RFR: 8303279: C2: crash in SubTypeCheckNode::sub() at IGVN split if [v3]

[Tobias Hartmann]

On Thu, 29 Jun 2023 07:38:15 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> The crash occurs because at split if during IGVN, a `SubTypeCheck` is
>> created with null as input. That happens because the control path the
>> `SubTypeCheck` is cloned for is dead. To fix that I propose delaying
>> split if until dead paths are collapsed.
>> 
>> I added an assert to check a nullable first input to `SubTypeCheck`
>> nodes (which should be impossible because it should be null
>> checked). When I ran testing, a number of cases showed up with known
>> non null values non properly marked as non null. I fixed them.
>
> Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review

Looks good to me too.

test/hotspot/jtreg/compiler/splitif/TestCrashAtIGVNSplitIfSubType.java line 28:

> 26:  * @bug 8303279
> 27:  * @summary C2: crash in SubTypeCheckNode::sub() at IGVN split if
> 28:  * @run main/othervm -XX:-TieredCompilation -XX:-BackgroundCompilation -XX:+UnlockDiagnosticVMOptions -XX:+StressIGVN -XX:StressSeed=598200189 TestCrashAtIGVNSplitIfSubType

Maybe add a `@run` without a fixed seed to give this a chance to still trigger in the future.

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Marked as reviewed by thartmann (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/14678#pullrequestreview-1510392072
PR Review Comment: https://git.openjdk.org/jdk/pull/14678#discussion_r1250249977