RFR: 8374180: C2 crash in PhaseCCP::verify_type - fatal error: Not monotonic [v3]

[Hannes Greule]

On Thu, 8 Jan 2026 08:52:32 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Hi,
>> 
>> The issue here is the inconsistency in computing the `_widen` field of the `TypeInt`. At the first step, the types of the operands are:
>> 
>>     t1 = int:0
>>     t2 = int:-2..3, widen = 3
>> 
>> Since the type of the first operand is a constant zero, `AddNode::Value` returns the type of the second operand directly, as `x ^ 0 == x for all x`. In the second step, `t1` is widened to `0..2`. This triggers the real computation of the result. The algorithm then splits `t2` into `t21 = int:-2..-1` and `t22 = int:0..3`. The `Xor` of these with `t1` are `r1 = int:-4..-1` and `r2 = int:0..3`. As both have `_hi - _lo <= SMALL_TYPEINT_THRESHOLD == 3`, their `_widen`s are normalized to `0`. As a result, their `meet` also has `_widen == 0`. This value is smaller than that from the previous step, which was `3`, which leads to the failure.
>> 
>> The root cause here is that, the `_widen` value of a node should be computed and normalized on the whole range of the node, not on its subranges, which may normalize it to `0` in more cases than what is expected. As a result, my proposed solution is to ignore the `_widen` value of the subranges, and pass the expected `_widen` value when composing the final result.
>> 
>> Please take a look and leave your reviews, thanks a lot.
>
> Quan Anh Mai has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:
> 
>  - Merge branch 'master' into widen
>  - copyright year
>  - Merge branch 'master' into widen
>  - RangeInference::infer should ensure correct value of _widen

The change looks good. I wonder if it's worth to add a comment about it somewhere?

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Marked as reviewed by hgreule (Committer).

PR Review: https://git.openjdk.org/jdk/pull/28952#pullrequestreview-3654272073