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 could see this being a valid optimization for -Os or -Oz mode, though,
> where the stated goal of optimization is to make the program fast and
> minimize code size. If you agree, you can file a bug at
> http://llvm.org/bugs/, or against Apple at http://bugreport.apple.com.
Will do. I might even gin up a patch myself; I'm on a bit of an exploratory
kick.
Thanks,
--Kyle Sluder
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