On Mon, Feb 18, 2019 at 7:25 PM Segher Boessenkool <
seg...@kernel.crashing.org> wrote:
> On Mon, Feb 18, 2019 at 10:48:35AM +0200, Janne Blomqvist wrote:
> > I wonder if we shouldn't exorcise all the varargs stuff, it seems to
> > cause more problems than benefits? But not in stage4 if we can avoid
> > it..
>
> On the Power ABIs at least, unprototyped functions (a K&R thing for C) are
> handled the same as varargs (with zero fixed arguments). How does this
> tie in with Fortran requirements?
>
Varargs don't exist in Fortran. But we need some kind of support for
so-called "implicit interfaces" (which was the only thing available before
Fortran 90), which I guess are pretty similar to the K&R unprototyped
functions. E.g. something like
subroutine foo
call bar(1, 2, 3.0)
end subroutine foo
is perfectly valid code, even though discouraged by modern programming
practice. Here the compiler can only deduce from the syntax that bar must
be a subroutine that takes (int, int, float) arguments. And bar can be in
another translation unit, so we have no idea what it's actual interface is,
the onus is on the programmer that they match. Similarly, from
subroutine foo
f = bar(1, 2)
print *, f
end subroutine foo
the compiler can deduce that bar is a function that takes (int, int)
arguments, and returns a float (due to implicit typing rules). However, as
previously mentioned in this thread
subroutine foo
call bar(1, 2)
f = bar(1, 2)
print *, f
end subroutine foo
is invalid since bar cannot be both a subroutine and a function. Also,
getting back to my first statement
subroutine foo
call bar(1, 2)
call bar(1, 2, 3)
end subroutine foo
is invalid since Fortran doesn't have vararg functions (well, with the
newer "explicit interfaces", optional arguments are possible, but that's
still not the same thing as varargs).
I'm not really sure if there is any good reason why GFortran occasionally
generates these varargs declarations, hence my suggestion to get rid of
them. Unless the middle-end is planning to get rid of untyped function
decls?
--
Janne Blomqvist
