What is "fnspec function type attribute"?

[FX]

In fortran/trans-decl.c, we have a comment above the code building function decls, saying:

>    The SPEC parameter specifies the function argument and return type
>    specification according to the fnspec function type attribute.  */

I was away from GCC development for some time, so this is news to me. The syntax is not immediately clear, and neither a Google search nor a grep of the trunk’s numerous .texi files reveals any information. I’m creating new decls, what I am to do with it?

FX