[jdk16] Integrated: 8260709: C2: assert(false) failed: unscheduable graph

[Roland Westrelin]

On Tue, 2 Feb 2021 14:53:15 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> The v = field load in the test case is found anti-dependent with the
> memory phi that merges the exception state of the 2 array
> allocation. Since JDK-8258393, anti-dependence computation only
> considers the Phi inputs that are reachable from the memory input of a
> load. As a consequence, the late control for the load is the control
> projection of the array allocation in the loop. When loop opts run,
> PhaseIdealLoop::split_if_with_blocks_post() finds that the load's late
> control is different from its current control (which is inside the
> outer loop). It tries to sink the load out of loop but ends up pinning
> it at its late control, the projection of the second AllocateNode.
> 
> The logic that expands the AllocateNode doesn't expect a pinned node
> on the control projection and the result is a broken graph. I think
> the fix for this would be to clone the load along both the exception
> and the fallthrough paths. But as noted in JDK-8252372, the whole
> process of sinking loads out of loops doesn't seem to work as expected
> (for instance in this case it sinks the load from the outer loop into
> the inner loop). So instead of going with a complicated fix, I propose
> simply to detect this corner and that no attempt be made to sink the
> load. Note that the current logic computes the late control for the
> load (which should be in the loop), will create a clone for each use
> and assign the dom_lca of the use's control and the load late control
> to that use, that is the load late control. So all uses end up at the
> same location, the load late control. So to detect that case, it's
> sufficient to test the load late control.

This pull request has now been integrated.

Changeset: 4de3a6be
Author:    Roland Westrelin <roland at openjdk.org>
URL:       https://git.openjdk.java.net/jdk16/commit/4de3a6be
Stats:     70 lines in 3 files changed: 69 ins; 0 del; 1 mod

8260709: C2: assert(false) failed: unscheduable graph

Reviewed-by: thartmann, chagedorn

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PR: https://git.openjdk.java.net/jdk16/pull/144