RFR: 8372302: C2: IGVN verification fails because ModXNode::Ideal creates unused intermediate nodes [v3]

[Emanuel Peter]

On Fri, 12 Dec 2025 19:05:15 GMT, Benoît Maillard <bmaillard at openjdk.org> wrote:

>> This PR addresses a failure in IGVN verification with `ModI` and `ModL` nodes.
>> 
>> In `ModeXNode::Ideal`, we have code to optimize a modulo expression by expressing it in terms of other operations. There are actually two distinct cases, one where the divisor is a constant and is equal to `modulo 2^k-1` for some integer `k`, and a more general case where other transformations do not succeed. Because these transformations involve creating several new nodes (sometimes in a loop) and calling `phase->transform(...)` on them, we want to avoid accidentally triggering optimizations on the "unfinished" state of the subgraph. For this, we create a temporary dummy node and add edges to the nodes being constructed.
>> 
>> There are some execution paths where the node is not destroyed before `Ideal` returns, and this creates issues during IGVN verification, as the verification code checks if the number of nodes has changed after having called `Ideal` on a given node and not expecting changes.
>> 
>> The path in question is when we exit because the divisor is a constant and is the minimum value:
>> https://github.com/openjdk/jdk/blob/c19b12927d2ac901ec8ccaa2de5897ee4c47af56/src/hotspot/share/opto/divnode.cpp#L1146-L1147
>> 
>> The zero case does not cause problems (this seems to be because it would hide behind a `div0_check` anyway).
>> 
>> The fix is simply to only create the temporary node when it is needed, and thus avoiding returning without destroying it.
>> 
>> ### Testing
>> - [x] [GitHub Actions](TODO)
>> - [x] tier1-4, plus some internal testing
>> 
>> Thank you for reviewing!
>
> Benoît Maillard has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Update package
>  - Move to compiler/c2/igvn

Marked as reviewed by epeter (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/28488#pullrequestreview-3643323542