遍查GCC文档，发现了-O0, -O1, -O2, -O3，就是没有发现-Os。

-Os相当于-O2.5。是使用了所有-O2的优化选项，但又不缩减代码尺寸的方法。

详细的说明如下：
Level 2.5 (-Os)

The special optimization level (-Os or size) enables all -O2 optimizations that do not increase code size; it puts the emphasis on size over speed. This includes all second-level optimizations, except for the alignment optimizations. The alignment optimizations skip space to align functions, loops, jumps and labels to an address that is a multiple of a power of two, in an architecture-dependent manner. Skipping to these boundaries can increase performance as well as the size of the resulting code and data spaces; therefore, these particular optimizations are disabled. The size optimization level is enabled as:

gcc -Os -o test test.c

In gcc 3.2.2, reorder-blocks is enabled at -Os, but in gcc 3.3.2 reorder-blocks is disabled

参考文档:  http://www.linuxjournal.com/article/7269

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补充：在GCC的官方文档里又发现了关于-Os的说明：

http://gcc.gnu.org/onlinedocs/gcc-3.4.6/gcc/Optimize-Options.html#Optimize-Options