RFR: 8295788: C2 compilation hits "assert((mode == ControlAroundStripMined && use == sfpt) || !use->is_reachable_from_root()) failed: missed a node"

[Vladimir Kozlov]

On Tue, 15 Nov 2022 11:42:00 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> This failure is similar to previous failures with loop strip mining: a
> node is encountered that has control set in the outer strip mined loop
> but is not reachable from the safepoint. There's already logic in loop
> cloning to find those and fix their control to be outside the
> loop. Usually a node ends up in the outer loop because some of its
> inputs is in the outer loop. The current logic to catch nodes that are
> erroneously assigned control in the outer loop is to start from
> safepoint's inputs and look for uses with incorrect control. That
> doesn't work in this case because: 1) the node is created by
> IdealLoopTree::reassociate in the outer loop because its inputs are
> indeed there 2) but a pass of split if updates the control to be
> inside the inner loop.
> 
> To fix this, I propose reusing the existing clone_outer_loop_helper()
> but apply it to the loop body as well. I had to tweak that method
> because I ran into cases of dead nodes still reachable from a node in
> the loop body but removed from the _body list by
> IdealLoopTree::DCE_loop_body() (and as a result not cloned).

Make sense.

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Marked as reviewed by kvn (Reviewer).

PR: https://git.openjdk.org/jdk/pull/11162