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

[Roland Westrelin]

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.

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Commit messages:
 - test & fix

Changes: https://git.openjdk.java.net/jdk16/pull/144/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=144&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260709
  Stats: 70 lines in 3 files changed: 69 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/144.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/144/head:pull/144

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