Re: Java and Objective-C

[Bill Bumgarner]

On Jun 7, 2008, at 2:10 PM, Denis Bohm wrote:

I don't think the same level of dynamism could be added to any other language without changing the nature of the language. For Java, adding such degrees of dynamism would change the fundamental nature of the virtual machines and JIT compilers in that they could no longer eliminate call sites as a part of optimizations. Actually, any object oriented language that has the ability to inline methods such that they cannot be "out of lined" again at runtime cannot support the dynamism offered by Objective-C.

Can you give a specific example of that specific point using some Objective-C code?

Sure-- in Objective-C I can dynamically load...

        [[NSBundle bundleWithPath: bundlePath] load];



So, consider the case where you start with this:

@interface Foo : NSObject
- (void) doSomething;
@end

@interface Bar : Foo
@end

barInstance = [[Bar alloc] init];
[barInstance doSomething];





@implementation Bar (MySpecialGoop)
- (void) doSomething
{
        ....
        [super doSomething];
}
@end

[barInstance doSomething];





b.bum

... code at any time that can override or extend any class throughout the runtime (and yes, this can be exceedingly dangerous when overriding existing functionality). An optimizing compiler and/or JIT might choose to inline the invocation of -doSomething on barInstance. Now consider what happens after I load a bundle that contains something like this: Then a static inline or other optimization that involves bypassing the standard messenger (objc_msgSend() for Objective-C) would fail to invoke the newly added -doSomething. More subtly, consider what would happen if an accessor method were inlined by the JIT or compiler. Such an action would effectively make it impossible to do KVO against said accessor unless the inlining optimization could be undone.

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