RFR: 8375653: C2: CmpUNode::sub is not monotonic [v5]

[Marc Chevalier]

On Thu, 22 Jan 2026 15:01:36 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Hi,
>> 
>> This PR fixes the issue that `CmpUNode::sub` is not monotonic. The root cause is that it returns different values for several cases, but the cases are not mutually exclusive and the return values are not a subset of each other. This leads to the possibilities that a node satisfying both cases will return the first value, but if upon being widen it ceases to satisfy the first case but still satisfies the second case, the method will return the second value, which is not a superset of the previous result.
>> 
>> For example, given `r = CmpU(x, y)`.
>> 
>> At the first iteration, `type(x) = {0}` and `type(y) = {1, -1}`, then `CmpUNode::sub` returns `TypeInt::CC_LE` since it sees that `x` is the constant `0`.
>> 
>> At the second iteration, `type(x) = {0, 2}` and `type(y) = {-1, 1}`, then `CmpUNode::sub` returns `TypeInt::CC_NE` since it sees that `x` and `y` do not overlap. This is not a superset of `TypeInt::CC_LE`, which leads to an assertion.
>> 
>> Please take a look and leave your reviews, thanks a lot.
>
> Quan Anh Mai has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Improve IR tests

Seems good, sound, and as precise as we can be. I just have a cosmetic detail.

src/hotspot/share/opto/subnode.cpp line 749:

> 747: // If both inputs are constants, compare them.
> 748: const Type* CmpUNode::sub(const Type* t1, const Type* t2) const {
> 749:   const TypeInt *r0 = t1->is_int();

We prefer `TypeInt* r0`. Same under.

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Marked as reviewed by mchevalier (Committer).

PR Review: https://git.openjdk.org/jdk/pull/29308#pullrequestreview-3696381918
PR Review Comment: https://git.openjdk.org/jdk/pull/29308#discussion_r2720237259