Re: Problem implementing faults in Objective-C

[Frederic Stark <fstark at almonde dot com>]

Timothy J. Wood wrote:

[crunch]
> > The code works correctly under Mac OS X.

I just checked under linux/gcc 3.4 and the code works fine there. Maybe 
this is a gcc 3.2 specific problem. I'll check gcc 3.4 windows one of 
those days.

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

You are right. I just disassembled a small variation on the thing, and 
gcc 3.2 generates:

0x401406 <x>:   push   %ebp
0x401407 <x+1>: mov    %esp,%ebp
0x401409 <x+3>: push   %edi
0x40140a <x+4>: push   %esi
0x40140b <x+5>: push   %ebx
0x40140c <x+6>: sub    $0xc,%esp
0x40140f <x+9>: sub    $0x4,%esp
0x401412 <x+12>:        sub    $0x4,%esp
0x401415 <x+15>:        push   $0x4041f0
0x40141a <x+20>:        mov    0x8(%ebp),%esi
0x40141d <x+23>:        push   %esi
0x40141e <x+24>:        call   0x4017b0 <objc_msg_lookup>
0x401423 <x+29>:        add    $0xc,%esp
0x401426 <x+32>:        mov    %eax,%edi
0x401428 <x+34>:        sub    $0x4,%esp
0x40142b <x+37>:        push   $0x4041e8
0x401430 <x+42>:        mov    0x8(%ebp),%ebx
0x401433 <x+45>:        push   %ebx
0x401434 <x+46>:        call   0x4017b0 <objc_msg_lookup>
0x401439 <x+51>:        add    $0x8,%esp
0x40143c <x+54>:        push   $0x4041e8
0x401441 <x+59>:        push   %ebx
0x401442 <x+60>:        call   *%eax
0x401444 <x+62>:        add    $0xc,%esp
0x401447 <x+65>:        push   %eax
0x401448 <x+66>:        push   $0x4041f0
0x40144d <x+71>:        push   %esi
0x40144e <x+72>:        call   *%edi
0x401450 <x+74>:        add    $0x10,%esp
0x401453 <x+77>:        lea    0xfffffff4(%ebp),%esp
0x401456 <x+80>:        pop    %ebx
0x401457 <x+81>:        pop    %esi
0x401458 <x+82>:        pop    %edi
0x401459 <x+83>:        pop    %ebp
0x40145a <x+84>:        ret

It looks like the two objc_msg_lookup are made before the actual call 
(0x401442 and 0x40144e)

>   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 :)

As in my code the real classes are all subclass of a specific one, I 
worked around the problem by implementing:

- (void)forwardInvocation:(NSInvocation *)anInvocation
{
	if (![self respondsToSelector:[anInvocation selector]])
		[super forwardInvocation:anInvocation];

	[anInvocation invoke];
}

in the 'B' class.

Thanks for the help (and it have been a loooong time since we last 
exchanged emails on the omni dev list...)

--fred