RFR: 8280126: C2: detect and remove dead irreducible loops [v3]

[Emanuel Peter]

On Wed, 18 Jan 2023 07:49:20 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

>> Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Improvements after review (Vladimir K)
>
> src/hotspot/share/opto/cfgnode.cpp line 608:
> 
>> 606:       // The irreducible loop is dead - must remove it
>> 607:       PhaseIterGVN* igvn = phase->is_IterGVN();
>> 608:       remove_unreachable_subgraph(igvn);
> 
> Since we are now always eagerly removing unreachable subgraphs in this case and for normal dead loops, I guess we can get rid of the caching of the `is_unreachable_from_root()` result managed with the `_is_unreachable_region` field.

I don't think that this is true. Note that only `is_unreachable_region` does the caching, and assumes that we have a canonical loop (one entry-control, one backedge). It is used both in `RegionNode::Ideal` (twice, so caching makes sense already. Without the caching the logic of that if becomes much more complex) and in `PhiNode::is_data_loop` (a phi might realize the loop is dead before the loop-head does, and already do the traversal - if the result is "unreachable", then we would like to have that value cached and immediately returned for the loop head).
Do you agree?

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PR: https://git.openjdk.org/jdk/pull/11764