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What does IF_ICMPNE mean?

For the following Java class:

public class ArtClass {
   public boolean foo(int x) {
      if(x == 3956681)
        return true;
      else if(x == 9855021)
        return true;
      else if(x == 63085561)
          return true;
      else
        return false;
   }
}

Its JVM instructions are:

I4 Branch 1 IF_ICMPNE L3
I13 Branch 2 IF_ICMPNE L5
I22 Branch 3 IF_ICMPNE L7

I understand that the first branch is in the third line and the same for the second and third branches but what does IF_ICMPNE mean and also what do I4, I13, and I22 mean?

Solution

  • This is the output, javap -c produced for your class (javap is a tool that ships with each standard JDK):

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

  public boolean foo(int);
    Code:
       0: iload_1
       1: ldc           #2                  // int 3956681
       3: if_icmpne     8
       6: iconst_1
       7: ireturn
       8: iload_1
       9: ldc           #3                  // int 9855021
      11: if_icmpne     16
      14: iconst_1
      15: ireturn
      16: iload_1
      17: ldc           #4                  // int 63085561
      19: if_icmpne     24
      22: iconst_1
      23: ireturn
      24: iconst_0
      25: ireturn
}

    The meaning of all instructions has been specified in the “Instruction Set” chapter of the The Java® Virtual Machine Specification. The instruction if_icmpne will pop two int values, compare them, and branch to the specified target if not equal.

    The output of javap makes it pretty clear, which targets have been specified by the branch instructions, as they match the numbers printed before each instruction.

    If you use a different tool producing different output, you have to refer to the tool’s documentation regarding, how to decipher the output. Comparing with javap’s output suggests that these prefixes like I4 also refer to bytecode offsets, but without further context, e.g. seeing the other instructions of the method, that’s quiet meaningless.