How To Stop JVM Skipping Loop - jvm

Question

I have my own test class that is supposed to do timing without JVM deleting anything. Some example test times of 100,000,000 reps comparing the native that Java calls from StrictMath.sin() to my own:
30 degrees
sineNative(): 18,342,858 ns (#1), 1,574,331 ns (#10)
sinCosTanNew6(): 13,751,140 ns (#1), 1,569,848 ns (#10)
60 degrees
sineNative(): 2,520,327,020 ns (#1), 2,520,108,337 ns (#10)
sinCosTanNew6(): 12,935,959 ns (#1), 1,565,365 ns (#10)
From 30 to 60 native time skyrockets * 137 while mine is ~constant. Also, some of the times are impossibly low even when repsDone returns == reps. I expect they should be > 1*reps.
CPU: G3258 # 4GHz
OS: Windows 7 HB SP1
Build Path: jre1.8.0_211
Reprex:
public final class MathTest {
private static int sysReps = 1_000_000;
private static double value = 0;
private static final double DRAD_ANGLE_30 = 0.52359877559829887307710723054658d;
private static final double DRAD_ANGLE_60 = 1.0471975511965977461542144610932d;
private static double sineNative(double angle ) {
int reps = sysReps * 100;
//int repsDone = 0;
value = 0;
long startTime, endTime, timeDif;
startTime = System.nanoTime();
for (int index = reps - 1; index >= 0; index--) {
value = Math.sin(angle);
//repsDone++;
}
endTime = System.nanoTime();
timeDif = endTime - startTime;
System.out.println("sineNative(): " + timeDif + "ns for " + reps + " sine " + value + " of angle " + angle);
//System.out.println("reps done: "+repsDone);
return value;
}
private static void testSines() {
sineNative(DRAD_ANGLE_30);
//sinCosTanNew6(IBIT_ANGLE_30);
}
/* Warm Up */
private static void repeatAll(int reps) {
for (int index = reps - 1; index >= 0; index--) {
testSines();
}
}
public static void main(String[] args) {
repeatAll(10);
}
}
I tried adding angle++ in the loop and that multiplies the times to a more reasonable level, but that messes with the math. I need a way to trick it into the running all of the code all x times. Single pass times are extremely volatile and calling nanotime() takes time, so I need the average of a large number.

Answer 1

The problem is that you never use/refer to the results returned by sineNative. The JIT compiler is clever enough to work out that you never use the return value, so it will just do nothing eventually. A very simple way to fix this is to add a dummy check for your return value. (e.g. if (Math.sin(angle) > 1) { System.out.println("Impossible!"); })
If you are writing benchmark like this it would be useful to use something like JMH (https://github.com/openjdk/jmh) which would automatically create a blackhole for your return variable, so that the JIT compiler will not optimise the value. (see example https://github.com/openjdk/jmh/blob/master/jmh-samples/src/main/java/org/openjdk/jmh/samples/JMHSample_09_Blackholes.java)