re PR libstdc++/47913 ([C++0x] improve ratio_add to overflow less often)

2011-03-02  Marc Glisse  <marc.glisse@normalesup.org>

	PR libstdc++/47913
	* include/std/ratio (ratio_add): Avoid denominator overflow.
	* testsuite/20_util/ratio/operations/47913.cc: New.

From-SVN: r170616

libstdc++-v3/ChangeLog

+2011-03-02  Marc Glisse  <marc.glisse@normalesup.org>
+
+	PR libstdc++/47913
+	* include/std/ratio (ratio_add): Avoid denominator overflow.
+	* testsuite/20_util/ratio/operations/47913.cc: New.
+
 2011-02-28  Benjamin Kosnik  <bkoz@redhat.com>
 
 	* testsuite/20_util/hash/chi2_quality.cc: Use C++0x mode on simulators.

libstdc++-v3/include/std/ratio

@@ -177,15 +177,19 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     struct ratio_add
     {
     private:
-      static const intmax_t __gcd =
+      static constexpr intmax_t __gcd =
         __static_gcd<_R1::den, _R2::den>::value;
+      static constexpr intmax_t __n = __safe_add<
+        __safe_multiply<_R1::num, (_R2::den / __gcd)>::value,
+        __safe_multiply<_R2::num, (_R1::den / __gcd)>::value>::value;
+
+      // The new numerator may have common factors with the denominator,
+      // but they have to also be factors of __gcd.
+      static constexpr intmax_t __gcd2 = __static_gcd<__n, __gcd>::value;
       
     public:
-      typedef ratio<
-        __safe_add<
-          __safe_multiply<_R1::num, (_R2::den / __gcd)>::value,
-          __safe_multiply<_R2::num, (_R1::den / __gcd)>::value>::value,
-        __safe_multiply<_R1::den, (_R2::den / __gcd)>::value> type;
+      typedef ratio<__n / __gcd2,
+        __safe_multiply<_R1::den / __gcd2, _R2::den / __gcd>::value> type;
 
       static constexpr intmax_t num = type::num;
       static constexpr intmax_t den = type::den;

libstdc++-v3/testsuite/20_util/ratio/operations/47913.cc

+// { dg-options "-std=gnu++0x" }
+// { dg-require-cstdint "" }
+
+// Copyright (C) 2011 Free Software Foundation
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+#include <ratio>
+#include <testsuite_hooks.h>
+
+// libstdc++/47913
+void test01()
+{
+  bool test __attribute__((unused)) = true;
+  using namespace std;
+
+  const intmax_t m = (intmax_t)1 << (4 * sizeof(intmax_t) - 1);
+  typedef ratio_add<ratio<1, (m - 1) * (m - 2)>,
+                    ratio<1, (m - 3) * (m - 2)> > ra_type;
+
+  VERIFY( ra_type::num == 2 );
+  VERIFY( ra_type::den == (m - 1) * (m - 3) );
+}
+
+int main()
+{
+  test01();
+  return 0;
+}