RFR: 8271340: Crash PhaseIdealLoop::clone_outer_loop

[Vladimir Kozlov]

On Mon, 23 Aug 2021 14:46:15 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> The test has a counted loop and an infinite loop. The infinite loop is
> reachable from multiple paths and its loop head is a Region with more
> than 3 inputs. One of these paths is from the counted loop. When loop
> opts run, a NeverBranch is added to the infinite loop that's removed
> by NeverBranchNode::Ideal() next because in(0) of the NeverBranch is a
> Region and not a Loop.
> 
> When CCP runs, it finds the counted loop exit is never reached because
> a test in the loop body that depends on a loop phi is never taken. As
> a consequence nodes along the path from the counted loop end to the
> infinite loop Region have type top. One of these nodes is a Call
> node. PhaseCCP::do_transform() should then cause the path between the
> counted loop and the infinite loop to optimize out but that doesn't
> happen because do_transform() starts from Root by following inputs.
> The dead path is only reachable from the infinite loop but the there's
> no edge between Root and the infinite loop.
> 
> IGVN next runs, processes the Call Node, finds it's dead, kills
> everything around it which causes the OuterStripMinedLoopEnd to loose
> a projection. That later triggers the crash.
> 
> The fix I propose is to be more conservative in
> NeverBranchNode::Ideal() and to check for a in(0) that's a Region. As
> a consequence, at CCP time, the infinite loop is reachable from Root.
> 
> This change also requires some adjustments to Shenandoah specific code
> that makes assumptions about the shape of infinite loops.

src/hotspot/share/opto/cfgnode.cpp line 2720:

> 2718: // Check for no longer being part of a loop
> 2719: Node *NeverBranchNode::Ideal(PhaseGVN *phase, bool can_reshape) {
> 2720:   if (can_reshape && !in(0)->is_Region()) {

I assume this is the actual fix.

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PR: https://git.openjdk.java.net/jdk/pull/5220