8279837: C2: assert(is_Loop()) failed: invalid node class: Region

[chhagedorn]

In the test case, we are attempting to apply iteration_split_impl() to a loop head which is a RegionNode and not a LoopNode. We then fail with an assertion when attempting to cast it.

This is quite an edge case. build_loop_tree() is initially adding a NeverBranch to an infinite inner loop. Afterwards, beautify_loop() makes progress on some other loop nest where the inner and outer loop share the same loop head. As a result, build_loop_tree() is called again:

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

Lines 4184 to 4189 in 2bbeae3

	if( _ltree_root->_child->beautify_loops( this ) ) {
	// Re-build loop tree!
	_ltree_root->_child = NULL;
	_nodes.clear();
	reallocate_preorders();
	build_loop_tree();

Through Root -> Halt -> CProj -> NeverBranch, the infinite loop is now reachable and is built as child of _ltree_root. However, the Region is not yet a Loop node. This will only happen in the next iteration of loop opts when beautify_loop() is called again. We then fail with the assertion assuming that loop tree head nodes are always LoopNodes. Some more details can be found in the comments of the test.

The fix is straight forward to bail out of iteration_split_impl() for non-LoopNodes. We already do a similar bailout in other optimizations like loop predication:

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

Lines 1427 to 1430 in 2bbeae3

	if (!loop->_head->is_Loop()) {
	// Could be a simple region when irreducible loops are present.
	return false;
	}

Thanks,
Christian

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Issue

• JDK-8279837: C2: assert(is_Loop()) failed: invalid node class: Region

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[chhagedorn]

This should actually go into JDK 18 - I'm closing this and re-open a PR for the JDK 18 fork.