Avoiding Dead-Code Elimination
Blackhole and state.
Avoiding Dead-Code Elimination
The single biggest microbenchmark trap is dead-code elimination (DCE): if the JIT proves a result is never used, it deletes the computation. Your benchmark then measures nothing. JMH gives you two tools to prevent this — returning values and the Blackhole.
The DCE Trap
This loop computes a square root a million times but never uses the result. A smart compiler can delete the entire loop.
public class Main {
public static void main(String[] args) {
long t = System.nanoTime();
for (int i = 0; i < 1_000_000; i++) {
double ignored = Math.sqrt(i); // result thrown away
}
System.out.println("ns: " + (System.nanoTime() - t));
}
}All lessons in this course
- Why JMH
- Writing a Benchmark
- Warmup and Iterations
- Avoiding Dead-Code Elimination