8373525: C2: assert(_base == Long) failed: Not a Long

[dafedafe]

Issue

Olivier's fuzzer found a test that makes C2 crash while running the optimization that collapses the addition with overflow-protection (fold_subI_no_underflow_pattern).

Causes

The crash happens because during fold_subI_no_underflow_pattern the first input of the AddL node (see comment below) becomes top.

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

Lines 1525 to 1533 in 82b04f0

	// x con2
	// | |
	// AddL clamp2
	// | |
	// Max/MinL con1
	// | |
	// AddL clamp1
	// | |
	// Max/MinL (n)

This happens because of a whole IfFalse subgraph that dies and nodes are being removed. AddL is not removed immediately as it has another input which is still alive but it is put in the IGVN worklist instead.

Unfortunately the fold_subI_no_underflow_pattern optimization runs before the next GVN pass and triggers the assert.

Fix

fold_subI_no_underflow_pattern should actually take into account that we could have the graph in such a state and that x could be top. So, the sensible fix is not to presume x to be of type long and bailout if it is not.

Testing

Tier 1-3+
(also checked for new regression test failure before the change)

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8373525: C2: assert(_base == Long) failed: Not a Long (Bug - P3)

Reviewers

• Manuel Hässig (@mhaessig - Committer)

Reviewing

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[bridgekeeper]

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[openjdk]

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[mlbridge]

Webrevs

• 01: Full - Incremental (3219ce4f)
• 00: Full (20fe00fd)

[mhaessig]

Thank you for fixing this, @dafedafe. The fix looks good. I only have some minor comments on the test.

[mhaessig]

Thanks for addressing my comments. Looks good to me.

[chhagedorn]

Otherwise, looks good to me, too, thanks!

[chhagedorn]

I suggest to to add a comment about when x_long is not a long as described in the PR description.

[chhagedorn]

Should be main to allow to pass flags in.

Suggested change

	* @run driver ${test.main.class}
	* @run main ${test.main.class}