RFR: 8359603: Missed optimization in PhaseIterGVN for redundant ConvX2Y->ConvY2X->ConvX2Y sequences due to missing notification in PhaseIterGVN::add_users_of_use_to_worklist [v4]

[Christian Hagedorn]

On Mon, 28 Jul 2025 12:35:15 GMT, Benoît Maillard <bmaillard at openjdk.org> wrote:

>> This PR addresses a missed optimization in `PhaseIterGVN` due to the lack of change notification to indirect users within `PhaseIterGVN::add_users_of_use_to_worklist` (again). This is similar to [JDK-8361700](https://bugs.openjdk.org/browse/JDK-8361700?filter=-1).
>> 
>> The optimization in question is the removal of redundant `ConvX2Y->ConvY2X->ConvX2Y` sequences (where `X` and `Y` are primitive number types), which get replaced by a single `ConvX2Y` as an identity optimization. This missing optimization was originally reported only for `ConvD2LNode`, but it turns out that other conversion nodes have analog optimization patterns. After manual inspection of identity optimizations in `convertnode.cpp`, I was able to reproduce missing optimizations for the following conversion sequences:
>> - `ConvD2L->ConvL2D->ConvD2L`
>> - `ConvF2I->ConvI2F->ConvF2I`
>> - `ConvF2L->ConvL2F->ConvF2L`
>> - `ConvI2F->ConvF2I->ConvI2F`
>> 
>> Similar optimization patterns exist for additional conversion nodes. However, it is not clear if these nodes are subject to the same missed optimization issue. Further investigation may be needed, as I was unable to reproduce such cases with simple tests.
>> 
>> This is again a case where an optimization depends on the input of its input. Currently, `PhaseIterGVN::add_users_of_use_to_worklist` contains specific logic to handle similar dependencies for other cases, but this specific scenario is not addressed. The proposed fix adds the necessary logic in `add_users_of_use_to_worklist` to ensure proper notification for this optimization pattern. 
>> 
>> ### Testing
>> - [x] [GitHub Actions](https://github.com/benoitmaillard/jdk/actions?query=branch%3AJDK-8359603)
>> - [x] tier1-3, plus some internal testing
>> - [x] Added test from the fuzzer, and tests for other sequences (manually derived from the original one)
>> 
>> Thank you for reviewing!
>
> Benoît Maillard has updated the pull request incrementally with one additional commit since the last revision:
> 
>   8359603: Add note

Update looks good, thanks!

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

Marked as reviewed by chagedorn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/26368#pullrequestreview-3062939535