Its faster(if you do addition like 1e6(1,000,000) or more times in a program, writing an addition function using this if instead of % will save a lot of time),but this can be used only for addition and a similar if for subtraction. It can not be used for multiplication.

The plausible reason should be that the execution time of the statement if(a >= b) a -= b; is more likely to be less than the execution time of the statement a %= b; when it is guaranteed that the expression a < 2*b is true. You may write a short C++ program to compare the average execution time of both statements so as to confirm or refute this hypothesis.

because

  const uint64_t maxn = 1000000009;
  uint64_t ans = 0;
  for (int i = 0; i < maxn; ++i) {
    ans += i;
    if (ans >= maxn) ans -= maxn;
  }

1000 ms

  const uint64_t maxn = 1000000009;
  uint64_t ans = 0;
  for (int i = 0; i < maxn; ++i) {
    ans = (ans + i) % maxn;
  }

8700 ms