What does “sibling calls” mean?

Question

On GCC manual,

-foptimize-sibling-calls

Optimize sibling and tail recursive calls.

I kn

Answer 1

the compiler considers two functions as being siblings if they share the same structural equivalence of return types, as well as matching space requirements of their arguments.

http://www.drdobbs.com/tackling-c-tail-calls/184401756