gdb/fortran: don't access non-existent type fields

When attempting to call a Fortran function for which there is no debug
information we currently trigger undefined behaviour in GDB by
accessing non-existent type fields.

The reason is that in order to prepare the arguments, for a call to a
Fortran function, we need to know the type of each argument.  If the
function being called has no debug information then obviously GDB
doesn't know about the argument types and we should either give the
user an error or pick a suitable default.  What we currently do is
just assume the field exist and access undefined memory, which is
clearly wrong.

The reason GDB needs to know the argument type is to tell if the
argument is artificial or not, artificial arguments will be passed by
value while non-artificial arguments will be passed by reference.

An ideal solution for this problem would be to allow the user to cast
the function to the correct type, we already do this to some degree
with the return value, for example:

  (gdb) print some_func_ ()
  'some_func_' has unknown return type; cast the call to its declared return type
  (gdb) print (integer) some_func_ ()
  $1 = 1

But if we could extend this to allow casting to the full function
type, GDB could figure out from the signature what are real
parameters, and what are artificial parameters.  Maybe something like
this:

  (gdb) print ((integer () (integer, double)) some_other_func_ (1, 2.3)

Alas, right now the Fortran expression parser doesn't seem to support
parsing function signatures, and we certainly don't have support for
figuring out real vs artificial arguments from a signature.

Still, I think we can prevent GDB from accessing undefined memory and
provide a reasonable default behaviour.

In this commit I:

  - Only ask if the argument is artificial if the type of the argument
  is actually known.

  - Unknown arguments are assumed to be artificial and passed by
  value (non-artificial arguments are pass by reference).

  - If an artificial argument is prefixed with '&' by the user then we
  treat the argument as pass-by-reference.

With these three changes we avoid undefined behaviour in GDB, and
allow the user, in most cases, to get a reasonably natural default
behaviour.

gdb/ChangeLog:

	PR fortran/26155
	* f-lang.c (fortran_argument_convert): Delete declaration.
	(fortran_prepare_argument): New function.
	(evaluate_subexp_f): Move logic to new function
	fortran_prepare_argument.

gdb/testsuite/ChangeLog:

	PR fortran/26155
	* gdb.fortran/call-no-debug-func.f90: New file.
	* gdb.fortran/call-no-debug-prog.f90: New file.
	* gdb.fortran/call-no-debug.exp: New file.

1 files changed, 35 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.fortran/call-no-debug-prog.f90 b/gdb/testsuite/gdb.fortran/call-no-debug-prog.f90
new file mode 100644
index 0000000..514c9c8
--- /dev/null
+++ b/gdb/testsuite/gdb.fortran/call-no-debug-prog.f90
@@ -0,0 +1,35 @@
+! Copyright 2020-2021 Free Software Foundation, Inc.
+!
+! This program is free software; you can redistribute it and/or modify
+! it under the terms of the GNU General Public License as published by
+! the Free Software Foundation; either version 3 of the License, or
+! (at your option) any later version.
+!
+! This program is distributed in the hope that it will be useful,
+! but WITHOUT ANY WARRANTY; without even the implied warranty of
+! MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+! GNU General Public License for more details.
+!
+! You should have received a copy of the GNU General Public License
+! along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+program main
+  implicit none
+
+  interface
+     integer function some_func (arg)
+       integer :: arg
+     end function some_func
+
+     integer function string_func (str)
+       character(len=*) :: str
+     end function string_func
+  end interface
+
+  integer :: val
+
+  val = some_func (1)
+  print *, val
+  val = string_func ('hello')
+  print *, val
+end program main