::set_prec
> 
> 
> Added `igvn->_worklist.push(node)` in various places that modified a `node` but did not add it to the igvn worklist.
> 
> 7 times I had to push `Root`, 5 of these it was because of the creation of a `HaltNode`, which means we have a `root->add_req(halt)`. 
> 
> In one case we have a MergeMemStream node, which gets two MergeMem nodes as input, and streams over them.
> Unfortunately, it modifies one of the two, which then can trigger our assertion code. I now push this node to the igvn worklist, but a better fix would be to make MergeMemStream leave the MergeMem nodes unmodified. I think that should be possible, filed an RFE [JDK-8293358](https://bugs.openjdk.org/browse/JDK-8293358)
> 
> FYI:
> What I am NOT doing here, and leave to a future RFE/independent change: investigate / implement these assertions for late/incremental inlining.
> 
> Ran larger regression tests, and 7-9h of fuzzing on 3 platforms.

src/hotspot/share/opto/loopnode.cpp line 5135:

> 5133:           set_loop(halt, l);
> 5134:           C->root()->add_req(halt);
> 5135:           _igvn._worklist.push(C->root());

Maybe add a method to PhaseIterGVN that does the add_req + push similar to replace_input_of?

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PR: https://git.openjdk.org/jdk/pull/9439