tree-optimization/110204 - second level redundancy and simplification

When PRE discovers a full redundancy during insertion it cannot unite
the two value sets.  Instead it inserts a copy old-val = new-val where
new-val can also be a constant.  The following looks through such
copies during elimination, providing one extra level of constant and
copy propagation.  For the PR this helps avoiding a bogus diagnostic
that's emitted on unreachable code during loop optimization.

Bootstrapped and tested on x86_64-unknown-linux-gnu, pushed.

Richard.

	PR tree-optimization/110204
	* tree-ssa-sccvn.cc (eliminate_dom_walker::eliminate_avail):
	Look through copies generated by PRE.

gcc/tree-ssa-sccvn.cc

@@ -6594,7 +6594,22 @@ eliminate_dom_walker::eliminate_avail (basic_block, tree op)
       if (SSA_NAME_IS_DEFAULT_DEF (valnum))
 	return valnum;
       if (avail.length () > SSA_NAME_VERSION (valnum))
-	return avail[SSA_NAME_VERSION (valnum)];
+	{
+	  tree av = avail[SSA_NAME_VERSION (valnum)];
+	  /* When PRE discovers a new redundancy there's no way to unite
+	     the value classes so it instead inserts a copy old-val = new-val.
+	     Look through such copies here, providing one more level of
+	     simplification at elimination time.  */
+	  gassign *ass;
+	  if (av && (ass = dyn_cast <gassign *> (SSA_NAME_DEF_STMT (av))))
+	    if (gimple_assign_rhs_class (ass) == GIMPLE_SINGLE_RHS)
+	      {
+		tree rhs1 = gimple_assign_rhs1 (ass);
+		if (CONSTANT_CLASS_P (rhs1) || TREE_CODE (rhs1) == SSA_NAME)
+		  av = rhs1;
+	      }
+	  return av;
+	}
     }
   else if (is_gimple_min_invariant (valnum))
     return valnum;