RFR: 8315920: C2: "control input must dominate current control" assert failure [v4]

[Roland Westrelin]

> The assert fires during range check eliminationin code that was added
> recently. It catches a bug that's been going unnoticed. The `array[0]`
> load of the test case ends up on the exit of the loop above it. After
> pre/main/post loops are created for this loop (the inner loop is
> optimized out), the load is at a region merging control from the just
> created pre/main/post loops but its control (as returned by
> `PhaseIdealLoop::get_ctrl()`) is not updated by pre/main/post loops
> creation code so it's still the exit projection of one of the
> loops. This causes the assert to fire (the load has bad control) when
> RC runs next in the same loop opts pass.
> 
> The current logic for pre/main/post loops creation does update the
> control of nodes on an exit projection but only if the node is
> referenced from some other node in the loop body.
> 
> I think a similar bug with a node assigned control at the exit but not
> pinned is likely to exist. So I doubt simply going over nodes pinned
> at exits during pre/main/post and updating their control is good
> enough. What I propose instead is to `lazy_replace()` the projection
> by the region so that, for all nodes that are assigned the exit as
> control, `get_ctrl()` will then return the region. It's not entirely
> straightforward to implement because the `lazy_replace()` mechanism
> assumes the control being replaced is dead which is not the case
> here. That's why in the patch the exit projection is cloned so the
> existing one is dead and can be used in `lazy_replace()`. This also
> makes the calls to `sink_use()` redundant.

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

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

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15720/files
  - new: https://git.openjdk.org/jdk/pull/15720/files/321a7244..886f964b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15720&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15720&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15720.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15720/head:pull/15720

PR: https://git.openjdk.org/jdk/pull/15720