[llvm-dev] multi-entry function (debug info)

[Roger Ferrer Ibáñez via llvm-dev]

Yep, I think wasn't clear enough when I said "an integer parameter to
discriminate". Perhaps an example may help

module moo
contains
    function bad_cos(x) result(y)
        real, value :: x   ! degrees
        real :: y
        real, parameter :: pi = 3.14159265359

        x = x + pi / 2

        entry bad_sin(x)

        y = ...

    end function bad_cos
end module moo

program main
    use moo
    implicit none

    real :: c, s

    c = bad_cos(0.6)
    s = bad_sin(0.6)

    print *, c, s, c**2 + s**2
end program main

the code above might be lowered into something like

float moo_bad_cos__(float x, int discriminator)
{
  switch (discriminator) {
   case 0: goto moo_bad_cos;
   case 1: goto moo_bad_sin;
  }
moo_bad_cos:
   x = x - pi;
moo_bad_sin:
  y = ...;
  return y;
}

int main() {
   float c = moo_bad_cos__(0.6, 0);
   float s = moo_bad_cos__(0.6, 1);

  // print etc.
}

I'm afraid I don't know enough about the consequences of describing the
ENTRY as a subprogram of its own in the debug info, so apologies if what
follows is unrelated to that.

In my (definitely biased) view of the Fortran things, I'd probably be OK if
the debugger knows I'm inside (the original) "bad_cos" given that an ENTRY
statement is always going to be found inside a function/procedure (where
the enclosing function/procedure acts as the "principal" entry), so it
would surprise me a bit if the debugger thinks I'm in a function called
"bad_sin" as I don't have any procedure (at least at the "program function"
or "module procedure", in Fortran parlance, level) with that name in the
code. But that's my view.

Perhaps flang/f18 folks want to weigh in here.

Kind regards,
Roger

Missatge de Krzysztof Parzyszek via llvm-dev <llvm-dev at lists.llvm.org> del
dia dt., 23 d’oct. 2018 a les 18:31:

> On 10/23/2018 11:21 AM, via llvm-dev wrote:
> > Interesting. So the FORTRAN subprogram would have an implicit parameter
> > and effectively have a computed GOTO at the beginning to dispatch to the
> > correct ENTRY?
>
> At least in Fortran 77, each ENTRY has its own name, so there is no
> "dynamic dispatch". They are really more like separate functions with a
> common tail.
>
> -Krzysztof
>
> --
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-- 
Roger Ferrer Ibáñez
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