Improve int128::operator+

Author: mborland

include/boost/decimal/detail/emulated128.hpp

@@ -645,8 +645,8 @@ struct int128
     constexpr auto operator==(int rhs) const noexcept -> bool;
     constexpr auto operator>(int rhs) const noexcept -> bool;
 
-    friend constexpr auto operator+(int128 lhs, int128 rhs) noexcept -> int128;
-    friend constexpr auto operator-(int128 lhs, int128 rhs) noexcept -> int128;
+    friend constexpr auto operator+(const int128& lhs, const int128& rhs) noexcept -> int128;
+    friend constexpr auto operator-(const int128& lhs, const int128& rhs) noexcept -> int128;
 
     #if !defined(BOOST_DECIMAL_DISABLE_IOSTREAM)
     template <typename charT, typename traits>
@@ -764,8 +764,8 @@ constexpr auto operator+(uint128 lhs, uint128 rhs) noexcept -> uint128
         unsigned long long low {};
         unsigned long long high {};
 
-        const auto carry {_addcarry_u64(0, lhs.low, rhs.low, &low)};
-        _addcarry_u64(carry, lhs.high, rhs.high, &high);
+        const auto carry {_addcarryx_u64(0, lhs.low, rhs.low, &low)};
+        _addcarryx_u64(carry, lhs.high, rhs.high, &high);
 
         return uint128{high, low};
     }
@@ -1326,17 +1326,29 @@ constexpr auto int128::operator>(int rhs) const noexcept -> bool
     return *this > static_cast<std::int64_t>(rhs);
 }
 
-constexpr auto operator+(int128 lhs, int128 rhs) noexcept -> int128
+constexpr auto operator+(const int128& lhs, const int128& rhs) noexcept -> int128
 {
+    #ifdef BOOST_DECIMAL_HAS_INT128
+
+    const auto lhs_full {(static_cast<__uint128_t>(lhs.high) << 64) | lhs.low};
+    const auto rhs_full {(static_cast<__uint128_t>(rhs.high) << 64) | rhs.low};
+    const auto result {lhs_full + rhs_full};
+
+    return {static_cast<std::int64_t>(result >> 64), static_cast<std::uint64_t>(result)};
+
+    #else
+
     const auto new_low {lhs.low + rhs.low};
-    const auto new_high {lhs.high + rhs.high + (new_low < lhs.low)};
+    const auto new_high {lhs.high + rhs.high + static_cast<std::int64_t>(new_low < lhs.low)};
     return int128{new_high, new_low};
+
+    #endif
 }
 
-constexpr auto operator-(int128 lhs, int128 rhs) noexcept -> int128
+constexpr auto operator-(const int128& lhs, int128& rhs) noexcept -> int128
 {
     const auto new_low {lhs.low - rhs.low};
-    const auto new_high {lhs.high - rhs.high - (lhs.low < rhs.low ? 1 : 0)};
+    const auto new_high {lhs.high - rhs.high - static_cast<std::int64_t>(lhs.low < rhs.low)};
     return int128{new_high, new_low};
 }