Avoid 128-bit math

Author: mborland

include/boost/int128/numeric.hpp

@@ -315,6 +315,14 @@ constexpr uint128_t gcd(uint128_t a, uint128_t b) noexcept
         }
 
         b -= a;
+
+        // Stop doing 128-bit math as soon as we can
+        if (a.high == 0U && b.high == 0U)
+        {
+            const auto g {detail::gcd64(a.low, b.low)};
+            return uint128_t{0, g} << shift;
+        }
+
     } while (b != 0U);
 
     return a << shift;