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I'm wondering if the Java compiler will optimize the run of the for-loop if there no changes within the for-loop body?

For instance, let's suppose I have the following code:

double res = 0;
for (int i = 0; i < 1000; i++) {
    res = calculateResValue();
}

Will the compiler shortcut the for-loop into a single calculateResValue() call?

I tried to modify the upper bound of the iteration counter, and I noticed that the time spent within for loop doesn't change proportionally to the boundary increase, i.e. let's suppose I run the loop not for 1000 times but 10000 times. I expect the timing to be increased in 10 times, but it doesn't happen.

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    The compiler: no. The JIT: perhaps. Commented Oct 10, 2024 at 12:50
  • 3
    Suppose calculateResValue() modifies some external state. Would you want it to be optimized away? Commented Oct 10, 2024 at 13:00
  • @FedericoklezCulloca not really. Actually, I want it to show the fair time execution. I'm running code as a JUnit unit test, and I think it can use multithreading under the hood, so the real time of the execution could be compromised Commented Oct 10, 2024 at 13:23
  • My guess is that you are running into Loop Invariant Code Motion. In this case, if a calculation within a loop doesn't change with each iteration, the compiler might move it outside. Have a look at the AI Overview -> type java for loop compiler optimization into Google. Commented Oct 10, 2024 at 14:50
  • 6
    "single calculateResValue() call" - the method must be called that many times, the (javac) compiler does not really know what the method is doing (or it does not analyze it) - it cannot decide if calling once will be a good replacement for all calls (the developer should know that ... ) Also check this question and its answer: How do I write a correct micro-benchmark in Java? Commented Oct 10, 2024 at 14:53
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