RFR: 8364970: Redo JDK-8327381 by updating the CmpU type instead of the Bool type [v3]

[Francisco Ferrari Bihurriet]

On Wed, 13 Aug 2025 07:01:13 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

>> Hmm, I was oversimplifying the problem, my way of thinking it was the following one:
>> 
>> 
>> m    x  m    1
>>  \  /    \  /
>>  AndI    AddI       grandparents
>>     \    /
>>      CmpU              parent
>>       |
>>      Bool            grandchild
>> 
>> 
>> _"As we were updating a grandchild based on its grandparents, we needed an ad-hoc worklist push for the grandchild. Since we now update the type of `CmpU` based on its parents, the canonical parent-to-child propagations should work, and we don't need any ad-hoc grandparents-to-grandchild worklist push anymore."_
>> 
>> But as you noted, non-immediate `CmpU` inputs such as `m` or `1` can change and should affect the `CmpU` type. Luckily, this already was the case for previous `CmpU` optimizations.
>> 
>> ---
>> 
>> For case **1a**, we don't need `PhaseCCP::push_cmpu` because `m` is also an immediate input of `CmpU`.
>> 
>> 
>> m    x
>>  \  /
>>  AndI     m
>>     \    /
>>      CmpU
>>       |
>>      Bool
>> 
>> 
>> ---
>> 
>> I'm now realizing this was a very lucky situation. The `AndI` input isn't problematic even when `PhaseCCP::push_cmpu` doesn't handle the `use_op == Op_AndI` case, because:
>> 
>> * `x` does not affect the application of `Value_cmpu_and_mask()`
>> * In case **1a**, `m` is a direct input of `CmpU`
>> * In case **1b**, the `AddI` input is handled in `PhaseCCP::push_cmpu` (`use_op == Op_AddI`)
>> 
>> Please let me know if you think we should add a comment in the code.
>
> That's a good summary! Thanks for double-checking again. It's indeed only for **1b** a probably that's handled by `push_cmpu()`. It probably would not hurt to add a comment that `push_cmpu` handles this case, just to be sure.

Great, I added the comments in 32a7940b8eefe56c3aa603be65da7b32981d2ab7.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26666#discussion_r2275569858