8367627: C2: Missed Ideal() optimization opportunity with MemBar

[benoitmaillard]

This PR addresses a missed optimization in PhaseIterGVN for MemBarAcquire nodes caused by a missing notification during parsing.

The missed optimization in question is the removal of the the in(MemBarNode::Precedent) edge for
MemBarAcquire nodes when the the MemBar is the only user of its input. This was intially
introduced to get rid of unused Load nodes that were only kept alive by such an edge.

jdk/src/hotspot/share/opto/memnode.cpp

Lines 4254 to 4259 in eeb7c3f

	Node *MemBarNode::Ideal(PhaseGVN *phase, bool can_reshape) {
	if (remove_dead_region(phase, can_reshape)) return this;
	// Don't bother trying to transform a dead node
	if (in(0) && in(0)->is_top()) {
	return nullptr;
	}

In our case, it happens that the Load node gets folded to a constant during the initial
_gvn.transform call in GraphKit::make_load. Because the value is converted before being
returned, we end up with two constant nodes: one ConL and one ConI. The ConL only
has one usage, and this triggers the optimization during verification.

static int test0() {
    var c = new MyClass();
    // the conversion ensures that the ConL node only has one use
    // in the end, which triggers the optimization
    return (int) c.l;
}

The optimization is not triggered earlier during when we apply _gvn.transform on the membar,
because it requires can_reshape, which is set to false in when we call apply_ideal in
PhaseGVN::transform.

For this reason, we should call record_for_igvn(membar) after the MemBar is created
and transformed in GraphKit::insert_mem_bar to make sure it gets an Ideal pass with
can_reshape later.

This issue was initially filed for Valhalla, because a condition in GraphKit::make_load
prevents its from occurring when boxing elimination is enabled. Boxing elimination is
disabled temporarily in Valhalla (see JDK-8328675),
which caused the issue to appear, but by using -XX:-EliminateAutoBox it became clear
that the issue was on mainline.

Testing

• GitHub Actions
• tier1-3, plus some internal testing

Thank you for reviewing!

---

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Issue

• JDK-8367627: C2: Missed Ideal() optimization opportunity with MemBar (Bug - P4)

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