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
