RFR: 8357726: C2 fails to recognize the counted loop when induction variable range is changed multiple times

[Emanuel Peter]

On Tue, 3 Jun 2025 01:48:47 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

>> test/micro/org/openjdk/bench/vm/compiler/CountedLoopCastIV.java line 54:
>> 
>>> 52:         Random r = new Random();
>>> 53:         start = r.nextInt(LEN >> 2);
>>> 54:         limit = r.nextInt(LEN >> 1, LEN - 3);
>> 
>> Does this not mean that we use a different seed every time, and therefore the loop has different lengths, and so the results can be influenced accordingly?
>
> Yes, I just want to make sure the loop length is different with each time running, so that it will not be influenced by some profiling related optimizations.

I see. Did you have any direct issues with profiling here?

I'm worried that the trip count is really very variant here. We could have:

start = 0 and limit = LEN - 3      -> count ~ LEN
start = LEN/4 and limit = LEN/2  -> count = LEN/4


That is a 4x variance, am I right? Can you run the benchmark a few times, and see what the error term looks like?

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PR Review Comment: https://git.openjdk.org/jdk/pull/25539#discussion_r2122954605