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

[Hannes Greule]

On Mon, 18 Aug 2025 15:49:48 GMT, Hannes Greule <hgreule 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.

test/hotspot/jtreg/compiler/c2/gvn/InvolutionIdentityTests.java line 211:

> 209: 
> 210:     @Test
> 211:     @IR(counts = {IRNode.REVERSE_BYTES_S, "2"})

I'm not sure if this is fine. The intrinsics might not apply to all platforms, in which case this would fail I think?

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PR Review Comment: https://git.openjdk.org/jdk/pull/26823#discussion_r2282799804