RFR: 8315066: Add unsigned bounds and known bits to TypeInt/Long [v12]

[Quan Anh Mai]

On Thu, 5 Sep 2024 14:38:20 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:
>> 
>>   fix build
>
> src/hotspot/share/opto/rangeinference.hpp line 46:
> 
>> 44:  * Bits that are known to be 0 or 1. A value v satisfies this constraint iff
>> 45:  * (v & zeros) == 0 && (~v & ones) == 0. I.e, all bits that is set in zeros
>> 46:  * must be unset in v, and all bits that is set in ones must be set in v.
> 
> That is quite counter-intuitive. Is there a good reason for this?
> I would have expected that `zero[i] = 1` would mean that a zero is allowed, and `ones[i] = 1` that a one is allowed.
> 
> Basically, when I see `zero[i]` I expect it to be a boolean that answers me this question: "can it be a zero?". But you are telling me I'm supposed to ask "Must it not be a zero"?
> 
> You are telling me that `zero[i] = 1` and `ones[i] = 0` means that it must be a `1`.
> 
> I know that changing it now would be a lot of effort. But the risk of being unintuitive is that even less people can quickly fix bugs in this code.
> 
> @vnkozlov what do you think about this?

You are a bit confused, the `zeros` and `ones` answer the question: Must this bit is 0 (or 1). Which means that `zero[i] = 1` means that the bit must be a `0`.

> src/hotspot/share/opto/type.hpp line 564:
> 
>> 562:   // Dual sets are only used to compute the join of 2 sets, and not used
>> 563:   // outside.
>> 564:   const bool _is_dual;
> 
> Do you have a clear definition of what a dual is somewhere?

Tbh I'm not entirely sure what does "dual" serve other than: it is to compute the join of 2 sets. Looking at the code suggests that a `Type` is somehow a symmetric space with all normal types on one side and the "dual" types on the other. This really confuses me and I cannot figure out what exactly a dual type represents.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17508#discussion_r1745773937
PR Review Comment: https://git.openjdk.org/jdk/pull/17508#discussion_r1745777943