RFR: 8371558: C2: Missing optimization opportunity in AbsNode::Ideal [v2]

[Roberto Castañeda Lozano]

On Wed, 12 Nov 2025 07:49:42 GMT, Benoît Maillard <bmaillard at openjdk.org> wrote:

>> This PR addresses another missed optimization in `PhaseIterGVN` due to a missing notification to indirect users within `PhaseIterGVN::add_users_of_use_to_worklist`.
>> 
>> The affected optimization is the transformation of `abs(0-x)` into `abs(x)`. This transformation is implemented in `AbsNode::Ideal`.
>> 
>> The bug was found by the fuzzer. At some point during IGVN, we have the following setup:
>> 
>> 
>> Phi  ... 
>>  \  /
>>  SubI
>>   |
>>  AbsI
>> 
>> 
>> The `Phi` node gets folded into a `ConI`, and we call `replace_node(phi, zero)`, which ends up calling `add_users_to_worklist(phi)`, and `add_users_of_use_to_worklist(phi, zero, ...)`. However the case for this specific notification was missing there, and the `AbsI` node is never notified (not added to the worklist).
>> 
>> This PR brings the following changes:
>> - Detect the optimization pattern in `add_users_of_use_to_worklist` for `AbsI`, `AbsL`, `AbsF` and `AbsD`
>> - Add new test `TestMissingOptAbsZeroMinusX.java`, initially obtained from the fuzzer and then heavily reduced, both with the usual tools and manually. In addition to `AbsI`, I have also added test cases for `AbsF` and `AbsD`, but was not able to reproduce for `AbsL` despite my best efforts.
>> 
>> ### Testing
>> - [x] [GitHub Actions](https://github.com/benoitmaillard/jdk/actions?query=branch%3AJDK-8371534)
>> - [ ] tier1-4, plus some internal testing
>> 
>> Thank you for reviewing!
>
> Benoît Maillard has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update test/hotspot/jtreg/compiler/c2/TestMissingOptAbsZeroMinusX.java
>   
>   Co-authored-by: Tobias Hartmann <tobias.hartmann at oracle.com>

Looks good!
At some point, we will have to think about some kind of system to enforce notification of indirect users "by construction", or at least make it possible to somehow detect when such notification may be needed.

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Marked as reviewed by rcastanedalo (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/28237#pullrequestreview-3452442885