RFR: 8254887: C2: assert(cl->trip_count() > 0) failed: peeling a fully unrolled loop

[Christian Hagedorn]

On Fri, 6 Nov 2020 16:41:28 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> A loop's trip count is computed to have exact trip count 6. Then:
> 
> 1- pre/main/post loops are created which brings the trip count from 6
>   to 5
> 2- main loop is unrolled which brings the trip count to 2
> 3- main loop is peeled: Trip count is 1
> 4- pre/main/post loops are created again. Trip count of main loop is 0.
> 5- peeling is attempted again and the assert fires
> 
> IdealLoopTree::policy_peeling() doesn't attempt peeling if the trip
> count is 1. I propose that IdealLoopTree::policy_range_check() (that
> causes the pre/main/post loops insertion the second time) performs the
> same check so step 4 doesn't happen.

Looks good to me!

src/hotspot/share/opto/loopTransform.cpp line 1066:

> 1064:   Node *trip_counter = cl->phi();
> 1065: 
> 1066:   // check for vectorized loops, some opts are no longer needed

Maybe you can update this comment to also include the new check.

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Marked as reviewed by chagedorn (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1096