[Bug fortran/109575] Implement runtime check for valid function result

[anlauf at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109575

--- Comment #2 from anlauf at gcc dot gnu.org ---
I have some idea how (and where) the runtime checks need to be implemented,
but I am confused by the following observations on the occurence of an
explicit RETURN statement and the use of a RESULT variable:

function f()
  integer :: f
end

function f1()
  integer :: f1
  f1 = 1
end

function q() result (f)
  integer :: f
end

function q2() result (f)
  integer :: f
  f = 2
end

function g()
  integer :: g
  return
end

function g3()
  integer :: g3
  g3 = 3
  return
end

function r() result (f)
  integer :: f
  return
end

function r4() result (f)
  integer :: f
  f = 4
  return
end

While the dump-fortran-original does not show any surprises, the tree-dump
looks somewhat irregular:

__attribute__((fn spec (". ")))
integer(kind=4) f ()
{
  (void) 0;
}


__attribute__((fn spec (". ")))
integer(kind=4) f1 ()
{
  integer(kind=4) __result_f1;

  __result_f1 = 1;
  return __result_f1;
}


__attribute__((fn spec (". ")))
integer(kind=4) q ()
{
  (void) 0;
}


__attribute__((fn spec (". ")))
integer(kind=4) q2 ()
{
  integer(kind=4) f;

  f = 2;
  return f;
}


__attribute__((fn spec (". ")))
integer(kind=4) g ()
{
  integer(kind=4) __result_g;

  return __result_g;
  return __result_g;
}


__attribute__((fn spec (". ")))
integer(kind=4) g3 ()
{
  integer(kind=4) __result_g3;

  __result_g3 = 3;
  return __result_g3;
  return __result_g3;
}


__attribute__((fn spec (". ")))
integer(kind=4) r ()
{
  return;
}


__attribute__((fn spec (". ")))
integer(kind=4) r4 ()
{
  integer(kind=4) f;

  f = 4;
  return f;
  return f;
}


This seems to correspond to two different paths to gfc_generate_return().