[patch, fortran] Fix PR 87689, wrong decls / ABI violation on POWER

[Bob Deen via gcc-patches]

On 2/19/19 2:44 PM, Thomas Koenig wrote:
> Bob,
> 
>> Some of us still use varargs interfaces (in my case, Fortran calling C
>> stdarg subroutines).
> 
> The problem for us is that that sometimes using varargs made standard-
> conforming Fortran code like, in file a.f
> 
>         subroutine foo(a)
>         print *,a
>         end
> 
> and in file main.f
> 
>         programme main
>         call foo(1.0)
>         end
> 
> depend ABI details: The call to foo used to be called using
> the varargs convention, and the subroutine foo was compiled
> as a non-varargs function.
> 
> This "worked" until PR 87689 showed that this breaks
> standard-conforming Fortran code on a primary gcc platform.
> 
> I don't know if that makes a difference for the platform you work
> on.  For the System V AMD64 ABI, I suspect it actually might not
> matter (at least from glancing at the corresponding Wikipedia
> article), but I am _not_ an expert in this field, so please take this
> with a chunk of rock salt of appropriate size.
> 
> So, we cannot really keep this as a feature (note that varargs
> are also not C interoperable).

Okay.  But I hope you don't let the perfect be the enemy of the good. 
That particular platform is not a concern for us, so if you break it 
there I'm not happy, but it's not the end of the world either.  But 
please don't break it other places just because it doesn't work on that 
one platform.  I know it's not good design practice to have that kind of 
platform-dependent behavior, but sometimes practicalities force less 
than ideal choices.

Thanks...

-Bob