RFR: 8280799: С2: assert(false) failed: cyclic dependency prevents range check elimination [v2]

[Roland Westrelin]

> The loop exit condition of the test method:
> 
> if (i == stop) {
>     break;
> }
> 
> requires insertion of a loop limit predicate when the loop is turned
> into a counted loop. stop is a LoadI. Next:
> 
> array[stop - i + j] = 0;
> 
> is transformed to:
> 
> array[stop - i] = 0;
> 
> and the range check for that array access becomes candidate for
> predication in a subsequent loop opts pass. stop has control just
> above the loop limit check when that happens (because it is assigned
> control as late as possible). But the loop predicate for the bound
> check needs to be above the loop limit check and that causes the
> assert failure.
> 
> There's already logic in PhaseIdealLoop::build_loop_late_post_work()
> to assign control to nodes above predicates so this sort of issues
> doesn't occur. But it only applies to node initially on the entry
> projection right above the loop head. The fix I propose is to remove
> that restriction.
> 
> That logic was added by JDK-8203197 and looking back at this change I
> noticed I replaced some existing logic with the current logic but,
> while the 2 overlap, the current logic is not guaranteed to always
> cover some cases handled by the old logic. So I resurrected that old
> logic here.
> 
> Finally, when running tests, I hit failures because Opaque nodes for
> skeleton predicates can now end up above a predicate that is split
> thru phi. That causes the Opaque nodes to be split up and breaks
> pattern matching. I'm actually not sure this issue is specific to the
> change here so I think it's best to treat it as a general issue and
> fix it by cloning the chain of nodes that lead to the Opaque node
> down.

Roland Westrelin has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - fix
 - Merge branch 'master' into JDK-8280799
 - fix & test

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7319/files
  - new: https://git.openjdk.java.net/jdk/pull/7319/files/7b606869..accc338e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7319&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7319&range=00-01

  Stats: 9465 lines in 353 files changed: 6414 ins; 1819 del; 1232 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7319.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7319/head:pull/7319

PR: https://git.openjdk.java.net/jdk/pull/7319