re PR c++/34459 (incorrect code when compiled with optimization (-O1))

PR c++/34459
	* tree-ssa-dse.c (dse_optimize_stmt): Don't eliminate store if
	USE_STMT not only stores into the same object as STMT, but might
	read it too.

	* gcc.c-torture/execute/20071219-1.c: New test.

From-SVN: r131101

gcc/ChangeLog

@@ -1,3 +1,10 @@
+2007-12-20  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/34459
+	* tree-ssa-dse.c (dse_optimize_stmt): Don't eliminate store if
+	USE_STMT not only stores into the same object as STMT, but might
+	read it too.
+
 2007-12-19  Sebastian Pop  <sebastian.pop@amd.com>
 
 	PR tree-optimization/34413

gcc/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2007-12-20  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c++/34459
+	* gcc.c-torture/execute/20071219-1.c: New test.
+
 2007-12-20  Tobias Burnus  <burnus@net-b.de>
 
 	PR fortran/34530

gcc/testsuite/gcc.c-torture/execute/20071219-1.c

@@ -0,0 +1,71 @@
+/* PR c++/34459 */
+
+extern void abort (void);
+extern void *memset (void *s, int c, __SIZE_TYPE__ n);
+
+struct S
+{
+  char s[25];
+};
+
+struct S *p;
+
+void __attribute__((noinline))
+foo (struct S *x, int set)
+{
+  int i;
+  for (i = 0; i < sizeof (x->s); ++i)
+    if (x->s[i] != 0)
+      abort ();
+    else if (set)
+      x->s[i] = set;
+  p = x;
+}
+
+void __attribute__((noinline))
+test1 (void)
+{
+  struct S a;
+  memset (&a.s, '\0', sizeof (a.s));
+  foo (&a, 0);
+  struct S b = a;
+  foo (&b, 1);
+  b = a;
+  b = b;
+  foo (&b, 0);
+}
+
+void __attribute__((noinline))
+test2 (void)
+{
+  struct S a;
+  memset (&a.s, '\0', sizeof (a.s));
+  foo (&a, 0);
+  struct S b = a;
+  foo (&b, 1);
+  b = a;
+  b = *p;
+  foo (&b, 0);
+}
+
+void __attribute__((noinline))
+test3 (void)
+{
+  struct S a;
+  memset (&a.s, '\0', sizeof (a.s));
+  foo (&a, 0);
+  struct S b = a;
+  foo (&b, 1);
+  *p = a;
+  *p = b;
+  foo (&b, 0);
+}
+
+int
+main (void)
+{
+  test1 ();
+  test2 ();
+  test3 ();
+  return 0;
+}

gcc/tree-ssa-dse.c

@@ -470,6 +470,26 @@ dse_optimize_stmt (struct dom_walk_data *walk_data,
 	  vuse_vec_p vv;
 	  tree stmt_lhs;
 
+	  if (LOADED_SYMS (use_stmt))
+	    {
+	      tree use_base
+		= get_base_address (GIMPLE_STMT_OPERAND (use_stmt, 0));
+	      /* If use_stmt is or might be a nop assignment, e.g. for
+		 struct { ... } S a, b, *p; ...
+		 b = a; b = b;
+		 or
+		 b = a; b = *p; where p might be &b, then USE_STMT
+		 acts as a use as well as definition, so store in STMT
+		 is not dead.  */
+	      if (TREE_CODE (use_base) == VAR_DECL
+		  && bitmap_bit_p (LOADED_SYMS (use_stmt),
+				   DECL_UID (use_base)))
+		{
+		  record_voperand_set (dse_gd->stores, &bd->stores, ann->uid);
+		  return;
+		}
+	    }
+
 	  if (dump_file && (dump_flags & TDF_DETAILS))
             {
               fprintf (dump_file, "  Deleted dead store '");