Attempting to avoid all overflow in diagram checks

src/policy/rbf.cpp

@@ -57,13 +57,19 @@ inline std::pair<int64_t, uint64_t> Mul128(int64_t a, int64_t b)
 
 inline std::pair<int64_t, uint64_t> Add128(const std::pair<int64_t, uint64_t>& a,
                                            const std::pair<int64_t, uint64_t>& b) {
-    // Add the low parts
-    uint64_t low = a.second + b.second;
+    uint32_t low_a_low = (uint32_t)a.second;
+    uint32_t low_a_high = a.second >> 32;
+    uint32_t low_b_low = (uint32_t)b.second;
+    uint32_t low_b_high = b.second >> 32;
 
-    // Calculate carry from the low part (if the addition of low parts overflows, it will carry to the high part)
-    int64_t carry = (low < a.second) ? 1 : 0;
+    uint64_t low_sum_low = static_cast<uint64_t>(low_a_low) + low_b_low;
+    uint64_t carry = low_sum_low >> 32; // carry for the next 32 bits
+
+    uint64_t low_sum_high = static_cast<uint64_t>(low_a_high) + low_b_high + carry;
+    carry = low_sum_high >> 32; // carry for the high part
+
+    uint64_t low = (low_sum_high << 32) | (low_sum_low & 0xFFFFFFFF);
 
-    // Add the high parts and the carry
     int64_t high = a.first + b.first + carry;
 
     return {high, low};