RFR: 8269820: C2 PhaseIdealLoop::do_unroll get wrong opaque node [v2]

[Christian Hagedorn]

On Wed, 30 Nov 2022 10:19:42 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> A main loop loses its pre loop. The Opaque1 node for the zero trip
>> guard of the main loop is assigned control at a Region through which
>> an If is split. As a result, the Opaque1 is cloned and the zero trip
>> guard takes a Phi that merges Opaque1 nodes. One of the branch dies
>> next and as, a result, the zero trip guard has an Opaque1 as input but
>> at the wrong CmpI input. The assert fires next.
>> 
>> The fix I propose is that if an Opaque1 node that is part of a zero
>> trip guard is encountered during split if, rather than split if up or
>> down, instead, assign it the control of the zero trip guard's
>> control. This way the pattern of the zero trip guard is unaffected and
>> split if can proceed. I believe it's safe to assign it a later
>> control:
>> 
>> - an Opaque1 can't be shared
>> 
>> - the zero trip guard can't be the If that's being split
>> 
>> As Vladimir noted, this bug used to not reproduce with loop strip
>> mining disabled but now always reproduces because the loop
>> strip mining nest is always constructed. The reason is that the
>> main loop in this test is kept alive by the LSM safepoint. If the
>> LSM loop nest is not constructed, the loop is optimized out. I
>> filed:
>> 
>> https://bugs.openjdk.org/browse/JDK-8297724
>> 
>> for this issue.
>
> Roland Westrelin has updated the pull request incrementally with three additional commits since the last revision:
> 
>  - more
>  - more
>  - review

That looks reasonable to me. Good idea to introduce a new opaque node type!

-------------

Marked as reviewed by chagedorn (Reviewer).

PR: https://git.openjdk.org/jdk/pull/11391