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

[Emanuel Peter]

On Wed, 4 Sep 2024 19:50:01 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Hi,
>> 
>> This patch adds unsigned bounds and known bits constraints to TypeInt and TypeLong. This opens more transformation opportunities in an elegant manner as well as helps avoid some ad-hoc rules in Hotspot.
>> 
>> In general, a TypeInt/Long represents a set of values x that satisfies: x s>= lo && x s<= hi && x u>= ulo && x u<= uhi && (x & zeros) == 0 && (~x & ones) == 0. These constraints are not independent, e.g. an int that lies in [0, 3] in signed domain must also lie in [0, 3] in unsigned domain and have all bits but the last 2 being unset. As a result, we must normalize the constraints (tighten the constraints so that they are optimal) before constructing a TypeInt/Long instance.
>> 
>> This is extracted from #15440 , node value transformations are left for later PRs. I have also added unit tests to verify the soundness of constraint normalization.
>> 
>> Please kindly review, thanks a lot.
>> 
>> Testing
>> 
>> - [x] GHA
>> - [x] Linux x64, tier 1-4
>
> Quan Anh Mai has updated the pull request incrementally with one additional commit since the last revision:
> 
>   fix build

Just a very quick scan this time.

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?

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?

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

Changes requested by epeter (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/17508#pullrequestreview-2283269454
PR Review Comment: https://git.openjdk.org/jdk/pull/17508#discussion_r1745673844
PR Review Comment: https://git.openjdk.org/jdk/pull/17508#discussion_r1745669003