cintegermodulus

Is fmod faster than % for integer modulus calculation

Just found the following line in some old src code:

int e = (int)fmod(matrix[i], n);

where matrix is an array of int, and n is a size_t

I'm wondering why the use of fmod rather than % where we have integer arguments, i.e. why not:

int e = (matrix[i]) % n;

Could there possibly be a performance reason for choosing fmod over % or is it just a strange bit of code?

Solution

  • Could there possibly be a performance reason for choosing fmod over % or is it just a strange bit of code?

    The fmod might be a bit faster on architectures with high-latency IDIV instruction, that takes (say) ~50 cycles or more, so fmod's function call and int <---> doubleconversions cost can be amortized.

    According to Agner's Fog instruction tables, IDIV on AMD K10 architecture takes 24-55 cycles. Comparing with modern Intel Haswell, its latency range is listed as 22-29 cycles, however if there are no dependency chains, the reciprocal throughput is much better on Intel, 8-11 clock cycles.