Speed up mul_impl for decimal32 and add additional impl for dec32_fast

include/boost/decimal/detail/mul_impl.hpp

@@ -26,14 +26,32 @@ namespace detail {
 
 template <typename ReturnType, typename T, typename U>
 BOOST_DECIMAL_FORCE_INLINE constexpr auto mul_impl(T lhs_sig, U lhs_exp, bool lhs_sign,
- T rhs_sig, U rhs_exp, bool rhs_sign) noexcept -> std::enable_if_t<detail::is_decimal_floating_point_v<ReturnType>, ReturnType>
+ T rhs_sig, U rhs_exp, bool rhs_sign) noexcept -> std::enable_if_t<std::is_same<ReturnType, decimal32_fast>::value, ReturnType>
 {
  using mul_type = std::uint_fast64_t;
 
- auto res_sig {static_cast<mul_type>(lhs_sig) * static_cast<mul_type>(rhs_sig)};
- auto res_exp {lhs_exp + rhs_exp};
+ const auto res_sig {static_cast<mul_type>(lhs_sig) * static_cast<mul_type>(rhs_sig)};
+ const auto res_exp {lhs_exp + rhs_exp};
+
+ return {res_sig, res_exp, lhs_sign != rhs_sign && res_sig != static_cast<mul_type>(0)};
+}
+
+template <typename ReturnType, typename T, typename U>
+BOOST_DECIMAL_FORCE_INLINE constexpr auto mul_impl(T lhs_sig, U lhs_exp, bool lhs_sign,
+ T rhs_sig, U rhs_exp, bool rhs_sign) noexcept -> std::enable_if_t<std::is_same<ReturnType, decimal32>::value, ReturnType>
+{
+ using mul_type = std::uint_fast64_t;
+
+ // The constructor needs to calculate the number of digits in the significand which for uint128 is slow
+ // Since we know the value of res_sig is constrained to [1'000'000^2, 9'999'999^2] which equates to
+ // either 13 or 14 decimal digits we can use a single division to make binary search occur with
+ // uint32_t instead. 14 - 5 = 9 or 13 - 5 = 8 which are both still greater than or equal to
+ // digits10 + 1 for rounding which is 8 decimal digits
+
+ auto res_sig {(static_cast<mul_type>(lhs_sig) * static_cast<mul_type>(rhs_sig)) / pow10(static_cast<mul_type>(5))};
+ auto res_exp {lhs_exp + rhs_exp + static_cast<U>(5)};
 
- return {res_sig, res_exp, lhs_sign != rhs_sign && res_sig != 0};
+ return {static_cast<std::uint32_t>(res_sig), res_exp, lhs_sign != rhs_sign && res_sig != static_cast<mul_type>(0)};
 }
 
 template <typename ReturnType, typename T, typename U>