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Matt_S_

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01-21-2015
01:37 PM

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Fortran FFT with MKL

I'm currently messing with some code that uses an old F77 looking FFT subroutine full of goto's and other nasty stuff. I have been looking around at using MKL's FFT routines, but I am having trouble finding some examples of how they are used. My current FFT routine uses only double precision data, so I don't need a FFT dealing with complex. Can anyone point me to any example code using a 2D MKL FFT routine? (possibly within /opt/intel/mkl/examples)

Thanks in advance!

(For any interested, here is the current FFT routine being used)

SUBROUTINE FFT(X,Y,N,ITYPE) implicit none INTEGER N,M,ITYPE real*8 X(*), Y(*) INTEGER I, J, K, N1, N2, N4, IS, ID, I0, I1, I2, I3 real*8 TWOPI, E, A, A3, CC1, SS1, CC3, SS3 real*8 R1, R2, S1, S2, S3, XT INTRINSIC SIN, COS PARAMETER ( TWOPI = 6.2831853071795864769 ) IF ( N .EQ. 1 ) RETURN M=nint(log(real(N))/log(2.0)) IF ( ITYPE .EQ. -1 ) THEN DO 1, I = 1, N Y(I) = - Y(I) 1 CONTINUE ENDIF N2 = 2 * N DO 10, K = 1, M-1 N2 = N2 / 2 N4 = N2 / 4 E = TWOPI / N2 A = 0.0 DO 20, J = 1, N4 A3 = 3 * A CC1 = COS( A ) SS1 = SIN( A ) CC3 = COS( A3 ) SS3 = SIN( A3 ) A = J * E IS = J ID = 2 * N2 40 DO 30, I0 = IS, N-1, ID I1 = I0 + N4 I2 = I1 + N4 I3 = I2 + N4 R1 = X(I0) - X(I2) X(I0) = X(I0) + X(I2) R2 = X(I1) - X(I3) X(I1) = X(I1) + X(I3) S1 = Y(I0) - Y(I2) Y(I0) = Y(I0) + Y(I2) S2 = Y(I1) - Y(I3) Y(I1) = Y(I1) + Y(I3) S3 = R1 - S2 R1 = R1 + S2 S2 = R2 - S1 R2 = R2 + S1 X(I2) = R1 * CC1 - S2 * SS1 Y(I2) = - S2 * CC1 - R1 * SS1 X(I3) = S3 * CC3 + R2 * SS3 Y(I3) = R2 * CC3 - S3 * SS3 30 CONTINUE IS = 2 * ID - N2 + J ID = 4 * ID IF ( IS .LT. N ) GOTO 40 20 CONTINUE 10 CONTINUE !C--------LAST STAGE, LENGTH-2 BUTTERFLY ----------------------C IS = 1 ID = 4 50 DO 60, I0 = IS, N, ID I1 = I0 + 1 R1 = X(I0) X(I0) = R1 + X(I1) X(I1) = R1 - X(I1) R1 = Y(I0) Y(I0) = R1 + Y(I1) Y(I1) = R1 - Y(I1) 60 CONTINUE IS = 2 * ID - 1 ID = 4 * ID IF ( IS .LT. N ) GOTO 50 !C-------BIT REVERSE COUNTER-----------------------------------C 100 J = 1 N1 = N - 1 DO 104, I = 1, N1 IF ( I .GE. J ) GOTO 101 XT = X(J) X(J) = X(I) X(I) = XT XT = Y(J) Y(J) = Y(I) Y(I) = XT 101 K = N / 2 102 IF ( K .GE. J ) GOTO 103 J = J - K K = K / 2 GOTO 102 103 J = J + K 104 CONTINUE IF ( ITYPE .EQ. -1 ) THEN do I = 1, N Y(I) = - Y(I) end do else do i=1,n x(i)=x(i)/n y(i)=y(i)/n end do ENDIF RETURN END

I am unfamiliar with "length 2 butterfly" and "bit reverse counter" in FFT. If there is any specific MKL FFT routine that performs these same calculations?

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Zhang_Z_Intel

Employee

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01-21-2015
04:34 PM

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Hi

Looks like this page is exactly what you need. 2D FFT examples are found in the middle of the page.

https://software.intel.com/en-us/node/522290#AC30C86F-9E01-4A5F-B086-CE7FBCE2126A ;

Matt_S_

Beginner

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01-23-2015
09:04 AM

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Zhang, thank you for the link! I have discovered where the FFT currently used came from and it is no standard FFT. I could not replicate the results from any of the MKL FFT routine examples. The FFT used can be found here: https://github.com/syntheticpp/benchfft/blob/master/benchees/burrus/sffteu.f and it is described as, "a slight modification of a complex split radix FFT routine presented by C.S. Burrus."

Does anyone know if a variation of MKL's FFT performs anything of the sort?

Thanks again.

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