On Nov 30, 2009, at 1:33 PM, Ben Haller wrote:
> On 30-Nov-09, at 4:06 PM, Bill Bumgarner wrote:
>> On Nov 30, 2009, at 12:50 PM, Dennis Munsie wrote:
>>
>>> Not that I'm advocating it, but you can also declare a field as @public to
>>> allow you to access it via the -> operator. Of course, I could be missing
>>> some compiler magic going on behind the scene as well, and it may not
>>> actually be the same speed wise as @defs was.
>>
>> The compiler preserves the "non-fragile" part of "non-fragile ivars" when
>> using the -> operator.
>>
>>> Not to mention that it's just plain nasty :)
>>
>> Yup, that it is.
>
> This is interesting to me, since I am in fact using @public and -> with some
> ivars to allow faster use of one of my classes. (Yes, I've confirmed that
> this is significant in Sampler; it is, in fact, quite a large percentage of
> the total time of my app, which has runtimes measured in days, so I do care
> :->). I understand the concept of the fragile base class problem and so
> forth; I just didn't realize Apple had done something to fix it that might
> affect the performance of my app. :->
>
> What I want is essentially a struct with methods; I need super-fast access
> to ivars for clients of the class in some places in my code. But I also want
> functionality provided by the class itself via methods, and I want a class
> hierarchy (so just using functions instead of methods gets ugly fast). Is
> there a better solution than @public? Or is there some reason I shouldn't
> worry about it -- is all the overhead just on the first pass through any
> given code path, with back-patching, for example?
@public is the best way to do this. (Well, @package may be slightly better, but
it's probably only a theoretical improvement in launch time.)
The details are CPU-specific, but non-fragile ivar access is expected to use
two or three loads where fragile ivar access used one load.
For ObjC code like this:
x = self->YourIvar;
Fragile ivar access looks like this when compiled (in a fake assembly language
I just made up):
load r1 = self+offsetof(YourIvar) // offsetof(YourIvar) is a
compile-time constant
Non-fragile ivar access looks roughly like this:
load r1 = &_OBJC_IVAR_$_YourClass.YourIvar
load r2 = *r1
load r3 = self+r2
--
Greg Parker gpar...@apple.com Runtime Wrangler
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