RFR: 8269820: C2 PhaseIdealLoop::do_unroll get wrong opaque node [v3]

[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 one additional commit since the last revision:

  Update src/hotspot/share/opto/subnode.cpp
  
  Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11391/files
  - new: https://git.openjdk.org/jdk/pull/11391/files/26a002f5..953958ad

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11391&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11391&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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