RFR: 8315066: Add unsigned bounds and known bits to TypeInt/Long [v59]

[Hannes Greule]

On Fri, 2 May 2025 14:52:27 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Quan Anh Mai has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   add some more sanity static_asserts
>
> src/hotspot/share/opto/rangeinference.cpp line 378:
> 
>> 376:     // it directly without going through i.
>> 377:     //           0 0 1 0 0 0 0 0
>> 378:     U alignment = tmp & (-tmp);
> 
> Suggestion:
> 
>     // We now want to select the last one of these candidates, which is
>     // exactly the last index i upto first_violation such that lo[i] == zeros[i] == 0.
>     // In our example we have i == 2.
>     //           0 0 1 0 0 0 0 0
>     U alignment = alignment_candidates & (-alignment_candidates);
> 
> Ok, well what is still missing is how the bit trick with `-alignment_candidates` works here.
> 
> Let me try:
> 
>  alignment_candidates =  0 1 1 0 0 0 0 0
> -alignment_candidates =  1 0 1 0 0 0 0 0
> 
> Hmm yeah I'm not sure about this one....
> I suppose I would have tried this via trailing zeros, but not sure about it...
> 
> What is the argument here, why this extracts the last bit?

I think it gets easier when splitting the `-` into a `~` and a `+ 1`. After `~`, the trailing zeros become ones, and adding 1 has a cascading effect of producing zeros again that is stopped by the lowest zero bit (which is a one in the original value, and now is becoming a one again). Due to the `~`, all higher bits are flipped, so they are just becoming zeros when `&`-ing.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17508#discussion_r2071855928