RFR: 8364407: [REDO] Consolidate Identity of self-inverse operations

[Hannes Greule]

On Mon, 18 Aug 2025 16:40:11 GMT, Manuel Hässig <mhaessig at openjdk.org> wrote:

>> The previous approach was flawed for `short` and `char` as these are int-subtypes and truncate the result (see the backout issue https://bugs.openjdk.org/browse/JDK-8364409 for a reproducer).
>> 
>> This change now first ensures that the input type is small enough so no truncation gets lost when dropping the operations.
>> 
>> The previous implementation also used an `InvolutionNode` superclass with one `Identity(...)` implementation, but there were some reservations whether this is the right way to go. As we now have a `ReverseBytesNode`, there is also less benefit in having the supertype, as this covers 4 in 1 already.
>> 
>> I also added test cases on top of the original ones that ensure the nodes stay when we can't prove the input type is small enough.
>
> src/hotspot/share/opto/subnode.cpp line 2080:
> 
>> 2078:     if (type == nullptr || involution->bottom_type()->is_int()->contains(type)) {
>> 2079:       return involution->in(1)->in(1);
>> 2080:     }
> 
> Instead of skipping the optimization, could you "clean" `involution->in(1)->in(1)` using `mask_int_value()`? That would follow the semantics of [JVMS§6.5 `ireturn`](https://docs.oracle.com/javase/specs/jvms/se24/html/jvms-6.html#jvms-6.5.ireturn).

I think this would need to happen in `Ideal()` then instead. Doing that would work too, but it further complicates things with little benefits imo.

> test/hotspot/jtreg/compiler/c2/gvn/InvolutionIdentityTests.java line 1:
> 
>> 1: /*
> 
> Why are you not testing involution on `NegL/I` nodes? Can this not be optimized?

`NegL/I` aren't used currently (see e.g. https://bugs.openjdk.org/browse/JDK-8262346).

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

PR Review Comment: https://git.openjdk.org/jdk/pull/26823#discussion_r2282958185
PR Review Comment: https://git.openjdk.org/jdk/pull/26823#discussion_r2282962673