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

diff --git a/gcc/c-family/ChangeLog b/gcc/c-family/ChangeLog
index 639fd2254cc0f537cae7dbcded7edd4909075029..f70b6f838322ac9d80c847db095e4c92368c5c62 100644 (file)
--- a/gcc/c-family/ChangeLog
+++ b/gcc/c-family/ChangeLog
@@ -1,3 +1,9 @@
+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

diff --git a/gcc/c-family/c-warn.c b/gcc/c-family/c-warn.c
index f86de10fdd9477d0a72eb503c807d3f6f42932de..2eb4cf5dd41537ab498dbe608b5ca60f85ec24b2 100644 (file)
--- a/gcc/c-family/c-warn.c
+++ b/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)

diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index f01325a3f0adc09c5a91ebe4af9bfd5992826ae6..1021c6ab563c590ff134a9a31fb238f4e871208e 100644 (file)
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+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.

diff --git a/gcc/testsuite/c-c++-common/Wtautological-compare-6.c b/gcc/testsuite/c-c++-common/Wtautological-compare-6.c
new file mode 100644 (file)
index 0000000..fcdca72
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/Wtautological-compare-6.c
@@ -0,0 +1,11 @@
+/* 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;
+}