Here’s a reference table that might help. This shows an example of every possible combination of the 4 arithmetic flags that can result from the ADD and SUB instructions on x86. ‘h’ ‘ud’ and ‘d’ stand for hex, unsigned decimal and signed decimal representations of each value. For example, the first row for SUB says … Read more
Actually gcc will use the carry automatically if you write your code carefully… Current GCC can optimize hiWord += (loWord < loAdd); into add/adc (x86’s add-with-carry). This optimization was introduced in GCC5.3. With separate uint64_t chunks in 64-bit mode: https://godbolt.org/z/S2kGRz. And the same thing in 32-bit mode with uint32_t chunks: https://godbolt.org/z/9FC9vc (editor’s note: Of course … Read more
The bits of the result are obtained from the truncated sum of unsigned integers. The add instruction doesn’t care about the sign here nor does it care about your own interpretation of the integers as signed or unsigned. It just adds as if the numbers were unsigned. The carry flag (or borrow in case of … Read more