8370502: C2: segfault while adding node to IGVN worklist

[krk]

Do not try to replace fallthrough_memproj when it is null, fixes crash.

Test case is simplified from the ticket. Verified that the case crashes without the fix.

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Issue

• JDK-8370502: C2: segfault while adding node to IGVN worklist (Bug - P3)

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[bridgekeeper]

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[openjdk]

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[openjdk]

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[krk]

/issue 8370502

[openjdk]

@krk The issue 8370502 was not found in the JDK project - make sure you have entered it correctly.
As there were validation problems, no additional issues will be added to the list of solved issues.

[krk]

/issue JDK-8370502

[openjdk]

@krk The issue 8370502 was not found in the JDK project - make sure you have entered it correctly.
As there were validation problems, no additional issues will be added to the list of solved issues.

[mlbridge]

Webrevs

• 01: Full - Incremental (e8699d79)
• 00: Full (80111b97)

[mhaessig]

Thank you for working on this, @krk. And nice job reducing the test further!

I have a few questions and style comments below.

[mhaessig]

The indentation in Java files should be 4 spaces.

[krk]

I followed another test's format style. Now I can see that there is a mixture of 2 and 4 space Test*.java files.

Fixing.

[mhaessig]

Do I understand correctly, that mem is never nullptr because UnlockNode is a subclass of a SafepointNode which always has a memory input?

[krk]

UnlockNode is created at

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

Line 3542 in b3acc48

unlock->init_req( TypeFunc::Memory , memory(raw_idx) );

and will assert under GraphKit:memory if memory is somehow null.

[mhaessig]

Why do we not have to hook up the memory input to the fall through projection if it does not exist in the first place?

[krk]

Could you clarify the question?

[mhaessig]

The code before assumed that fallthrough_memproj is always not null. So does the rest of the code also expect this? If so, then we should perhaps use mem for that purpose. This is related to @dean-long's question below. There is an invariant that is being violated and your fix should take into account why it is violated.

[dean-long]

Why is fallthrough_memproj null, and why is this an issue only for expand_unlock_node but not expand_lock_node or other code that tries to replace fallthrough_memproj?

[TobiHartmann]

Drive-by comment: Please rename the test to something more descriptive. We don't use bug numbers for test names anymore.

[TobiHartmann]

Also, +1 to Dean's question. We need a better understanding of how we ended up in this situation.