RFR: 8336702: C2 compilation fails with "all memory state should have been processed" assert [v2]

[Roland Westrelin]

> When converting a `LongCountedLoop` into a loop nest, c2 needs jvm
> state to add predicates to the inner loop. For that, it peels an
> iteration of the loop and uses the state of the safepoint at the end
> of the loop. That's only legal if there's no side effect between the
> safepoint and the backedge that goes back into the loop. The assert
> failure here happens in code that checks that.
> 
> That code compares the memory states at the safepoint and at the
> backedge. If they are the same then there's no side effect. To check
> consistency, the `MergeMem` at the safepoint is cloned. As the logic
> iterates over the backedge state, it clears every component of the
> state it encounters from the `MergeMem`. Once done, the cloned
> `MergeMem` should be "empty". In the case of this failure, no side
> effect is found but the cloned `MergeMem` is not empty. That happens
> because of EA: it adds edges to the `MergeMem` at the safepoint that
> it doesn't add to the backedge `Phis`.
> 
> So it's the verification code that fails and I propose dealing with
> this by ignoring memory state added by EA in the verification code.

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:

 - Merge branch 'master' into JDK-8336702
 - test indentation
 - fix & test

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/21009/files
  - new: https://git.openjdk.org/jdk/pull/21009/files/463d6a21..a4263e28

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21009&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21009&range=00-01

  Stats: 193974 lines in 1550 files changed: 175338 ins; 10446 del; 8190 mod
  Patch: https://git.openjdk.org/jdk/pull/21009.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21009/head:pull/21009

PR: https://git.openjdk.org/jdk/pull/21009