First it normalizes to if (num <= 0) { return 0; } int a = 0; int loop_n = (unsigned)(num - 1) / 5; for (int x = 0; x <= loop_n; x+

First it normalizes to if (num <= 0) { return 0; } int a = 0; int loop_n = (unsigned)(num - 1) / 5; for (int x = 0; x <= loop_n; x++) { a += x * 5 - 1; } return a; Division by 5 is done by multiplying by the modular inverse, 3435973837. From there you just have a summation, which has two components. ((loop_n + 1) * loop_n / 2) * 5 // 5x - loop_n - 1 // -1 The assembly between these the manual and automatic methods is slightly different, but the difference is fairly trivial and opaque.