re PR c/39855 (Shift optimization discards operands' side effects)

	PR c/39855
	* fold-const.c (fold_binary) <case LSHIFT_EXPR>: When optimizing
	into 0, use omit_one_operand.

	* gcc.dg/torture/pr39855.c: New test.

From-SVN: r146617

gcc/ChangeLog

@@ -3,6 +3,10 @@
 	* alias.c (find_base_term): Move around LO_SUM case, so that
 	CONST falls through into PLUS/MINUS handling.
 
+	PR c/39855
+	* fold-const.c (fold_binary) <case LSHIFT_EXPR>: When optimizing
+	into 0, use omit_one_operand.
+
 2009-04-23  Ben Elliston  <bje@au.ibm.com>
 
 	* config/rs6000/linux-unwind.h (get_regs): Remove type

gcc/fold-const.c

@@ -11880,7 +11880,8 @@ fold_binary (enum tree_code code, tree type, tree op0, tree op1)
 	      if (code == LROTATE_EXPR || code == RROTATE_EXPR)
 	        low = low % TYPE_PRECISION (type);
 	      else if (TYPE_UNSIGNED (type) || code == LSHIFT_EXPR)
-	        return build_int_cst (type, 0);
+		return omit_one_operand (type, build_int_cst (type, 0),
+					 TREE_OPERAND (arg0, 0));
 	      else
 		low = TYPE_PRECISION (type) - 1;
 	    }

gcc/testsuite/ChangeLog

+2009-04-22  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c/39855
+	* gcc.dg/torture/pr39855.c: New test.
+
 2009-04-22  Dodji Seketeli  <dodji@redhat.com>
 
 	PR c++/39639

gcc/testsuite/gcc.dg/torture/pr39855.c

+/* PR c/39855 */
+/* { dg-do run { target { int32plus } } } */
+
+extern void abort (void);
+
+int i, j, k;
+
+int
+foo (void)
+{
+  return ++i;
+}
+
+int
+main ()
+{
+  if (__CHAR_BIT__ != 8 || sizeof (int) != 4)
+    return 0;
+  j = foo () << 30 << 2;
+  k = (unsigned) foo () >> 16 >> 16;
+  if (i != 2 || j != 0 || k != 0)
+    abort ();
+  return 0;
+}