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

[Paul Thomas]

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

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