Division for signed ac_fixed could get incorrect result

[zpan007]

Below is an example to show the division difference between signed ac_fixed and double:

#include "ac_types/include/ac_fixed.h"

int main()
{
    ac_fixed<40,30,true> num = -46800;
    ac_fixed<40,30,true> den = 2136;
    ac_fixed<40,30,true> q = num / den;

    std::cout << "division using ac_fixed division = " << q.to_string(AC_BIN) << ", " << q << std::endl;

    q = static_cast<double>(num.to_int64())/den.to_int64();

    std::cout << "division using double   = " << q.to_string(AC_BIN) << ", " << q << std::endl;

   return 0;
}

The output of the above code snippet.

division using ac_fixed = 0b101010.0001011101, -21.9091796875
division using double   = 0b101010.0001011100, -21.91015625

[andres-takach]

Good observation. However, as Table 3 in the documentation indicates the ac_fixed division rounds towards zero. This was done to be consistent with the integer division (both ac_int and C native integers). So if you add the following lines to your code: ac_fixed<40,30,true,*AC_TRN_ZERO*> q3 = static_cast<double>(num.to_int64())/den.to_int64(); std::cout << "division using double with TRN_ZERO = " << q3.to_string(AC_BIN) << ", " << q3 << std::endl; you will find that the results are as expected: division using ac_fixed division = 0b101010.0001011101, *-21.9091796875* division using double = 0b101010.0001011100, -21.91015625 division using double with *TRN_ZERO* = 0b101010.0001011101, *-21.9091796875 * Andres Takach

…

On Thu, Feb 20, 2020 at 10:20 AM Zhengjun Pan ***@***.***> wrote: Below is an example to show the division difference between signed ac_fixed and double: #include "ac_types/include/ac_fixed.h" int main() { ac_fixed<40,30,true> num = -46800; ac_fixed<40,30,true> den = 2136; ac_fixed<40,30,true> q = num / den; std::cout << "division using ac_fixed division = " << q.to_string(AC_BIN) << ", " << q << std::endl; q = static_cast<double>(num.to_int64())/den.to_int64(); std::cout << "division using double = " << q.to_string(AC_BIN) << ", " << q << std::endl; return 0; } The output of the above code snippet. division using ac_fixed = 0b101010.0001011101, -21.9091796875 division using double = 0b101010.0001011100, -21.91015625 — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#9?email_source=notifications&email_token=AEVVDJDFXKYQPC5DXMYQNF3RD2NSLA5CNFSM4KYQ7VB2YY3PNVWWK3TUL52HS4DFUVEXG43VMWVGG33NNVSW45C7NFSM4IPAW64Q>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AEVVDJEOEZAUB6ZST3TEY3DRD2NSLANCNFSM4KYQ7VBQ> .

[zpan007]

Ah, thanks for the documentation details. That explains.