目前ios的指令集有以下几种：
i386：mac
armv6：iPhone，iPhone2，iPhone3G，第一代和第二代iPod Touch
armv7：iPhone 3GS, iPhone 4, iPhone 4S, iPod 3G/4G/5G, iPad, iPad 2, iPad 3, iPad Mini
armv7s：iPhone5，iPhone5C， iPad 4
ARMv8/ARM64: iPhone 6(Plus), iPhone 5s, iPad Air(2), Retina iPad Mini(2,3)
机器对指令集的支持是向下兼容的，因此armv7的指令集是可以运行在iphone5S的，只是效率没那么高而已~
************************************************************************
Architecture ： 指你想支持的指令集。
Valid architectures : 指即将编译的指令集。
Build Active Architecture Only : 只是否只编译当前适用的指令集。
************************************************************************
现在是2014年初，其实4和4S的用户还是蛮多的，而iphone3之类的机器几乎没有了，所以我们的指令集最低必须基于armv7.
因此,Architecture的值选择：armv7 armv7s arm64
PS:选arm64时需要最低支持5.1.1:
Convert Your App to a 64-Bit Binary After Updating It for iOS 7
Xcode 5.0.1 can build your app with both 32-bit and 64-bit binaries included. This combined binary requires a minimum deployment target of iOS 5.1.1 or later. The 64-bit binary runs only on 64-bit devices running iOS 7.0.3 and later. If you have an existing
app, you should first update your app for iOS 7 and then port it to run on 64-bit processors. By updating it first for iOS 7, you can remove deprecated code paths and use modern practices. If you’re creating a new app, target iOS 7 and compile 32-bit and 64-bit
versions of your app.
The architecture for 64-bit apps on iOS is almost identical to the architecture for OS X apps, making it easy to create a common code base that runs in both operating systems. Converting a Cocoa Touch app to 64-bit follows a similar transition process as the
one for Cocoa apps on OS X. Pointers and some common C types change from 32 bits to 64 bits. Code that relies on the NSInteger and CGFloat types
needs to be carefully examined.
Start by building the app for the 64-bit runtime, fixing any warnings that occur as well as searching your code for specific 64-bit issues. For example:
- Make sure all function calls have a proper prototype.
- Avoid truncating 64-bit values by accidentally assigning them to a 32-bit data type.
- Ensure that calculations are performed correctly in the 64-bit version of your app.
- Create data structures whose layouts are identical in the 32-bit and 64-bit versions of your app (such as when you write a data file to iCloud).
1，如果想自己的app在各个机器都能够最高效率的运行，则需要将Build Active Architecture Only改为NO,Valid architectures选择对应的指令集：armv7 armv7s arm64。这个会为各个指令集编译对应的代码，因此最后的 ipa体积基本翻了3倍,Release版本必须NO。
2，如果想让app体积保持最小，则现阶段应该选择Valid architectures为armv7,这样Build Active Architecture Only选YES或NO就无所谓了
对于64位的支持
苹果明年2月强制64位支持
对于支持64-bit,我们可以设置Architectures为 Standard architectures，在最新的Xcode 6上，它包括 armv7和arm64。
让App支持32-bit和64-bit基本步骤
1.确保Xcode版本号>=5.0.1
2.更新project settings, minimum deployment target >= 5.1.1
3.改变Architectures为 Standard architectures（include 64-bit）
4.运行测试代码，解决编译warnings and errors，对照本文档或者官方文档 64-Bit
Transition Guide for Cocoa Touch对相应地方做出修改。（编译器不能告诉我们一切）
5.在真实的64-bit机器上测试。
6.使用Instruments查看内存使用问题。
64-bit主要的变化
64-bit运行时环境和32-bit运行时环境主要有以下两点的不同：
数据类型的改变
方法调用上的改变
数据类型的改变