re PR middle-end/69547 (no-op array initializer emits an empty loop)

2016-01-29  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/69547
	* tree-ssa-dce.c (mark_aliased_reaching_defs_necessary_1):
	Do not mark clobbers necessary.
	(mark_all_reaching_defs_necessary_1): Likewise.

	* g++.dg/tree-ssa/pr69547.C: New testcase.

From-SVN: r232976

gcc/ChangeLog

+2016-01-29  Richard Biener  <rguenther@suse.de>
+
+	PR tree-optimization/69547
+	* tree-ssa-dce.c (mark_aliased_reaching_defs_necessary_1):
+	Do not mark clobbers necessary.
+	(mark_all_reaching_defs_necessary_1): Likewise.
+
 2016-01-29  Dominik Vogt  <vogt@linux.vnet.ibm.com>
 
 	* config/s390/s390-c.c (s390_resolve_overloaded_builtin): Format

gcc/testsuite/ChangeLog

+2016-01-29  Richard Biener  <rguenther@suse.de>
+
+	PR tree-optimization/69547
+	* g++.dg/tree-ssa/pr69547.C: New testcase.
+
 2016-01-29  Jakub Jelinek  <jakub@redhat.com>
 
 	PR debug/66869

gcc/testsuite/g++.dg/tree-ssa/pr69547.C

+// { dg-do compile }
+// { dg-options "-O2 -fdump-tree-cddce1" }
+
+struct A { A () { } };
+
+void foo (void*, int);
+
+void bar ()
+{
+  enum { N = 64 };
+  A a [N];
+  foo (&a, N);
+}
+
+// { dg-final { scan-tree-dump-not "if" "cddce1" } }

gcc/tree-ssa-dce.c

@@ -462,7 +462,8 @@ mark_aliased_reaching_defs_necessary_1 (ao_ref *ref, tree vdef, void *data)
   gimple *def_stmt = SSA_NAME_DEF_STMT (vdef);
 
   /* All stmts we visit are necessary.  */
-  mark_operand_necessary (vdef);
+  if (! gimple_clobber_p (def_stmt))
+    mark_operand_necessary (vdef);
 
   /* If the stmt lhs kills ref, then we can stop walking.  */
   if (gimple_has_lhs (def_stmt)
@@ -584,7 +585,8 @@ mark_all_reaching_defs_necessary_1 (ao_ref *ref ATTRIBUTE_UNUSED,
 	  }
     }
 
-  mark_operand_necessary (vdef);
+  if (! gimple_clobber_p (def_stmt))
+    mark_operand_necessary (vdef);
 
   return false;
 }