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

[Chen Liang]

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.

Created https://bugs.openjdk.org/browse/JDK-8347645 for you. The causes are #2776 and #4136.

I think the right way forward is to remove XorI/LNode::Value and move the code to their add_ring, which has lower priority than constant folding, instead. However I am not a hotspot/JIT engineer, so don't take my words for granted.

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PR Comment: https://git.openjdk.org/jdk/pull/23089#issuecomment-2589292750