RFR: 8307927: C2: "malformed control flow" with irreducible loop [v2]

[Emanuel Peter]

On Thu, 29 Jun 2023 07:44:01 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> The test contains a loop nest with 2 loops. The outer loop is an
>> irreducible loop. The safepoint for that loop is also in the inner
>> loop. Because `IdealLoopTree::check_safepts()` ignores irreducible
>> loops, that safepoint is not marked as being required and is
>> eliminated from the inner loop. The inner loop is then optimized out
>> and the outer loop becomes an infinite loop with no safepoint (a
>> single node loop). That, in turn, causes the loop to be eliminated
>> because it has not use and the assert fires.
>> 
>> The fix I propose is to make `IdealLoopTree::check_safepts()` work
>> with irreducible loops. I think
>> `IdealLoopTree::allpaths_check_safepts()` can be used for that. When
>> working on this I wondered if that method could be called with a loop
>> whose head has more than 3 inputs. I couldn't write a test case with
>> an irreducible loop whose head had more than 3 inputs but I added an
>> assert in the method and ran some testing. That assert fired so I also
>> propose to tweak the method so it's robust in that case.
>
> Roland Westrelin 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 three additional commits since the last revision:
> 
>  - review
>  - Merge branch 'master' into JDK-8307927
>  - test

Thanks for the changes @rwestrel , looks good now!

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Marked as reviewed by epeter (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/14522#pullrequestreview-1504735398