Give the PowerPC long double force_rounding operand as output and input

include/boost/numeric/interval/detail/ppc_rounding_control.hpp

@@ -103,11 +103,14 @@ struct rounding_control<long double>:
 #if __SIZEOF_LONG_DOUBLE__ == 8
     // Same format as double.
     __asm__ __volatile__ ("" : "+d"(r_));
+#elif defined(__clang__)
+    // Clang has no register constraint that works for both 128-bit ABIs.
+    __asm__ __volatile__ ("" : "+m"(r_));
 #else
-    // Which register can hold a 128-bit long double depends on the ABI: FPRs
-    // for IBM double-double, a VSX register for IEEE 128-bit. Clang accepts
-    // neither in all cases, hence the memory fallback.
-    __asm__ __volatile__ ("" : "+wa,d,m"(r_));
+    // FPRs for IBM double-double, a VSX register for IEEE 128-bit. A single
+    // alternative, as GCC rejects a "+" operand with several when the value
+    // is a constant.
+    __asm__ __volatile__ ("" : "+wadm"(r_));
 #endif
     return r_;
   }

test/rounding_mode.cpp

@@ -45,9 +45,21 @@ void test() {
   BOOST_TEST(div_down<T>(one, three) < div_up<T>(one, three));
 }
 
+// Exact constants reach force_rounding once the division is folded; GCC
+// used to reject the PowerPC long double asm then.
+template<class T>
+void test_constant() {
+  typedef boost::numeric::interval<T> I;
+  I q = I(6) / I(2);
+  BOOST_TEST(q.lower() == 3 && q.upper() == 3);
+}
+
 int main() {
   test<float>();
   test<double>();
   test<long double>();
+  test_constant<float>();
+  test_constant<double>();
+  test_constant<long double>();
   return boost::report_errors();
 }