RFR: 8347645: C2: XOR bounded value handling blocks constant folding [v44]

[Vladimir Ivanov]

On Mon, 24 Mar 2025 18:29:34 GMT, Johannes Graham <duke at openjdk.org> wrote:

>> An interaction between xor bounds optimization and constant folding resulted in xor over constants not being optimized. 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 change moves logic from the `Xor(L|I)Node::Value` methods into the `add_ring` methods, and gives priority to constant-folding. A static method was separated out to facilitate direct unit-testing. It also (subjectively) simplified the  calculation of the upper bound and added an explanation of the reasoning behind it.
>> 
>> In addition to testing for constant folding over xor, IR tests were added to `XorINodeIdealizationTests` and `XorLNodeIdealizationTests` to cover these related items:
>>  - Bounds optimization of xor
>>  - A check for  `x ^ x = 0`
>>  - Explicit testing of xor over booleans.
>> 
>> Also `test_xor_node.cpp` was added to more extensively test the correctness of the bounds optimization. It exhaustively tests ranges of 4-bit numbers as well as at the high and low end of the affected types.
>
> Johannes Graham has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Undo accidental changes to Int tests

Overall, looks good. Very nice unit test!

2 comments:
  * naming: why not simply `xor_upper_bound` instead of `calc_xor_upper_bound_of_non_neg`?
  * ceremony around exposing `calc_xor_upper_bound_of_non_neg` to gtest: have you tried to move it under `src/share/hotspot/utilities/` and include from both `addnode.cpp` and `test_xor_node.cpp`.

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PR Review: https://git.openjdk.org/jdk/pull/23089#pullrequestreview-2726627039