static inline void _delay_us(double __us) __attribute__

__attribute__((always_inline))代表啥意思，查找所以头文件都没找到关于 _attribute__的定义

解决方案

https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
always_inline
Generally, functions are not inlined unless optimization is specified. For functions declared inline, this attribute inlines the function independent of any restrictions that otherwise apply to inlining. Failure to inline such a function is diagnosed as an error. Note that if such a function is called indirectly the compiler may or may not inline it depending on optimization level and a failure to inline an indirect call may or may not be diagnosed.
gcc 的扩展。

这是貌似delay_us的返回值类型？