what “inline __attribute__((always_inline))” means in the function?

The often referenced gcc documentation for always_inline is incomplete.

always_inline attribute makes gcc compiler:

• Ignore -fno-inline (this is what the documentation says).
• Ignore the inlining limits hence inlining the function regardless. It also inlines functions with alloca calls, which inline keyword never does.
• Not produce an external definition of a function with external linkage if marked with always_inline.

The source of the above information is gcc source code, and, hence, is subject to change with no warning.

An interesting bechmark: always_inline
performance.