re PR c/82437 (false-positive -Wtautological-compare warning with -std=gnu89)

	PR c/82437
	* c-warn.c (warn_tautological_bitwise_comparison): Instead of
	using to_widest use wide_int with the larger of the two precisions.

	* c-c++-common/Wtautological-compare-6.c: New test.

From-SVN: r253476

gcc/c-family/ChangeLog

+2017-10-06  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c/82437
+	* c-warn.c (warn_tautological_bitwise_comparison): Instead of
+	using to_widest use wide_int with the larger of the two precisions.
+
 2017-10-05  Bernd Edlinger  <bernd.edlinger@hotmail.de>
 
 	* c-pretty-print.c (pp_c_parameter_type_list): Print ... for variadic

gcc/c-family/c-warn.c

@@ -356,16 +356,24 @@ warn_tautological_bitwise_comparison (location_t loc, tree_code code,
     return;
 
   /* Note that the two operands are from before the usual integer
-     conversions, so their types might not be the same.  */
-  widest_int res;
+     conversions, so their types might not be the same.
+     Use the larger of the two precisions and ignore bits outside
+     of that.  */
+  int prec = MAX (TYPE_PRECISION (TREE_TYPE (cst)),
+		  TYPE_PRECISION (TREE_TYPE (bitopcst)));
+
+  wide_int bitopcstw = wide_int::from (bitopcst, prec, UNSIGNED);
+  wide_int cstw = wide_int::from (cst, prec, UNSIGNED);
+
+  wide_int res;
   if (TREE_CODE (bitop) == BIT_AND_EXPR)
-    res = wi::to_widest (bitopcst) & wi::to_widest (cst);
+    res = bitopcstw & cstw;
   else
-    res = wi::to_widest (bitopcst) | wi::to_widest (cst);
+    res = bitopcstw | cstw;
 
   /* For BIT_AND only warn if (CST2 & CST1) != CST1, and
      for BIT_OR only if (CST2 | CST1) != CST1.  */
-  if (res == wi::to_widest (cst))
+  if (res == cstw)
     return;
 
   if (code == EQ_EXPR)

gcc/testsuite/ChangeLog

+2017-10-06  Jakub Jelinek  <jakub@redhat.com>
+
+	PR c/82437
+	* c-c++-common/Wtautological-compare-6.c: New test.
+
 2017-10-06  Richard Biener  <rguenther@suse.de>
 
 	* gcc.dg/graphite/id-15.c: No longer expect a code generation error.

gcc/testsuite/c-c++-common/Wtautological-compare-6.c

+/* PR c/82437 */
+/* { dg-do compile { target int32plus } } */
+/* { dg-options "-Wtautological-compare" } */
+
+int
+foo (unsigned int x)
+{
+  if ((x & -1879048192) != -1879048192)	/* { dg-bogus "bitwise comparison always evaluates to" } */
+    return 0;
+  return 1;
+}