topic Yes, std:: can be omitted in Intel® oneAPI Math Kernel Library

Getting different output when calling the same FFT functions of MKL from C++ and Fortran

Zonglin_G_ — Thu, 14 Jan 2016 08:17:16 GMT

I got different results when calling same functions of MKL from intel C++ compiler and IVF separately. I'm in windows 10 64bit, and VS 2013. My C++ code is as follows:

#include <complex>
#include <iostream>
#include "mkl_dfti.h"
int main()
{
using namespace std;
complex <float>x[32];
int i;
DFTI_DESCRIPTOR_HANDLE my_desc1_handle;
MKL_LONG status;

for (i = 0; i < 32; i++)
{
x = ((i+1)*1.0, (i+1)*1.0);
}

status = DftiCreateDescriptor(&my_desc1_handle, DFTI_SINGLE, DFTI_COMPLEX, 1, 32);
status = DftiCommitDescriptor(my_desc1_handle);
status = DftiComputeForward(my_desc1_handle, x);
status = DftiFreeDescriptor(&my_desc1_handle);

for (i = 0; i < 32; i++)
{
cout << x << endl;
}

return 0;
}

My fortran code is as follows:

PROGRAM MAIN
    USE MKL_DFTI
    IMPLICIT NONE
    Complex :: X(32)
    INTEGER ::   I
    type(DFTI_DESCRIPTOR), POINTER :: My_Desc1_Handle
    Integer :: Status
    DO I=1,32
        X(I)=CMPLX(1d0*i,1d0*i)
    END DO
    Status = DftiCreateDescriptor( My_Desc1_Handle, DFTI_SINGLE, DFTI_COMPLEX, 1, 32 )
    Status = DftiCommitDescriptor( My_Desc1_Handle )
    Status = DftiComputeForward( My_Desc1_Handle, X )
    Status = DftiFreeDescriptor(My_Desc1_Handle)
    DO I=1,32
        write(*,*)x(i)
    END DO
    END

And results from these two programs are listed in the following columns. Left from C++, and right column from fortran.

(528,0)                    (528.0000,528.0000)
(-16,162.451)          (-178.4507,146.4507)
(-16,80.4374)          (-96.43743,64.43743)
(-16,52.7449)          (-68.74493,36.74493)
(-16,38.6274)          (-54.62742,22.62742)
(-16,29.9339)          (-45.93389,13.93389)
(-16,23.9457)          (-39.94569,7.945690)
(-16,19.4961)          (-35.49606,3.496048)
(-16,16)                   (-32.00000,0.0000000E+00)
(-16,13.1309)          (-29.13086,-2.869139)
(-16,10.6909)          (-26.69086,-5.309141)
(-16,8.55218)          (-24.55218,-7.447823)
(-16,6.62742)          (-22.62742,-9.372583)
(-16,4.85355)          (-20.85354,-11.14646)
(-16,3.1826)           (-19.18260,-12.81741)
(-16,1.57586)          (-17.57586,-14.42414)
(-16,0)                    (-16.00000,-16.00000)
(-16,-1.57586)         (-14.42414,-17.57586)
(-16,-3.18259)         (-12.81741,-19.18260)
(-16,-4.85355)         (-11.14646,-20.85355)
(-16,-6.62742)         (-9.372583,-22.62742)
(-16,-8.55217)         (-7.447824,-24.55218)
(-16,-10.6909)         (-5.309142,-26.69086)
(-16,-13.1309)         (-2.869135,-29.13086)
(-16,-16)                 (0.0000000E+00,-32.00000)
(-16,-19.4961)         (3.496052,-35.49606)
(-16,-23.9457)         (7.945692,-39.94569)
(-16,-29.9339)         (13.93390,-45.93390)
(-16,-38.6274)         (22.62742,-54.62742)
(-16,-52.7449)         (36.74493,-68.74493)
(-16,-80.4374)         (64.43743,-96.43742)
(-16,-162.451)         (146.4507,-178.4507)

Could someone enlighten me on this? I'm really obsessed with it. Thanks

Hi Zonglin G!

Evgueni_P_Intel — Thu, 14 Jan 2016 09:49:34 GMT

Hi Zonglin G!

Could you please check if casting 32 to MKL_LONG in the C++ code fixes the problem?

DftiCreateDescriptor is variadic, so the compiler cannot do this cast for you.

Evgueni.

Quote:Evgueni Petrov aka

Zonglin_G_ — Thu, 14 Jan 2016 12:24:12 GMT

Evgueni Petrov aka espetrov (Intel) wrote:

Hi Zonglin G!

Could you please check if casting 32 to MKL_LONG in the C++ code fixes the problem?

DftiCreateDescriptor is variadic, so the compiler cannot do this cast for you.

Evgueni.

Thank you very much for your reply!

I happen to be a beginner. So I'm wondering if you could be more specific?

Could you please add (MKL

Evgueni_P_Intel — Thu, 14 Jan 2016 12:29:15 GMT

Could you please add (MKL_LONG) so that DftiCreateDescriptor is called as follows?

status = DftiCreateDescriptor(&my_desc1_handle, DFTI_SINGLE, DFTI_COMPLEX, 1, (MKL_LONG)32);

Quote:Evgueni Petrov aka

Zonglin_G_ — Thu, 14 Jan 2016 13:52:58 GMT

Evgueni Petrov aka espetrov (Intel) wrote:

Could you please add (MKL_LONG) so that DftiCreateDescriptor is called as follows?

status = DftiCreateDescriptor(&my_desc1_handle, DFTI_SINGLE, DFTI_COMPLEX, 1, (MKL_LONG)32);

Thank you for your patience with me.

I tried your suggestion, but the problem remains.

Could you please also change

Evgueni_P_Intel — Thu, 14 Jan 2016 19:19:47 GMT

Could you please also change initialization of the input array as follows?

for (i = 0; i < 32; i++)
{
x = std::complex<float>((i+1)*1.0,(i+1)*1.0);
}

Quote:Evgueni Petrov aka

Zonglin_G_ — Fri, 15 Jan 2016 01:19:41 GMT

Evgueni Petrov aka espetrov (Intel) wrote:

Could you please also change initialization of the input array as follows?

for (i = 0; i < 32; i++)
{
x = std::complex<float>((i+1)*1.0,(i+1)*1.0);
}

Yes, it works now!

I really appreicate your help!

But why should we explicitly specify "std::"? Is it meant to avoid some conflicts of name?

Yes, std:: can be omitted

Evgueni_P_Intel — Fri, 15 Jan 2016 04:41:30 GMT

Yes, std:: can be omitted since 'using namespace std;' is present.