RFR: 8278857: C2: optimize (x << 2) & -4 to x (and similar patterns) -> KnownBits for LShift [v2]

[Ashay Rane]

On Thu, 26 Feb 2026 17:23:38 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Ashay Rane has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Addressed review comments
>>   
>>   1. Elided redundant check when type is known to not be `Type::BOTTOM`.
>>   
>>   2. Used `RangerInference` to AND the shift amount with 31 or 63 (based
>>      on the basic type) to match the Java shift semantics so that we have
>>      a higher chance of triggering the optimization.
>>   
>>   3. Added a `type_width()` function that can work with `uintn_t<N>`
>>      types, in addition to other unsigned types like `juint` and `julong`.
>>   
>>   4. Updated `RangeInference::infer_lshift()` to now accept masked shift,
>>      which works correctly because the caller (`LShiftINode::Ideal()`)
>>      ANDs the shift amount with 31 or 63.
>
> Thanks for extracting this into a separate PR, it will be much clearer for us to review and much less overwhelming for you to address all of the comments :)

@merykitty Thanks for taking a look and for your suggestions.  The code is so much cleaner now and I learnt a number of things related to the type system.  I've added a new commit that incorporates your suggestions, but let me know if you'd like to see more changes.  Thanks!

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

PR Comment: https://git.openjdk.org/jdk/pull/29898#issuecomment-3974732615