Re: Problem implementing faults in Objective-C

[Timothy J. Wood]

On Jun 21, 2005, at 4:12 AM, Frederic Stark wrote:

> I am sending this to gnustep-dev crossposted to gcc. Maybe this  
> isn't the right mailing list. See at the end of the post for a 40  
> line program that exhibit the bad behavior.
>
> Problem:
> If a is a fault (ie: changes its isa pointer during  
> forwardInvocation), then:
>
> [a method1:[a method2]]
>
> fails (a does not recognize 'method1:'), while:


  I believe the GNU runtime looks up the IMP and then calls it  
rather than always calling a dispatch function.  In this case, the  
code above is possibly getting miscompiled into something like

    IMP imp1 = getInstanceImp(a->isa, @selector(method1:))
    IMP imp2 = getInstanceImp(a->isa, @selector(method2))
    id result1 = imp2(a, @selector(method2))
    id result2 = imp1(a, @selector(method1:), result1)

  That is, the lookup of the IMP for -method1: may be happening too  
early.

> The code works correctly under Mac OS X.

  The Apple runtime doesn't have this design choice, so it can't  
really have this problem.

> Am I doing something horribly wrong ?

  I don't think so; seems like a bug in the GNU ObjC runtime support  
in the compiler.  I suppose the runtime maintainers might choose to  
define this as a bug in your code, but isa-swizzling is a fairly  
common and _extremely_ useful pattern in ObjC (see CoreData,  
NSZombie, etc.) so that'd not be my stance, obviously :)

-tim