RFR: 8375653: C2: CmpUNode::sub is not monotonic

[Dean Long]

On Tue, 20 Jan 2026 02:42:41 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.

src/hotspot/share/opto/subnode.cpp line 758:

> 756:   // (This is a gross hack, since the sub method never
> 757:   // looks at the structure of the node in any other case.)
> 758:   if (r0->_lo >= 0 && r1->_lo >= 0 && is_index_range_check()) {

Do we still need this after improvements like JDK-8356813?

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PR Review Comment: https://git.openjdk.org/jdk/pull/29308#discussion_r2715266363