Sorry guys - I don't know if any of this should work, but I tried to use gdb
with limited success - if the subroutine argument values would have printed, gdb
would have been working well enough to be useful.

[dir:~/tests/gfortran] dir% gfortran  -g -o debug debug.f
[dir:~/tests/gfortran] dir% gdb debug
GNU gdb 5.3-20030128 (Apple version gdb-330.1) (Fri Jul 16 21:42:28 GMT 2004)
Copyright 2003 Free Software Foundation, Inc.
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Type "show copying" to see the conditions.
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This GDB was configured as "powerpc-apple-darwin".
Reading symbols for shared libraries ..... done
(gdb) break sub1_
Breakpoint 1 at 0x1c90: file ../../../gcc/libgfortran/runtime/error.c, line 275.
(gdb) break sub1
Function "sub1" not defined.
(gdb) run
Starting program: /Users/dir/tests/gfortran/debug 
Reading symbols for shared libraries ++. done
Error in re-setting breakpoint 1:
Function "sub1_" not defined.
Re-enabling shared library breakpoints: 1

Breakpoint 1, 0x00001c90 in sub1_ (a=Invalid F77 type code 17 in symbol table.
) at debug.f:5
5             subroutine sub1(a,b)
(gdb) s
Current language:  auto; currently fortran
sub1_ (a=Invalid F77 type code 17 in symbol table.
) at debug.f:6
6             i=3
(gdb) s
7             write(*,*)a,b
(gdb) print a
$1 = (Invalid type code (17) in symbol table.
(gdb) print b
$2 = (Invalid type code (17) in symbol table.
(gdb) quit
The program is running.  Exit anyway? (y or n) y
[dir:~/tests/gfortran] dir% cat debug.f
      program main
      call sub1(1.0,2.0)
      stop
      end
      subroutine sub1(a,b)
      i=3
      write(*,*)a,b
      return
      end