re PR tree-optimization/64191 (indirect clobbers messes up dead code elimination…

re PR tree-optimization/64191 (indirect clobbers messes up dead code elimination in loop calling dtor)

2014-12-10  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/64191
	* tree-ssa-dce.c (mark_stmt_if_obviously_necessary): Do not
	mark clobbers as necessary.
	(eliminate_unnecessary_stmts): Keep clobbers live if we can.

	* g++.dg/pr64191.C: Make sure we can DCE empty loops with
	indirect clobbers.

From-SVN: r218566

gcc/ChangeLog

+2014-12-10  Richard Biener  <rguenther@suse.de>
+
+	PR tree-optimization/64191
+	* tree-ssa-dce.c (mark_stmt_if_obviously_necessary): Do not
+	mark clobbers as necessary.
+	(eliminate_unnecessary_stmts): Keep clobbers live if we can.
+
 2014-12-10  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/63594

gcc/testsuite/ChangeLog

+2014-12-10  Richard Biener  <rguenther@suse.de>
+
+	PR tree-optimization/64191
+	* g++.dg/pr64191.C: Make sure we can DCE empty loops with
+	indirect clobbers.
+
 2014-12-09  Alan Lawrence  <alan.lawrence@arm.com>
 
 	PR target/63870

gcc/testsuite/g++.dg/pr64191.C

+// { dg-do compile }
+// { dg-options "-O2 -fdump-tree-cddce1" }
+
+struct Bar
+{
+  int i;
+  ~Bar() { }
+};
+void bar_dtor_loop(Bar* p, unsigned int n)
+{
+  if (p) {
+      Bar* e = p + n;
+      while (e > p) {
+	  --e;
+	  e->~Bar();
+      }
+  }
+}
+
+// The clobber in ~Bar should persist but those inlined into
+// bar_dtor_loop not, nor should the loop therein
+
+// { dg-final { scan-tree-dump-times "CLOBBER" 1 "cddce1" } }
+// { dg-final { scan-tree-dump-times "if" 0 "cddce1" } }
+// { dg-final { cleanup-tree-dump "cddce1" } }

gcc/tree-ssa-dce.c

@@ -292,8 +292,7 @@ mark_stmt_if_obviously_necessary (gimple stmt, bool aggressive)
       break;
 
     case GIMPLE_ASSIGN:
-      if (TREE_CODE (gimple_assign_lhs (stmt)) == SSA_NAME
-	  && TREE_CLOBBER_P (gimple_assign_rhs1 (stmt)))
+      if (gimple_clobber_p (stmt))
 	return;
       break;
 
@@ -1362,6 +1361,25 @@ eliminate_unnecessary_stmts (void)
 	  /* If GSI is not necessary then remove it.  */
 	  if (!gimple_plf (stmt, STMT_NECESSARY))
 	    {
+	      /* Keep clobbers that we can keep live live.  */
+	      if (gimple_clobber_p (stmt))
+		{
+		  ssa_op_iter iter;
+		  use_operand_p use_p;
+		  bool dead = false;
+		  FOR_EACH_SSA_USE_OPERAND (use_p, stmt, iter, SSA_OP_USE)
+		    {
+		      tree name = USE_FROM_PTR (use_p);
+		      if (!SSA_NAME_IS_DEFAULT_DEF (name)
+			  && !bitmap_bit_p (processed, SSA_NAME_VERSION (name)))
+			{
+			  dead = true;
+			  break;
+			}
+		    }
+		  if (!dead)
+		    continue;
+		}
 	      if (!is_gimple_debug (stmt))
 		something_changed = true;
 	      remove_dead_stmt (&gsi, bb);