On Feb 12, 2014, at 9:54 , Kyle Sluder <k...@ksluder.com> wrote:
> On Feb 12, 2014, at 9:23 AM, Jordan Rose <jordan_r...@apple.com> wrote:
>
>> Hi, Kyle. It looks like Clang does not perform this optimization, even under
>> -O3 or -Oz. I can guess why: even though sending a message to nil will
>> result in 0, it's still more work to call objc_msgSend, have it do the
>> comparison to nil, and then have it clean things up properly to return 0.
>> (In fact, you can poke at the source for objc_msgSend and see exactly how
>> much work it is: not much, but still more than doing the check yourself.)
>
> Understood. Though I’m curious: is there a hard-and-fast rule against
> optimizations introducing function calls and/or branches where they would
> otherwise not appear?
>
> Nonetheless, this optimization also introduces the possibility of involving
> the nil message handler (which AFAIK is undocumented), so there is a semantic
> difference as well as a performance difference.
I didn't want to muddy the situation, but yes, that is exactly a case where
messaging nil will not be equivalent. That might actually mean you'd end up
having to hide your optimization behind a flag. Since the custom nil receiver
is so rare, though, it might be okay to be on by default. (Can you tell that's
not my area of the compiler? :-) )
Jordan
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