죽은 코드 제거 방지

죽은 코드 제거 방지

DCE 함정

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));
    }
}

해결 방법 1: 결과 반환

import org.openjdk.jmh.annotations.Benchmark;

public class Bench {
    @Benchmark
    public double sqrt() {
        return Math.sqrt(42.0); // returned -> consumed by JMH
    }
}

해결 방법 2: 블랙홀

import org.openjdk.jmh.annotations.Benchmark;
import org.openjdk.jmh.infra.Blackhole;

public class Bench {
    @Benchmark
    public void manyValues(Blackhole bh) {
        for (int i = 0; i < 100; i++) {
            bh.consume(Math.sqrt(i));
        }
    }
}

@State 객체

import org.openjdk.jmh.annotations.*;

public class Bench {
    @State(Scope.Thread)
    public static class Data {
        public int x = 21;
    }

    @Benchmark
    public int multiply(Data d) {
        return d.x * 2; // d.x is not a compile-time constant
    }
}

@Setup과 @TearDown

import org.openjdk.jmh.annotations.*;

@State(Scope.Thread)
public class Data {
    int[] arr;
    @Setup(Level.Trial)
    public void init() {
        arr = new int[1000];
        for (int i = 0; i < arr.length; i++) arr[i] = i;
    }
}

상수 입력 피하기

여러 값 반환하기

import org.openjdk.jmh.annotations.Benchmark;
import org.openjdk.jmh.infra.Blackhole;

public class Bench {
    @Benchmark
    public int twoResults(Blackhole bh) {
        int a = 3 * 7;
        int b = 5 + 9;
        bh.consume(b);
        return a;
    }
}

블랙홀 오버헤드

측정값 점검하기

종합하기

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