RFR: 8357726: Improve C2 to recognize counted loops with multiple casts in trip counter

[Emanuel Peter]

On Wed, 4 Jun 2025 08:04:00 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>>> Thanks for your suggestion! Sounds better to me. How about changing the title to Improve C2 to recognize counted loops with multiple casts in trip counter ?
>> 
>> @XiaohongGong Sounds good too :)
>
> Hi @eme64 , I'v updated the IR test and JMH based on your comments. Could you please help review whether it's fine to you. Thanks for all your suggestion!
> 
> Following shows the performance data of the new JMH test on Grace (the performance gain is almost the same on my x64 machine):
> 
> Benchmark                      Mode   Cnt limit Unit    Before    Error (99.9%)    After     Error (99.9%) Gain
> CountedLoopCastIV.loop_iv_int  thrpt  30  1024  ops/s 1225620.536 39505.158362   5778120.132 4781.602088   4.71
> CountedLoopCastIV.loop_iv_int  thrpt  30  1536  ops/s  830600.832 14758.561182   3839404.338 3362.727083   4.62
> CountedLoopCastIV.loop_iv_int  thrpt  30  2048  ops/s  618114.174 36999.511727   2890853.495  416.969862   4.67
> CountedLoopCastIV.loop_iv_long thrpt  30  1024  ops/s 1063902.078  4616.608855   1314828.963 1267.470199   1.23
> CountedLoopCastIV.loop_iv_long thrpt  30  1536  ops/s  714538.178   630.085477    870801.472  753.347684   1.21
> CountedLoopCastIV.loop_iv_long thrpt  30  2048  ops/s  536724.086   131.313178    652775.363  539.107806   1.21
> 
> 
> The error term is larger as before. But I don't think this is caused by the large variance of loop iterations. Does the new benchmark look fine to you? Thanks!

@XiaohongGong Nice, thanks for the updates! Especially the IR rules and reduction in JMH benchmark variance, excellent :)

Please ping me again once you have addressed my comment above, and then I can run some internal testing for you!

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PR Comment: https://git.openjdk.org/jdk/pull/25539#issuecomment-2939057374