RFR(S): 8232539: SIGSEGV in C2 Node::unique_ctrl_out

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8232539/webrev.00/

I couldn't come up with a test case because node processing order during
IGVN matters. Bug was reported against 11 but I see no reason it
wouldn't apply to current code as well.

At parse time, predicates are added by
Parse::maybe_add_predicate_after_if() but not loop is actually
created. Compile::_major_progress is cleared. On the next round of IGVN,
one input of a region points to predicates. The same region has an if as
use that can be split through phi during IGVN. The predicates are going
to be removed by IGVN. But that happens in multiple steps because there
are several predicates (for reason Deoptimization::Reason_predicate,
Deoptimization::Reason_loop_limit_check etc.) and because for each
predicate one IGVN iteration must first remove the Opaque1 node, then
another kill the IfFalse projection, finally another replace the IfTrue
projection by the If control input.

Split if occurs while predicates are in the process of being removed. It
sees predicates, tries to walk over them, encounters a predicates that's
been half removed (false projection removed) and we hit the assert/crash.

I propose we simply not apply IGVN split if if we're splitting through a
loop or if there's a predicate input to a region because:

- Making split if robust to dying predicates is not straightforward as
  far as I can tell

- Loop opts split if doesn't split through loop header so why would it
  make sense for IGVN split if?

- I'm wondering if there are other cases where handling of predicates in
  split if could be wrong (and so more trouble ahead):

  + What if we split through a Loop region, predicates were added by
  loop optimizations, loop opts are now over so the predicates added at
  parse time were removed: then PhaseIdealLoop::find_predicate()
  wouldn't report a predicate but cloning predicates would still be
  required for correctness?

  + What if we have no loop, a region has predicates as input,
  predicates are going to die but have not yet been processed, split if
  uselessly duplicates predicates but one of then is control dependent
  on the branch it is in so cloning predicates actually causes a broken
  graph?

So overall it feels safer to me to simply bail out from split if for
loops/predicates.

Roland.