RFR: 8347499: C2: Make `PhaseIdealLoop` eliminate more redundant safepoints in loops [v2]

[Qizheng Xing]

On Wed, 2 Apr 2025 07:22:13 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> The second question:
>> 
>>> If we now removed safepoints in places where we would actually have needed them: how would we find out? I suppose we would get longer time to safepoint - higher latency in some cases. How would we catch this with our tests?
>> 
>> I tried running tier1 tests with `JAVA_OPTIONS=-XX:+UnlockDiagnosticVMOptions -XX:+SafepointTimeout -XX:+AbortVMOnSafepointTimeout -XX:SafepointTimeoutDelay=1000`, and there were no failures.
>> 
>> Running with `-XX:SafepointTimeoutDelay=500` caused 1 random JDK test case to fail. But then I tried to build a JDK without this patch, and it still had the random failure with this option.
>
> @MaxXSoft Would you mind improving the documentation comments, so that they are easier to understand? Maybe you can even add more comments around your code change, to "prove" why it is ok to do what we would do with your change?

Hi @eme64, this PR is now ready for further reviews. Could you please continue to review this PR?

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PR Comment: https://git.openjdk.org/jdk/pull/23057#issuecomment-3087162955