RFR: 8305189: C2 failed "assert(_outcnt==1) failed: not unique" [v4]

[Tobias Hartmann]

On Tue, 23 May 2023 15:32:01 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> pre/main/post loops are created for an inner loop of a loop nest but
>> assert predicates cause the main and post loops to be removed. The
>> OpaqueZeroTripGuard nodes for the loops are not removed: there's no
>> logic to trigger removal of the opaque nodes once the loops are no
>> longer there. With the inner loops gone, the outer loop becomes
>> candidate for optimizations and is unrolled which causes the zero trip
>> guards of the now removed loops to be duplicated and the opaque nodes
>> to have more than one use.
>> 
>> The fix I propose is, using logic similar to
>> `PhaseIdealLoop::eliminate_useless_predicates()`, to check during loop
>> opts if every OpaqueZeroTripGuard node guards a loop and if not,
>> remove it.
>
> Roland Westrelin has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Update src/hotspot/share/opto/loopnode.cpp
>    
>    Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
>  - review

Looks reasonable to me.

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

Marked as reviewed by thartmann (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/13901#pullrequestreview-1445598078