RFR: 8308504: C2: "malformed control flow" after JDK-8303466 [v2]

[Roland Westrelin]

> I took that bug over from Emanuel because he's away:
> https://github.com/openjdk/jdk/pull/14331
> 
> I tried adding a `CastII` to narrow the limit of the loop as I
> suggested in a comment on the PR but I found that doesn't work in all
> cases: if the type of the initial value for the loop variable is not
> narrow enough, then the narrower type for the limit doesn't help
> narrow the loop phi type.
> 
> What I propose instead is to add an assert predicate that catches when
> the main loop is unreachable but the zero trip count doesn't constant
> fold. For that to work, the order of predicates must be preserved when
> they are copied or updated. I had to make some small changes to
> guarantee that.

Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:

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

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/14973/files
  - new: https://git.openjdk.org/jdk/pull/14973/files/46c60fc3..c65b4bf1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14973&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14973&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14973.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14973/head:pull/14973

PR: https://git.openjdk.org/jdk/pull/14973