RFR: 8315066: Add unsigned bounds and known bits to TypeInt/Long [v20]

[Quan Anh Mai]

On Thu, 19 Sep 2024 09:38:59 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 comments, refactor functions to helper class
>
> src/hotspot/share/opto/rangeinference.cpp line 104:
> 
>> 102:     // This means that the first bit that does not satisfy the bit requirement
>> 103:     // is a 0 that should be a 1, this may be the first different bit we want
>> 104:     // to find.
> 
> I would say something like this.
> `The msb violation is a one_violation - i.e. it is 0 instead of 1. Intuitively, we want to iteratively add 1 to the number, until that violating bit turns from 0 to 1. This is the lowest number that achieves to remove that one_violation. Note, that for this number all higher bits remain the same, the bit itself is 1, and the lower bits must be all zero.`

It would be tempting to do that, but it would make the second case harder to understand. The explanation I settled on makes the 2 cases similar, which I hope would help understand the whole picture more easily.

In short, since `result > lo`, there must be a bit position `i` such that `result[i] = 1` and `lo[i] = 0` and all bits before `i` would be the same in `result` and `lo`. In both cases, we find a position `j` such that `result[j] != lo[j]`, which leads to `i >= j`; and prove that `j` leads to an acceptable result.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17508#discussion_r1766936906