RFR: 8307927: C2: "malformed control flow" with irreducible loop

[Emanuel Peter]

On Fri, 16 Jun 2023 15:59:40 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.

src/hotspot/share/opto/loopnode.cpp line 3515:

> 3513: // Allpaths backwards scan from loop tail, terminating each path at first safepoint
> 3514: // encountered.  Helper for check_safepts.
> 3515: void IdealLoopTree::allpaths_check_safepts(VectorSet &visited, Node_List &stack) {

@rwestrel you should update the description here. Suggestion:

Allpaths backwards scan. Starting at the head, traversing all backedges, and the body. Terminating each path at first safepoint encountered.  Helper for check_safepts.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14522#discussion_r1239537606