re PR tree-optimization/63841 (Incorrect strlen optimization after complete unroll)

2014-11-13  Teresa Johnson  <tejohnson@google.com>

gcc:
	PR tree-optimization/63841
	* tree.c (initializer_zerop): A clobber does not zero initialize.

gcc/testsuite:
	PR tree-optimization/63841
	* g++.dg/tree-ssa/pr63841.C: New test.

From-SVN: r217505

gcc/ChangeLog

+2014-11-13  Teresa Johnson  <tejohnson@google.com>
+
+	PR tree-optimization/63841
+	* tree.c (initializer_zerop): A clobber does not zero initialize.
+
 2014-11-13  Ulrich Weigand  <Ulrich.Weigand@de.ibm.com>
 
 	* optabs.c (prepare_operand): Gracefully fail if the mode of X

gcc/testsuite/ChangeLog

+2014-11-13  Teresa Johnson  <tejohnson@google.com>
+	PR tree-optimization/63841
+	* g++.dg/tree-ssa/pr63841.C: New test.
+
 2014-11-13  Richard Biener  <rguenther@suse.de>
 
 	* gcc.dg/tree-ssa/forwprop-28.c: Adjust.

gcc/testsuite/g++.dg/tree-ssa/pr63841.C

+/* { dg-do run } */
+/* { dg-options "-O2" } */
+
+#include <cstdio>
+#include <string>
+
+std::string __attribute__ ((noinline)) comp_test_write() {
+  std::string data;
+
+  for (int i = 0; i < 2; ++i) {
+    char b = 1 >> (i * 8);
+    data.append(&b, 1);
+  }
+
+  return data;
+}
+
+std::string __attribute__ ((noinline)) comp_test_write_good() {
+  std::string data;
+
+  char b;
+  for (int i = 0; i < 2; ++i) {
+    b = 1 >> (i * 8);
+    data.append(&b, 1);
+  }
+
+  return data;
+}
+
+int main() {
+  std::string good = comp_test_write_good();
+  printf("expected: %hx\n", *(short*)good.c_str());
+
+  std::string bad = comp_test_write();
+  printf("got: %hx\n", *(short*)bad.c_str());
+
+  return good != bad;
+}

gcc/tree.c

@@ -10330,6 +10330,8 @@ initializer_zerop (const_tree init)
       {
 	unsigned HOST_WIDE_INT idx;
 
+	if (TREE_CLOBBER_P (init))
+	  return false;
 	FOR_EACH_CONSTRUCTOR_VALUE (CONSTRUCTOR_ELTS (init), idx, elt)
 	  if (!initializer_zerop (elt))
 	    return false;