[Patch, fortran] PR22304 - gfortran silently changes values in equivalencedvariables

[Paul Thomas <paulthomas2 at wanadoo dot fr>]

This patch, that Mike Albert found, is devastatingly simple; just one 
added line.  It is discussed in the list for the PR but not adopted 
because it was not clear how it worked.

In fact, it is also relatively easy to understand:

(i) Translate_common calls add_equivalences, which calls 
find_equivalences.  At present, this only marks, as used, the 
equivalence partners that appear in the common block.
(ii) Later, finish_equivalences does the same again.  This time it 
creates local unions for all equivalence variables that are not used; 
ie. those that are declared in the local scope.
(iii) Thus, where this happens, references in this scope point to the 
local union, rather than the common block.
(iv) The fix acts on step (i) by marking the partners in the local 
scope, as used, as well.

There is a strange side-effect of this fix, noted by Thomas Koenig, in 
the PR correspondence:  Where this fix operates, -fdump-tree-original 
produces a scope block that is not terminated by a right brace.  The 
gimple output is fine and the programs run fine.  Using one more 
variable in this scope clears the problem and restores the right brace.

Is this a gfortran or a tree-dump problem?

2005-08-05  Mike Albert  <albertm@uphs.upenn.edu>

   PR fortran/22304
   * trans-common.c (find_equivalences): Ensure that both partners
   in common block equivalences are marked as used. This prevents
   the second call to this function from making local unions.

2005-08-05  Paul Thomas  <pault@gcc.gnu.org>

   PR fortran/22304
   * gfortran.dg/common_equivalence_1.f: New.

Patch and test case are attached.

OK for mainline and 4.0?

Paul T

Index: gcc/gcc/fortran/trans-common.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/fortran/trans-common.c,v
retrieving revision 1.29
diff -c -3 -p -r1.29 trans-common.c
*** gcc/gcc/fortran/trans-common.c	20 Jul 2005 01:19:32 -0000	1.29
--- gcc/gcc/fortran/trans-common.c	5 Aug 2005 14:51:32 -0000
*************** find_equivalence (segment_info *n)
*** 700,705 ****
--- 700,706 ----
  	    {
  	      add_condition (n, eq, other);
  	      eq->used = 1;
+ 	      other->used = 1;
  	      found = TRUE;
  	      /* If this symbol is the first in the chain we may find other
  		 matches. Otherwise we can skip to the next equivalence.  */

c { dg-do run }
c This program tests the fix for PR22304.
c
c provided by Paul Thomas - pault@gcc.gnu.org
c
      integer a(2), b, c
      COMMON /foo/ a
      EQUIVALENCE (a(1),b), (c,a(2))
      a(1)=101
      a(2)=102
      if (b.ne.101) call abort ()
      if (c.ne.102) call abort ()
      END