Reduce operations in umul256

include/boost/decimal/detail/emulated256.hpp

@@ -551,26 +551,15 @@ constexpr uint256_t umul256_impl(std::uint64_t a_high, std::uint64_t a_low, std:
  const auto mid_product2 {static_cast<unsigned_int128_type>(a_high) * b_low};
  const auto high_product {static_cast<unsigned_int128_type>(a_high) * b_high};
 
- std::uint64_t carry {};
+ const auto mid_sum {mid_product1 + mid_product2};
+ bool carry {mid_sum < mid_product1};
 
- const auto mid_combined {mid_product1 + mid_product2};
- if (mid_combined < mid_product1)
- {
- carry = 1;
- }
-
- const auto mid_combined_high {mid_combined >> 64};
- const auto mid_combined_low {mid_combined << 64};
-
- const auto low_sum {low_product + mid_combined_low};
- if (low_sum < low_product)
- {
- carry += 1;
- }
+ const auto low_sum {low_product + (mid_sum << 64)};
+ carry += (low_sum < low_product);
 
  uint256_t result {};
  result.low = low_sum;
- result.high = high_product + mid_combined_high + carry;
+ result.high = high_product + (mid_sum >> 64) + carry;
 
  return result;
 }