RFR: 8255936: "parsing found no loops but there are some" assertion failure with Shenandoah [v4]

[Tobias Hartmann]

On Tue, 17 Nov 2020 12:38:23 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> This is a Shenandoah bug but the proposed fix is in shared code.
>> 
>> In an infinite loop, a barrier is located right after the loop head
>> and above the never branch. When the barrier is expanded, control flow
>> is added between the loop and the never branch. During loop
>> verification the assert fires because it doesn't expect any control
>> flow between the never branch and the loop head.
>> 
>> While it would have been nice to fix this Shenandoah issue in
>> Shenandoah code, I think the cleaner fix is to preserve the invariant
>> that the never branch is always right after the loop head in an
>> infinite loop. In the proposed patch, this is achieved by moving all
>> uses of the loop head to the never branch when it's constructed.
>
> 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 two additional commits since the last revision:
> 
>  - fix
>  - test

The reworked fix looks good to me too.

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

Marked as reviewed by thartmann (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1073