RFR: 8280126: C2: detect and remove dead irreducible loops [v2]

[Vladimir Kozlov]

On Wed, 11 Jan 2023 17:37:33 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Emanuel Peter has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:
>> 
>>  - Merge branch 'master' into JDK-8280126
>>  - 8280126: C2: detect and remove dead irreducible loops
>
> src/hotspot/share/ci/ciTypeFlow.cpp line 1867:
> 
>> 1865:   // Head of infinite loop, no parent was attached because no exit found
>> 1866:   // Since this infinite loop does not exit into another loop, it is not
>> 1867:   // nested in any loop, hence also not in an irreducible loop.
> 
> Do we need `_infinite_loop_head` for easy check here?

Also even if it do not have exit it may have entry from irreducible loop. I don't understand your statement that "it is not nested".

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