Differences between compiling for i386 vs x86_64 in Xcode?

What are the differences between compiling a Mac app in Xcode with the Active Architecture set to i386 vs x86_64 (chosen in the drop down at the top left of the main window)? In the Build settings for the project, the Architecture options are Standard (32/64-bit Universal), 32-bit Universal, and 64-bit Intel. Practically, what do these mean and how does one decide?

Assume one is targeting OS X 10.5 and above. I see in the Activity Monitor that compiling fo开发者_运维知识库r x86_64 results in an app that uses more memory than one compiled for i386. What is the advantage? I know 64-bit is "the future", but given the higher memory usage, would it make sense to choose 32-bit ever?

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32/64-bit Universal -- i386, x86_64, ppc

32-bit Universal -- i386, ppc

64-bit Intel -- 64 bit Intel only

ppc64 is no longer supported.

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x86_64 binaries are faster for a number of reasons; faster ABI, more registers, on many (most & all new machines) machines the kernel is 64 bit & kernel calls are faster, etc.etc.etc...

While 64 bit has a bit of memory overhead directly related, generally, to how pointer heavy your app's data structures are, keep in mind that 32 bit applications drag in the 32 bit versions of all frameworks. If yours is the only 32 bit app on the system, it is going to incur a massive amount of overhead compare to the 64 bit version.

64 bit apps also enjoy the latest and greatest Objective-C ABI; synthesized ivars, non-fragile ivars, unified C++/ObjC exceptions, zero-cost @try blocks etc... and there are a number of optimizations that are only possible in 64 bit, too.

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iOS apps need to run on many different architectures:

arm7: Used in the oldest iOS 7-supporting devices[32 bit]
arm7s: As used in iPhone 5 and 5C[32 bit]
arm64: For the 64-bit ARM processor in iPhone 5S[64 bit]

i386: For the 32-bit simulator
x86_64: Used in 64-bit simulator

Xcode basically emulates the environment of 32 bit or 64 bit based on what is set in the Valid Architecture - i386 or x86_64 respectively

Every architecture requires a different binary, and when you build an app Xcode will build the correct architecture for whatever you’re currently working with. For instance, if you’ve asked it to run in the simulator, then it’ll only build the i386 version (or x86_64 for 64-bit).

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Unless you have a reason to compile for x86_64, I recommend just compiling for i386 (and PPC if you support that). Read Apple's stance on the matter:

Although 64-bit executables make it easier for you to manage large data sets (compared to memory mapping of large files in a 32-bit application), the use of 64-bit executables may raise other issues. Therefore you should transition your software to a 64-bit executable format only when the 64-bit environment offers a compelling advantage for your specific purposes.