RFR: 8269820: C2 PhaseIdealLoop::do_unroll get wrong opaque node [v2]

[Roland Westrelin]

> 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.

Roland Westrelin has updated the pull request incrementally with three additional commits since the last revision:

 - more
 - more
 - review

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11391/files
  - new: https://git.openjdk.org/jdk/pull/11391/files/7d1e79cb..26a002f5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11391&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11391&range=00-01

  Stats: 117 lines in 9 files changed: 8 ins; 89 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/11391.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11391/head:pull/11391

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