Integrated: 8371581: C2: PhaseCCP should reach fixpoint by revisiting deeply-Value-d nodes

[Aleksey Shipilev]

On Thu, 13 Nov 2025 10:49:14 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 (!)

This pull request has now been integrated.

Changeset: 99be0e73
Author:    Aleksey Shipilev <shade at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/99be0e73ce9779e85c9ec6598e0a7ce964d62e82
Stats:     66 lines in 2 files changed: 45 ins; 1 del; 20 mod

8371581: C2: PhaseCCP should reach fixpoint by revisiting deeply-Value-d nodes

Reviewed-by: epeter, vlivanov, qamai

-------------

PR: https://git.openjdk.org/jdk/pull/28288