在使用优化选项/O1/O2/O3时，有些函数会被优化去掉，有没有什么方法可以让指定函数不会被优化去掉？

1） For GCC, please try below method:

#pragma GCC push_options
#pragma GCC optimize ("O0")

your code

#pragma GCC pop_options
to disable optimizations since GCC 4.4.

2） For Microsoft compiler, it also supports below grammar:

#pragma optimize( "[optimization-list]", {on | off} )

3）For ARM AC5/AC6, I do a quick go through but I don't find it.  Maybe I miss it. 
Anyway, you can try it as what GCC does.

4) The last method is to try a unnecessary "volatile" to your function.