RFR: 8269820: C2 PhaseIdealLoop::do_unroll get wrong opaque node

[Roland Westrelin]

On Mon, 28 Nov 2022 14:02:50 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.

Thanks for reviewing this.

> General question. Will it help (simplify changes) if we add specialized `class OpaqueZeroTripGuardNode : public Opaque1Node` class?

Good suggestion! Let me give it a try.

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

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