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Thread: What will the compiler remove from the code?

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    Default What will the compiler remove from the code?

    Hi there.

    Edit: I googled some more and found out, that the java compiler does almost no optimization at all.
    Java relies on the JIT to do all kinds of optimization.
    But there are 3rd party applications to optimize the byte code generated by a compiler a little more.

    I have a quick question: How good is the compiler at noticing dead code and removing it before creating byte-code?
    Lets assume I have the following code:
    public class ImportantClass {
     
    	private static final boolean DEBUG = false;
     
    	public void importantTask() {
    		doStuff();
    		debug("This is an important message!");
    	}
     
    	private void debug(String text) {
    		if (DEBUG) {
    			System.out.println(text);
    		}
    	}
     
    	private void doStuff() {
    		// This is very complicated stuff right here!
    	}
     
    }
    Would the compiler remove the call to "debug"?
    Would the compiler remove the method "debug(String)"?
    If it would not, is there some simple way for me to implement a conditional output which is removed from the final release?

    I am working with Eclipse Juno SR2 at the moment if this is important.

    Thank you very much.


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    Default Re: What will the compiler remove from the code?

    Unreachable statements cause a compiler error.

    Empty blocks, like a while( true ) block with nothing in it, are removed per the compiler's optimization logic.

    In your example above, I don't believe the call to debug() would be removed, because debug() contains logic that would have to be evaluated, but I can't point to a reference that confirms that, and I could be wrong.

    And, No, the IDE you're using is not important or even relevant.

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    Default Re: What will the compiler remove from the code?

    Quote Originally Posted by GregBrannon View Post
    Unreachable statements cause a compiler error.

    Empty blocks, like a while( true ) block with nothing in it, are removed per the compiler's optimization logic.

    In your example above, I don't believe the call to debug() would be removed, because debug() contains logic that would have to be evaluated, but I can't point to a reference that confirms that, and I could be wrong.
    I was just thinking that, since DEBUG is a final variable and set to false, the compile might realize that the entire if-block can be removed.
    And then, since the method "debug(...)" doesnt contain anything anymore (after the if-block has been removed) the entire method might get removed.
    And if the method is removed then obviously the call to the method would need to be removed as well.

    I know that some fancy C compilers can do stuff like this.

    Quote Originally Posted by GregBrannon View Post
    And, No, the IDE you're using is not important or even relevant.
    If I am not mistaken there are different Java-compilers out there. So perhaps a different IDE might use a different compiler. I dont really know, but thanks for clarifying.

    Edit: After googling a bit about it I read on wikipedia, that eclipse has its own java compiler. Whether the eclipse version I use actually uses this different java compiler is not known to me.
    Source: http://en.wikipedia.org/wiki/Java_compiler

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    Default Re: What will the compiler remove from the code?

    When in doubt, run your code through a dissassembler. The Oracle JDK includes a javap tool which allows you to decompile the output.

    I copied your code into test.java (default compiler options, Oracle JDK 1.7.0_40), compiled it, and ran javap -p -c test

    Here's the output:
    Compiled from "test.java"
    public class test {
      private static final boolean DEBUG;
     
      public test();
        Code:
           0: aload_0       
           1: invokespecial #1                  // Method java/lang/Object."<init>":()V
           4: return        
     
      public void importantTask();
        Code:
           0: aload_0       
           1: invokespecial #2                  // Method doStuff:()V
           4: aload_0       
           5: ldc           #3                  // String This is an important message!
           7: invokespecial #4                  // Method debug:(Ljava/lang/String;)V
          10: return        
     
      private void debug(java.lang.String);
        Code:
           0: return        
     
      private void doStuff();
        Code:
           0: return        
    }

    So Oracle's javac compiler did remove the dead code inside of debug(), but it did not remove the call to debug(). This is in general the case with Java because of the modular nature of Java that removing the call to debug() can't be done at compile time reliably.

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    Default Re: What will the compiler remove from the code?

    Okay, thank you very much.
    I have, however, found several programs which optimize the byte-code after compilation and claim to fix problems like these.

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