Unexpected performance of operator % vs &

[Scott Palmer]

I hope this is the appropriate list for this question.

Given the following Java code to test if a number is even or odd I observed
unexpected results.

boolean evenA = ((i % 2) == 0);

boolean evenB = ((i & 1) == 0);

I expect the bitwise AND to be the fastest, as modulo operations are
generally slower. The masking operation should never take more CPU cycles
than the modulo operation.
This in fact is true, but only until the code is JIT compiled, and then the
performance flips and the modulo version is notably faster.

This remains the case for checking if 'i' is evenly divisible by 4, using
(i % 4) vs (i & 3).  Only when I get to checking for divisibility by 8
using (i % 8) vs (i & 7) do I see the performance shift to masking's favour
after JIT compiling.

I suspect there is an optimization somewhere in the JIT compiler that sees
the modulo 2 pattern and outputs optimized code that is not in fact doing a
modulo calculation.  What I don't understand is how it ends up faster than
the bit-mask version.  The JIT compiler appears to be undoing my attempted
optimization.

Am I making a mistake here (other than assuming what is faster before
profiling)?
Is this something that could be improved/fixed in the compiler?

Regards,

Scott

My simple experiment:

  public static void main(String [] args) {
    for (int i = 0; i < 10; i++) {
      long start = System.nanoTime();
      long maskCount = mask();
      var maskTime = Duration.ofNanos(System.nanoTime()-start);
      System.out.printf("%d     mask method took: %s%n", maskCount,
maskTime);
      start = System.nanoTime();
      long moduloCount = modulo();
      var moduloTime = Duration.ofNanos(System.nanoTime()-start);
      System.out.printf("%d  modulo method took: %s%n", moduloCount,
moduloTime);
      System.out.println("fastest: " + ((maskTime.compareTo(moduloTime) <
0) ? "MASK" : "MODULO"));
    }
  }
  static long modulo () {
    long count = 0;
    for (int i = 0; i < 2_000_000_000; i++) {
      if ((i % 2) == 0)
        count++;
    }
    return count;
  }
  static long mask() {
    long count = 0;
    for (int i = 0; i < 2_000_000_000; i++) {
      if ((i & 1) == 0)
        count++;
    }
    return count;
  }
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/hotspot-dev/attachments/20230722/acbee0ec/attachment.htm>