RFR: 8347645: C2: XOR bounded value handling blocks constant folding

[Emanuel Peter]

On Mon, 13 Jan 2025 22:16:20 GMT, Johannes Graham <duke at openjdk.org> wrote:

> C2 does not eliminate XOR nodes with constant arguments. This has a noticeable effect on `Long.expand` with a constant mask, on architectures that don't have instructions equivalent  to `PDEP` to be used in an intrinsic.
> 
> This patch demonstrates a potential fix to the problem, but there might well be better ways to do it.

Thanks for working on this!

I have not yet looked at the VM cpp changes yet.

Some comments about the test:
Please move it to:
`test/hotspot/jtreg/compiler/c2/gvn/TestXor.java`

The comments sometimes mention `c3` etc, but there may only be a `c` or `x`. Please fix them ;)

The tests should also do result verification. Currently you only check that we have the expected nodes, but constant folding could have bugs we would not catch this way. What I usually do:
Compute some `GOLDEN`, which should be computed in interpreter, and then with a `@Check` method you can compare the result to that `GOLDEN` value.

Plus: it would be nice if the constants could be picked at random. You can do that with a `public static final int CON = random_value`.

Best would be if you could use the new `Generators`, see
`./test/hotspot/jtreg/compiler/lib/generators/Generators.java`

Let me know if you need any more help with that.

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PR Review: https://git.openjdk.org/jdk/pull/23089#pullrequestreview-2566427884