[Bug tree-optimization/30840] [4.3 Regression] ice for legal code with flags -O3 -fno-strict-aliasing

[rguenth at gcc dot gnu dot org]

------- Comment #4 from rguenth at gcc dot gnu dot org  2007-02-19 11:31 -------
It seems to be a problem with ssa-edge simulation ordering.  We have

# SMT.6_95 = VDEF <SMT.6_60>
# SMT.8_96 = VDEF <SMT.8_70>
this_22->firstfree = D.2527_46;

in particular, adding only the first output to simulate

      /* Note that for propagation purposes, we are only interested in
         visiting statements that load the exact same memory reference
         stored here.  Those statements will have the exact same list
         of virtual uses, so it is enough to set the output of this
         statement to be its first virtual definition.  */
      *output_p = first_vdef (stmt);
      if (changed)
        {
          if (val.lattice_val == VARYING)
            retval = SSA_PROP_VARYING;

will cause a missed propagation of the new lattice value to PHI results.
Another point is that get_value_loaded_by will return NULL for the two
VOPs where one has a CONSTANT and one a VARYING value and so we'll call
evaluate_stmt on the stmt which will return UNDEFINED which causes us to
go back from VARYING to UNDEFINED.  Boom.

Sth. like the following fixes it (note 'VARYING' is unknown to
tree-ssa-propagate.c):

Index: tree-ssa-propagate.c
===================================================================
*** tree-ssa-propagate.c        (revision 122127)
--- tree-ssa-propagate.c        (working copy)
*************** get_value_loaded_by (tree stmt, prop_val
*** 835,850 ****
    tree vuse;
    prop_value_t *prev_val = NULL;
    prop_value_t *val = NULL;

    FOR_EACH_SSA_TREE_OPERAND (vuse, stmt, i, SSA_OP_VIRTUAL_USES)
      {
        val = &values[SSA_NAME_VERSION (vuse)];
        if (prev_val && prev_val->value != val->value)
!       return NULL;
!       prev_val = val;
      }

!   return val;
  }


--- 841,860 ----
    tree vuse;
    prop_value_t *prev_val = NULL;
    prop_value_t *val = NULL;
+   bool valid = true;

+   /* Make sure to set the result to varying if any of the operands is
+      varying.  */
    FOR_EACH_SSA_TREE_OPERAND (vuse, stmt, i, SSA_OP_VIRTUAL_USES)
      {
        val = &values[SSA_NAME_VERSION (vuse)];
        if (prev_val && prev_val->value != val->value)
!       valid = false;
!       if (!prev_val || prev_val->lattice_val < val->lattice_val)
!         prev_val = val;
      }

!   return (valid || prev_val->lattice_val == 3) ? prev_val : NULL;
  }


Index: tree-ssa-ccp.c
===================================================================
*** tree-ssa-ccp.c      (revision 122127)
--- tree-ssa-ccp.c      (working copy)
*************** visit_assignment (tree stmt, tree *outpu
*** 1208,1215 ****
        prop_value_t *nval = get_value_loaded_by (stmt, const_val);

        if (nval
!         && nval->mem_ref
!         && operand_equal_p (nval->mem_ref, rhs, 0))
        val = *nval;
        else
        val = evaluate_stmt (stmt);
--- 1208,1216 ----
        prop_value_t *nval = get_value_loaded_by (stmt, const_val);

        if (nval
!         && ((nval->mem_ref
!              && operand_equal_p (nval->mem_ref, rhs, 0))
!             || nval->lattice_val == VARYING))
        val = *nval;
        else
        val = evaluate_stmt (stmt);


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dnovillo at redhat dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30840