LLDB中使用po打印变量时，变量为空，且会抛出如下报错信息：
error: warning: couldn’t get cmd pointer (substituting NULL): extracting data from value failed
Couldn’t materialize: couldn’t get the value of variable now: no location, value may have been optimized out
Errored out in Execute, couldn’t PrepareToExecuteJITExpression
解决方案：
1.Produc-> Scheme-> Edit Scheme -> run ，Build Configuration 改为Debug.
2.如果scheme已经是Debug, 则在Build Settings > Optimization Level项，将Debug的值设置为None[-O0]
什么是Optimization Level
下面是苹果的官方解释：
Specifies the degree to which the generated code is optimized for speed and binary size.
None[-O0]: Do not optimize. With this setting, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you would expect from the source code.
Fast[-O1]: Optimizing compilation takes somewhat more time, and a lot more memory for a large function. With this setting, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time. In Apple's compiler, strict aliasing, block reordering, and inter-block scheduling are disabled by default when optimizing.
Faster[-O2]: The compiler performs nearly all supported optimizations that do not involve a space-speed tradeoff. With this setting, the compiler does not perform loop unrolling or function inlining, or register renaming. As compared to the 'Fast' setting, this setting increases both compilation time and the performance of the generated code.
Fastest[-O3]: Turns on all optimizations specified by the 'Faster' setting and also turns on function inlining and register renaming options. This setting may result in a larger binary.
Fastest, Smallest[-Os]: Optimize for size. This setting enables all 'Faster' optimizations that do not typically increase code size. It also performs further optimizations designed to reduce code size.
Fastest, Aggressive Optimizations[-Ofast]: This setting enables 'Fastest' but also enables aggressive optimizations that may break strict standards compliance but should work well on well-behaved code.
在Xcode中，为了方便调试，通常Debug模式默认为None[-O0]，Release模式默认为Fastest, Smallest[-Os]，在实际的项目中，可以根据项目的情况设置优化级别。
