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.)
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. Jordan On Feb 11, 2014, at 19:24 , Kyle Sluder <k...@ksluder.com> wrote: > Just curious if clang optimizes ObjC messages that it knows have a nil > receiver? > > I'm converting a subclass of NSTextStorage to lazily create its backing > store. To make this very clear to readers, I want to wrap all accesses > to the backing store in conditional checks. > > For example: > > - (NSUInteger)length; > { > return _contents ? _contents.length : 0; > } > > Strictly speaking, this is unnecessary, because sending -length to nil > will result in 0 anyway. > > Part of the reason I'm making this change is for optimization, so I'd > feel a lot better if I knew that clang would optimize away conditional > expressions when it determines that they are 1) solely determined by the > NULL-ness of an object and 2) the result of evaluating either branch is > known to be exactly equal to sending the message to the receiver. > > Is this something that clang does already? > > --Kyle Sluder > _______________________________________________ > cfe-users mailing list > cfe-users@cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-users
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