Does LLVM convert Objective-C methods to inline functions?

No, because its impossible to know in the context of the Obj-C runtime if those kind of optimizations can be performed. The thing to remember is that Obj-C methods are invoked by a message send, these messages can come from more than just the [myObject doSomething] syntax.

Consider [obj performSelector:NSSelectorFromString(@"hello")] the fact that this can happen means that it would be impossible to ever inline any method.

There is also a chain of events that happens when a message is received by a class, these events can reroute, or even change the message that is being sent. This happens transparently underneath the message send.

Let's assume for a moment that the compiler inlines a method:

@implementation AwesomeClass 

- (void)doFoo OBJC_INLINE { // or some way to indicate "this is an inline method"
  NSLog(@"doing foo!");
}

- (void)doBar {
  [self doAwesomeStuff];
  [self doFoo];
}

@end

so that -doBar essentially becomes:

- (void)doBar {
  [self doAwesomeStuff];
  {
    NSLog(@"doing foo!");
  }
}

Awesome, that seems like it'd be faster, right? We save ourselves a whole dozen instructions by not calling objc_msgSend. So you package this up and post it online as a .a file.

NSCleverCoder comes along and says "but I want doFoo to do a little bit more", so he does:

@interface SuperAwesomeClass : AwesomeClass @end
@implementation SuperAwesomeClass
- (void)doFoo {
  NSLog(@"doing more foo!");
  [super doFoo];
}
@end

When he tries to run this, it never gets called, because AwesomeClass never actually invokes the -doFoo method.

"But," you say, "this is a contrived example!"

No, it's not. In Objective-C, it is perfectly legal to do this at any point in the development or execution of an app. I can do this when writing the code. Heck, I can even do this at runtime by using objc_allocateClassPair and class_addMethod to dynamically create a subclass and add a method override.

I can also swizzle method implementations. Don't like the existing implementation of -doFoo? That's cool; replace it with your own. Oh wait; if the method was inlined, your new implementation would never get called, because -doBar is never actually invoking the -doFoo method.

The only time I could see this being possible is if there were some sort of way to annotate a method as being un-overridable. answered 2011-11-30T18:11:54.693