RFR: 8294217: Assertion failure: parsing found no loops but there are some

[Roland Westrelin]

This was reported on 11 and is not reproducible with the current
jdk. The reason is that the PhaseIdealLoop invocation before EA was
changed from LoopOptsNone to LoopOptsMaxUnroll. In the absence of
loops, LoopOptsMaxUnroll exits earlier than LoopOptsNone. That wasn't
intended and this patch makes sure they behave the same. Once that's
changed, the crash reproduces with the current jdk.

The assert fires because PhaseIdealLoop::only_has_infinite_loops()
returns false even though the IR only has infinite loops. There's a
single loop nest and the inner most loop is an infinite loop. The
current logic only looks at loops that are direct children of the root
of the loop tree. It's not the first bug where
PhaseIdealLoop::only_has_infinite_loops() fails to catch an infinite
loop (8257574 was the previous one) and it's proving challenging to
have PhaseIdealLoop::only_has_infinite_loops() handle corner cases
robustly. I reworked PhaseIdealLoop::only_has_infinite_loops() once
more. This time it goes over all children of the root of the loop
tree, collects all controls for the loop and its inner loop. It then
checks whether any control is a branch out of the loop and if it is
whether it's not a NeverBranch.

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Commit messages:
 - test
 - fix
 - reproduce

Changes: https://git.openjdk.org/jdk/pull/10904/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10904&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294217
  Stats: 111 lines in 2 files changed: 96 ins; 3 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/10904.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10904/head:pull/10904

PR: https://git.openjdk.org/jdk/pull/10904