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

Question

According to a similar question, and multiple Google searches, declaring or initializing a variable outside or inside a loop "might" be the same in terms of performance.

I am running a computer simulation, and I am trying to optimize the code as much as possible, because it has to run a lot of times and it has multiple inner loops with a lot of iterations.

I am wondering if it worth moving all my variables outside of the loops. I have many variables that "theoretically" are constantly re-declared. I said "theoretically" because I don't know if the the Java compiler actually optimize the code and declare everything at, let's say, the method level.

Basically, what would be more efficient? CodeA or CodeB?

Code A:

private void codeA() {
    double a = 0.0;

    for (int i1 = 0; i < i1_N; i1++ ) {
        // Many other loops
             for (int im = 0; i < im_N; im++ ) {
                 a = 0; // a is used for the first time
                 for (int in = 0; i < in_N; in++ ) {
                     a++; // calculate something with a
                 }
             }
        // close many other loops
    }
}

Code B:

private void codeA() {
    double a;

    for (int i1 = 0; i < i1_N; i1++ ) {
        // Many other loops
             for (int im = 0; i < im_N; im++ ) {
                 a = 0; // a is used for the first time
                 for (int in = 0; i < in_N; in++ ) {
                     a++; // calculate something with a
                 }
             }
        // close many other loops
    }
}

Code C:

private void codeC() {

    for (int i1 = 0; i < i1_N; i1++ ) {
        // Many other loops
             for (int im = 0; i < im_N; im++ ) {
                 double a = 0; // a is used for the first time
                 for (int in = 0; i < in_N; in++ ) {
                     a++; // calculate something with a
                 }
             }
        // close many other loops
    }
}

I am just using the variable a in this example, but the computer simulation uses much more. Most of my variables are primitives (int, double, float). Does it make sense to declare primitives outside of the loop in order to improve performance?

Answer 1

javac will not optimize. Bytecode generated in all the 3 cases is different. (javac Test.java compiles and javap -c Test.class shows the bytecode)

Java Virtual Machine may optimize.

JMH benchmark shows the clear winner:

Benchmark           Mode  Cnt    Score    Error  Units
MyBenchmark.testA  thrpt   20  423.437 ±  6.745  ops/s
MyBenchmark.testB  thrpt   20  337.728 ± 56.363  ops/s
MyBenchmark.testC  thrpt   20  351.419 ± 70.290  ops/s

Units are operations per second, the more the better. Benchmark source code. OpenJDK IcedTea 2.5.4 Java Virtual Machine was used.

So, declaring and initializing variables outside should be the most efficient.