# Monday, June 2, 2003
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Invokespecial

One of the more interesting bytecode instructions is invokespecial. During Java's early days this instruction was called invokenonvirtual, but in JDK 1.0.2 it was renamed to invokespecial to indicate it has some very special semantics.

Invokespecial is used in three ways, to call instance initializers (constructors),  to call base class methods non-virtually and to call private methods. It's worth mentioning that, unlike the CLR call instruction, invokespecial cannot be used to call arbitrary methods, it can only call methods in the current class or in a base class of of the current class (and then only on references of the type of the current class, or subclasses of it). The JVM's invokevirtual is very similar to the CLR's callvirt instruction. In addition to invokespecial and invokevirtual, the JVM also has invokestatic and invokeinterface to invoke static methods and interface methods, respectively. The CLR has no special instructions for that, it uses the call and callvirt instructions to call static and interface methods.

Prior to JDK 1.1, invokespecial called the exact method specified in the instruction. In JDK 1.1 this behavior was changed, because it caused versioning problems. Here is an example of what could go wrong:

Component A - version 1

public class GrandParent {
  protected void myMethod() {
    // ...
  }
}
public class Parent extends GrandParent {
}

Component B

public class Child extends Parent {
  protected void myMethod() {
    // ...
    super.myMethod();
  }
}

The compiler would compile the super.myMethod() call in Child to invokespecial GrandParent.myMethod(). Now suppose a new version of Component A is released:

Component A - version 2

public class GrandParent {
  protected void myMethod() {
    // ...
  }
}
public class Parent extends GrandParent {
  protected void myMethod() {
    // ...
    super.myMethod();
  }
}

When Component B is used (without recompiling) with this new version of Component A, the super.myMethod call in Child will still go directly to GrandParent.myMethod and this is probably not what the author of Component A had intended.

To fix this, invokespecial was changed to search the class hierarchy if the called class is a base class of the caller (from the caller's base class on up), but only if the caller's class has the ACC_SUPER bit set in the class' access_flags mask. All Java compilers since JDK 1.1 always set the ACC_SUPER flag. It's interesting to note that the current Sun JRE 1.4.1 still honors a cleared ACC_SUPER flag.

Why doesn't the CLR have an equivalent of the ACC_SUPER flag? The reason is that it isn't needed. When, for example, the C# compiler compiles a base method call, it emits a call instruction to the immediate base class of the caller, even if that class isn't the one that implements the called method[1]. When the JIT is resolving the call it searches up the class hierarchy to find the actual method.

Comparison JVM and CLR call instructions

JVM

CLR

Notes

invokespecial

call

invokespecial checks the object reference for null, call doesn't.

invokevirtual

callvirt

invokeinterface

callvirt

Like in other places where the JVM consumes interface references, the object reference doesn't have to statically implement the called interface type.

invokestatic

call

One final issue relevant to IKVM here is that invokespecial requires the object reference to be checked for null, the CLR call instruction doesn't do this (for this reason the C# compiler uses callvirt when calling non-virtual methods, except when explicitly calling a base class method of course, it always makes sure the object reference isn't null). The IKVM compiler has to insert an explicit check for null references when it is compiling invokespecial. It took me a while to come up with an efficient way of doing this. Javac faces a similar issue when it compiles the explicit instantation of an inner class:

public class Foo {
  public class Inner {}
  static void method() {
    // next line throws a NullPointerException
    ((Foo)null).new Inner();
  }
}

Why does this throw a NullPointerException? The answer may be surprising, but it is because Javac inserts a call to ((Foo)null).getClass() to make it throw a NullPointerException if the outer this is null. If it didn't do this, you'd run into a NullPointerException later on when one of Inner's methods tried to use the outer this and this would be a very surprising and hard to find bug. However, I didn't want to use this trick because it isn't particularly efficient. What I came up with is the following trick:

ldvirtftn instance string System.Object::ToString()
pop

The JIT compiles this to:

mov   eax,dword ptr [ecx]
mov   eax,dword ptr [eax+28h]

If the reference in question is null, the first instruction causes an x86 trap that the CLR translates into a NullReferenceException. The second instruction is overhead and hopefully a future version of the JIT will stop emitting it, but it isn't very expensive anyway.

Note to self: Consider adding an optimization to the IKVM compiler to detect Javac's null reference check:

invokevirtual java/lang/Object.getClass()
pop

and replace it with the more efficient ldvirtftn based check above. This could actually be an important optimization, because getClass() on IKVM is pretty expensive.

[1] When the base classes are in the same module, the C# actually emits a call to the class the defines the method, instead of to the direct base class. This is an optimization, because it saves the JIT from having to search for the method in the class hierarchy. When all classes are in the same module, there obviously aren't any versioning issues, since the module is always built as one unit.

Monday, June 2, 2003 5:56:11 PM (W. Europe Daylight Time, UTC+02:00)  #    Comments [0]