RFR: 8347273: C2: VerifyIterativeGVN for Ideal and Identity [v5]

[Christian Hagedorn]

On Tue, 10 Jun 2025 12:16:13 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> **Past Work**
>> With https://github.com/openjdk/jdk/pull/11775 / [JDK-8298952](https://bugs.openjdk.org/browse/JDK-8298952) we added `Node::Value` verification.
>> 
>> **This PR**
>> I'm now adding verification for `Ideal` and `Identity`. I'm adding two bits to the flag `VerifyIterativeGVN`.
>> 
>> I found many many  node types that hit my verification assert, i.e. that could still be optimized after IGVN is over, just because these nodes were not put on the worklist any more.
>> 
>> My approach was to aggressively bail-out for all nodes that had an issue. This way, we can address one by one in follow-up RFEs. For many, I did some initial assessment, and left some comments about what issues I encountered.
>> 
>> **Future Work:**
>> In many cases, the issue is just a missing notification when inputs of inputs are changed. These would be good starter tasks. But there are probably also more complicated cases. And there are surely cases where verification will be impossible, because it is possible that the Idea / Identity optimizations traverse longer paths, and we cannot expect that notification makes it down that path. For those cases, we will have to leave the exception and document it well.
>> 
>> I filed:
>> [JDK-8359103](https://bugs.openjdk.org/browse/JDK-8359103) C2 VerifyIterativeGVN: Umbrella for extending Ideal and Identity verification (JDK-8347273)
>> (We can file subtasks for the nodes we want to fix. I don't want to file them all now, but we should file them as we are investigating, so that there is no duplicate work.)
>> 
>> Testing passed tier1-3, with extra timeout factor 20.
>
> Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:
> 
>   use stringStream instead of ttyLocker

src/hotspot/share/runtime/flags/jvmFlagConstraintsCompiler.cpp line 303:

> 301: JVMFlag::Error VerifyIterativeGVNConstraintFunc(uint value, bool verbose) {
> 302:   uint original_value = value;
> 303:   for (int i = 0; i < 4; i++) {

You might want to consider adding a `const int max_modes = 4` or something like that and use it also below in the error message.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22970#discussion_r2137843279