21.8.13 Permitting tail calls to functions

FUNCTION_OK_FOR_SIBCALL (decl)

A C expression that evaluates to true if it is ok to perform a sibling call to decl.

It is not uncommon for limitations of calling conventions to prevent tail calls to functions outside the current unit of translation, or during PIC compilation. Use this macro to enforce these restrictions, as the sibcall md pattern can not fail, or fall over to a "normal" call.