Java JIT-compiler: is dead code removed forever?

Question

I have follow test class for checking JIT-compiler logic:

public static final int COUNT = 2_000_000_000;

public static final MyLogger LOG = new MyLogger(false);

//Here IS_DEBUG is false
public static final boolean IS_DEBUG = LOG.isDebug();

private void run() throws Exception
{
    System.out.println(getSum(COUNT));

    //Compilation without OSR
    System.out.println(getSum(COUNT + 2));

    //Change value IS_DEBUG -> true over reflection
    setFinalStatic(TestDeadCode.class.getField("IS_DEBUG"), true);
    //Show true
    System.out.println(IS_DEBUG);
    
    COUNT = COUNT / 2;
    System.out.println(getSum(COUNT + 3));
}

private int getSum(int count)
{
    int result = 0;
    for (int j = 0; j < count; j++)
    {
        result = result + 1;
        if (IS_DEBUG)
        {
            //Dead code here
            System.out.println("debug: " + result);
        }
    }
    return result;
}

If i call method run() then "dead code"

System.out.println("debug: " + result);

is executed never. Is it JVM bug?

Java version:

Java(TM) SE Runtime Environment (build 1.7.0_17-b02)

Java HotSpot(TM) 64-Bit Server VM (build 23.7-b01, mixed mode)

UPDATE: PrintCompilation output:

79    1 %           com.nau.sample.deadcode.TestDeadCode::getSum @ 7 (34 bytes)

85    1             com.nau.sample.deadcode.TestDeadCode::getSum (34 bytes)

Actually most likely (if `LOG.isDebug()` is known at compile time) there is javac acts, not JIT compiler. — om-nom-nom, Apr 06 '13 at 22:15
as you can see System.out.println(IS_DEBUG) shows "true" after setFinalStatic() — anstarovoyt, Apr 06 '13 at 22:19
The value of a (primitive) `static final` variable is *replaced* at compile time everywhere it is referenced. So the code using the variable does no longer reference it, it just "uses" the value available at compile time. Not sure if this is also true in your case (because your static final is initialized by a method call), but it sure sounds as if exactly that would happen. — , Apr 06 '13 at 22:37
Looks like this is bad idea to change value of final attributes :) — MariuszS, Apr 06 '13 at 22:46
I wouldn't call this a bug; I'd call this exactly what you should expect if you go messing around with reflection. — Louis Wasserman, Apr 06 '13 at 23:01
I checked that assembly code doesn't contain "if (debug)" instruction. Therefore, It is dead code elimination really — anstarovoyt, Apr 07 '13 at 16:51

Java JIT-compiler: is dead code removed forever?

0 Answers0