Add special handling for u256 divided by a single word

Author: mborland

include/boost/decimal/detail/u256.hpp

@@ -1077,7 +1077,25 @@ BOOST_DECIMAL_FORCE_INLINE constexpr auto div_mod(const u256& lhs, const Unsigne
 
     BOOST_DECIMAL_DETAIL_INT128_ASSERT(m >= n);
 
-    int128::detail::impl::knuth_divide<true>(u, m, v, n, q);
+    // Simplify handling of single word division
+    // We run into this case with dividing by powers of 10 while rounding u256
+    if (n == 1U)
+    {
+        std::uint64_t remainder {};
+
+        for (std::size_t j = m; j-- > 0;)
+        {
+            const auto dividend {(remainder << 32) | u[j]};
+            q[j] = static_cast<std::uint32_t>(dividend / v[0]);
+            remainder = dividend % v[0];
+        }
+
+        u[0] = static_cast<std::uint32_t>(remainder);
+    }
+    else
+    {
+        int128::detail::impl::knuth_divide<true>(u, m, v, n, q);
+    }
 
      return {from_words(q), from_words(u)};
 }