[Bug fortran/35037] VOLATILE attribute not being honored with common block variable

[fxcoudert at gcc dot gnu dot org]

------- Comment #5 from fxcoudert at gcc dot gnu dot org  2008-02-04 22:51 -------
(In reply to comment #4)
> I did not test your patch, but without the common line, the dump contains
> "gfhzjf" until you remove the volatile. This saves you from reading assembler.

Thanks! This allowed me to realize that my patch didn't work: I was marking the
"fake" variable we create for debugging purposes, not the field of the
structure/union. The following patch works, this time:

Index: trans-common.c
===================================================================
--- trans-common.c      (revision 131944)
+++ trans-common.c      (working copy)
@@ -318,6 +318,15 @@ build_field (segment_info *h, tree union
       GFC_DECL_ASSIGN_ADDR (field) = pushdecl_top_level (addr);
     }

+  /* If this field is volatile, mark it.  */
+  if (h->sym->attr.volatile_)
+    {
+      tree new;
+      TREE_THIS_VOLATILE (field) = 1;
+      new = build_qualified_type (TREE_TYPE (field), TYPE_QUAL_VOLATILE);
+      TREE_TYPE (field) = new;
+    }
+
   h->field = field;
 }


It makes the variable volatile in the following examples (common and
equivalence, since they are handled commonly by the codepath I modified):

      subroutine wait4it ()
        logical event
        volatile event
        common /dd/ event
        event = .false.
        do
          if (event) print *, 'gfhzjf'
        end do
      end subroutine

      subroutine wait4it2 ()
        logical event
        integer i
        volatile event
        equivalence(event,i)
        event = .false.
        do
          if (event) print *, 'gfhzjf'
        end do
      end subroutine

We don't, however, handle the case where we mark volatile the other variable
involved in the equivalence:

      subroutine wait4it3 ()
        logical event
        integer i
        volatile i
        equivalence(event,i)
        event = .false.
        do
          if (event) print *, 'gfhzjf'
        end do
      end subroutine

Do we need it? I'm not sure. I've checked what other compilers do: Intel and
Sun do not optimize out the PRINT statement if variable "i" is marked as
volatile; g95 seems to never optimize out variables in equivalences. I've also
checked what GCC does for C code, and it is does behave the same as my patch
above, ie the following is indeed optimized out:

void wait4it ()
{
  union { int event; volatile int i; } dd;

  dd.event = 0;
  while (1)
  {
    if (dd.event)
      __builtin_puts ("gfhzjf");
  }
}

>From the standard point of view, when you equivalence A and B, you're not
supposed to read B after having written to A, so I would think the behaviour I
propose here is standard-conforming. I've asked for opinions on c.l.f
(http://groups.google.com/group/comp.lang.fortran/browse_thread/thread/9602660696e0170e).


-- 

fxcoudert at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |fxcoudert at gcc dot gnu dot
                   |dot org                     |org
             Status|NEW                         |ASSIGNED
  GCC build triplet|i686-pc-cygwin              |
           Keywords|                            |patch
   Last reconfirmed|2008-01-31 13:59:37         |2008-02-04 22:51:44
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35037