[cfe-users] Objective-C message-to-nil optimization?
jordan_rose at apple.com
Thu Feb 13 08:55:13 PST 2014
On Feb 12, 2014, at 9:54 , Kyle Sluder <kyle at ksluder.com> wrote:
> On Feb 12, 2014, at 9:23 AM, Jordan Rose <jordan_rose at 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? :-) )
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