RFR: 8305189: C2 failed "assert(_outcnt==1) failed: not unique" [v4]

[Roland Westrelin]

> pre/main/post loops are created for an inner loop of a loop nest but
> assert predicates cause the main and post loops to be removed. The
> OpaqueZeroTripGuard nodes for the loops are not removed: there's no
> logic to trigger removal of the opaque nodes once the loops are no
> longer there. With the inner loops gone, the outer loop becomes
> candidate for optimizations and is unrolled which causes the zero trip
> guards of the now removed loops to be duplicated and the opaque nodes
> to have more than one use.
> 
> The fix I propose is, using logic similar to
> `PhaseIdealLoop::eliminate_useless_predicates()`, to check during loop
> opts if every OpaqueZeroTripGuard node guards a loop and if not,
> remove it.

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

 - Update src/hotspot/share/opto/loopnode.cpp
   
   Co-authored-by: Christian Hagedorn <christian.hagedorn at oracle.com>
 - review

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13901/files
  - new: https://git.openjdk.org/jdk/pull/13901/files/8f37817c..5e26e81d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13901&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13901&range=02-03

  Stats: 33 lines in 3 files changed: 22 ins; 5 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/13901.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13901/head:pull/13901

PR: https://git.openjdk.org/jdk/pull/13901