0 Minimum optimization. Turns off most optimizations.It gives the best possible debug view and the lowest level of optimization. 近乎不优化，用于调试代码。出现代码行不能设置断点可如此设置试试。
1 Restrictedoptimization. Removes unused inline functions and unused static functions.Turns off optimizations that seriously degrade the debug view. Ifused with --debug, this option gives a satisfactorydebug view with good code density.部分优化。移除未调用的内联函数和静态函数，关闭debug窗口优化，此状态也能用于调试
2 Highoptimization. If used with--debug, the debug viewmight be less satisfactory because the mapping of object code tosource code is not always clear.
This is the default optimization level.
默认优化等级。如果处于debug状态，部分代码行将不能被调试，具体做了什么优化好像没说
3 Maximumoptimization.-O3 performs the same optimizationsas-O2 however the balance between space and timeoptimizations in the generated code is more heavily weighted towardsspace or time compared with-O2. That is:
-O3 -Otime aims to produce fastercode than-O2 -Otime, at the risk of increasingyour image size
-O3 -Ospace aims to produce smallercode than-O2 -Ospace, but performance might bedegraded.
In addition, -O3 performs extra optimizationsthat are more aggressive, such as:
High-levelscalar optimizations, including loop unrolling, for-O3-Otime. Thiscan give significant performance benefits at a small code size cost,but at the risk of a longer build time.
More aggressive inlining and automatic inliningfor-O3 -Otime.
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