RFR: 8371581: C2: PhaseCCP should reach fixpoint by revisiting deeply-Value-d nodes [v4]

[Aleksey Shipilev]

On Thu, 13 Nov 2025 19:04:56 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> I started this as investigation into one rare/intermittent CTW failure that I get with [JDK-8360557](https://bugs.openjdk.org/browse/JDK-8360557). The bug seems to reproduce on a very specific JAR with a very specific random seed, so no easy regression test.
>> 
>> At this point I believe we found that PhaseCCP does not reach the fix point for a peculiar reason: `LoadN` that looks deeply into the graph is not revisited and thus misses the chance to update its type. There is an exception for loads in `verify_Value_for`, but it seems to only apply to constants, and does not apply to `LoadN` in question. Revisiting `LoadN` shows that updating the types downstream performs type widenings (= current types are too narrow), which AFAICS says that this unsound analysis can lead to miscompilation. See more debugging breadcrumbs in the bug.
>> 
>> It looks like we can reach the fixpoint by recording the nodes we need to revisit and doing another CCP round. This also makes CCP verification stricter: we effectively move 2 exceptional cases recorded in `verify_Value_for` into the analysis itself.
>> 
>> Testing shows there are no ill effects on correctness doing this. But I would appreciate someone more savvy in this code to sanity check all of this.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 server fastdebug, CTW reproducer no longer fails
>>  - [x] Linux x86_64 server fastdebug, `all` tests pass
>>  - [x] Linux x86_64 server fastdebug, Maven Central CTW passes (!)
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:
> 
>  - Merge branch 'master' into JDK-8371581-ccp-spooky-nodes
>  - More comments
>  - More restrictive CmpP check
>  - Tighten up comments and signatures
>  - Do Value() once
>  - Fix

Thanks everyone! Here goes.

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PR Comment: https://git.openjdk.org/jdk/pull/28288#issuecomment-3569349918