Declaring/initializing variables inside or outside of a double (or multiple) loop

Bytecode

Compiled from "Test.java"
public class Test {
  public Test();
    Code:
       0: aload_0       
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return        

  public void testA();
    Code:
       0: dconst_0      
       1: dstore_1      
       2: iconst_0      
       3: istore_3      
       4: iload_3       
       5: sipush        1000
       8: if_icmpge     57
      11: iconst_0      
      12: istore        4
      14: iload         4
      16: sipush        1000
      19: if_icmpge     51
      22: dconst_0      
      23: dstore_1      
      24: iconst_0      
      25: istore        5
      27: iload         5
      29: sipush        1000
      32: if_icmpge     45
      35: dload_1       
      36: dconst_1      
      37: dadd          
      38: dstore_1      
      39: iinc          5, 1
      42: goto          27
      45: iinc          4, 1
      48: goto          14
      51: iinc          3, 1
      54: goto          4
      57: return        

  public void testB();
    Code:
       0: iconst_0      
       1: istore_3      
       2: iload_3       
       3: sipush        1000
       6: if_icmpge     55
       9: iconst_0      
      10: istore        4
      12: iload         4
      14: sipush        1000
      17: if_icmpge     49
      20: dconst_0      
      21: dstore_1      
      22: iconst_0      
      23: istore        5
      25: iload         5
      27: sipush        1000
      30: if_icmpge     43
      33: dload_1       
      34: dconst_1      
      35: dadd          
      36: dstore_1      
      37: iinc          5, 1
      40: goto          25
      43: iinc          4, 1
      46: goto          12
      49: iinc          3, 1
      52: goto          2
      55: return        

  public void testC();
    Code:
       0: iconst_0      
       1: istore_1      
       2: iload_1       
       3: sipush        1000
       6: if_icmpge     53
       9: iconst_0      
      10: istore_2      
      11: iload_2       
      12: sipush        1000
      15: if_icmpge     47
      18: dconst_0      
      19: dstore_3      
      20: iconst_0      
      21: istore        5
      23: iload         5
      25: sipush        1000
      28: if_icmpge     41
      31: dload_3       
      32: dconst_1      
      33: dadd          
      34: dstore_3      
      35: iinc          5, 1
      38: goto          23
      41: iinc          2, 1
      44: goto          11
      47: iinc          1, 1
      50: goto          2
      53: return        
}

MyBenchmark.java

package org.sample;

import org.openjdk.jmh.annotations.*;
import org.openjdk.jmh.runner.Runner;
import org.openjdk.jmh.runner.RunnerException;
import org.openjdk.jmh.runner.options.Options;
import org.openjdk.jmh.runner.options.OptionsBuilder;

import java.util.concurrent.TimeUnit;

@OutputTimeUnit(TimeUnit.SECONDS)
@BenchmarkMode({Mode.Throughput})
@Warmup(iterations = 10)
@Fork(value = 1)
@State(Scope.Benchmark)
public class MyBenchmark {

    private static final int LOOPS = 100;

    @Benchmark
    public double testA() {
        double b = 0;
        double a = 0.0;
        for (int i1 = 0; i1 < LOOPS; i1++) {
            for (int im = 0; im < LOOPS; im++) {
                a = 0; // a is used for the first time
                for (int in = 0; in < LOOPS; in++) {
                    a++; // calculate something with a
                    b += a;
                }
            }
        }
        return b;
    }

    @Benchmark
    public double testB() {
        double b = 0;
        double a;
        for (int i1 = 0; i1 < LOOPS; i1++) {
            for (int im = 0; im < LOOPS; im++) {
                a = 0; // a is used for the first time
                for (int in = 0; in < LOOPS; in++) {
                    a++; // calculate something with a
                    b += a;
                }
            }
        }
        return b;
    }

    @Benchmark
    public double testC() {
        double b = 0;
        for (int i1 = 0; i1 < LOOPS; i1++) {
            for (int im = 0; im < LOOPS; im++) {
                double a = 0; // a is used for the first time
                for (int in = 0; in < LOOPS; in++) {
                    a++; // calculate something with a
                    b += a;
                }
            }
        }
        return b;
    }

    public static void main(String[] args) throws RunnerException {
        Options opt = new OptionsBuilder()
                .include(MyBenchmark.class.getSimpleName())
                .forks(1)
                .build();
        new Runner(opt).run();
    }
}

Test.java

public class Test {

    private static final int LOOPS = 1_000;

    public void testA() {
        double a = 0.0;
        for (int i1 = 0; i1 < LOOPS; i1++) {
            for (int im = 0; im < LOOPS; im++) {
                a = 0; // a is used for the first time
                for (int in = 0; in < LOOPS; in++) {
                    a++; // calculate something with a
                }
            }
        }
    }

    public void testB() {
        double a;
        for (int i1 = 0; i1 < LOOPS; i1++) {
            for (int im = 0; im < LOOPS; im++) {
                a = 0; // a is used for the first time
                for (int in = 0; in < LOOPS; in++) {
                    a++; // calculate something with a
                }
            }
        }
    }

    public void testC() {
        for (int i1 = 0; i1 < LOOPS; i1++) {
            for (int im = 0; im < LOOPS; im++) {
                double a = 0; // a is used for the first time
                for (int in = 0; in < LOOPS; in++) {
                    a++; // calculate something with a
                }
            }
        }
    }

}