re PR middle-end/45705 (Useless store not optimized away)

2010-09-20  Richard Guenther  <rguenther@suse.de>

	PR tree-optimization/45705
	* tree-ssa-dom.c (optimize_stmt): Perform redundant store elimination.

	* gcc.dg/tree-ssa/ssa-dom-dse-1.c: New testcase.

From-SVN: r164434

gcc/ChangeLog

+2010-09-20  Richard Guenther  <rguenther@suse.de>
+
+	PR tree-optimization/45705
+	* tree-ssa-dom.c (optimize_stmt): Perform redundant store elimination.
+
 2010-09-20  Jakub Jelinek  <jakub@redhat.com>
 
 	PR rtl-optimization/45695

gcc/testsuite/ChangeLog

+2010-09-20  Richard Guenther  <rguenther@suse.de>
+
+	PR tree-optimization/45705
+	* gcc.dg/tree-ssa/ssa-dom-dse-1.c: New testcase.
+
 2010-09-20  Michael Matz  <matz@suse.de>
 
 	PR testsuite/45706

gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-dse-1.c

+/* { dg-do compile } */
+/* { dg-options "-O -fdump-tree-optimized" } */
+
+int x;
+void
+foo (void)
+{
+  if (x == 0)
+    x = 0;
+}
+void
+bar (int i)
+{
+  if (x == i)
+    x = i;
+}
+
+/* { dg-final { scan-tree-dump-not "x =" "optimized" } } */
+/* { dg-final { cleanup-tree-dump "optimized" } } */

gcc/tree-ssa-dom.c

@@ -2178,6 +2178,48 @@ optimize_stmt (basic_block bb, gimple_stmt_iterator si)
       update_stmt_if_modified (stmt);
       eliminate_redundant_computations (&si);
       stmt = gsi_stmt (si);
+
+      /* Perform simple redundant store elimination.  */
+      if (gimple_assign_single_p (stmt)
+	  && TREE_CODE (gimple_assign_lhs (stmt)) != SSA_NAME)
+	{
+	  tree lhs = gimple_assign_lhs (stmt);
+	  tree rhs = gimple_assign_rhs1 (stmt);
+	  tree cached_lhs;
+	  gimple new_stmt;
+	  if (TREE_CODE (rhs) == SSA_NAME)
+	    {
+	      tree tem = SSA_NAME_VALUE (rhs);
+	      if (tem)
+		rhs = tem;
+	    }
+	  /* Build a new statement with the RHS and LHS exchanged.  */
+	  if (TREE_CODE (rhs) == SSA_NAME)
+	    {
+	      gimple defstmt = SSA_NAME_DEF_STMT (rhs);
+	      new_stmt = gimple_build_assign (rhs, lhs);
+	      SSA_NAME_DEF_STMT (rhs) = defstmt;
+	    }
+	  else
+	    new_stmt = gimple_build_assign (rhs, lhs);
+	  gimple_set_vuse (new_stmt, gimple_vuse (stmt));
+	  cached_lhs = lookup_avail_expr (new_stmt, false);
+	  if (cached_lhs
+	      && rhs == cached_lhs)
+	    {
+	      basic_block bb = gimple_bb (stmt);
+	      int lp_nr = lookup_stmt_eh_lp (stmt);
+	      unlink_stmt_vdef (stmt);
+	      gsi_remove (&si, true);
+	      if (lp_nr != 0)
+		{
+		  bitmap_set_bit (need_eh_cleanup, bb->index);
+		  if (dump_file && (dump_flags & TDF_DETAILS))
+		    fprintf (dump_file, "  Flagged to clear EH edges.\n");
+		}
+	      return;
+	    }
+	}
     }
 
   /* Record any additional equivalences created by this statement.  */