add dfft benchmark

Author: cval26

example/bench03_dfft.f90

@@ -0,0 +1,64 @@
+program main
+use fftpack, only: dffti, dfftf, dfftb, rfft, irfft
+use fftpack_kind, only: rk
+
+implicit none
+integer, parameter :: N = 1024*135*77  !> (2**10)*(3**3)*5*7*11
+
+real(rk), dimension(N) :: x, y
+real(rk), dimension(2*N+15) :: w
+real(rk) :: err, time_i, time_f, time_b, t1, t2
+
+call random_number(x)
+y = x
+
+print *, "03: Benchmarking dfft & rfft"
+
+call cpu_time(t1)
+call dffti(N, w)
+call cpu_time(t2)
+time_i = t2-t1
+print *, "Initializing: done"
+
+call cpu_time(t1)
+call dfftf(N, y, w)
+call cpu_time(t2)
+time_f = t2-t1
+print *, "Forward: done"
+
+call cpu_time(t1)
+call dfftb(N, y, w)
+call cpu_time(t2)
+time_b = t2-t1
+print *, "Backward: done"
+print *, ""
+
+err = maxval(abs(x-y/N))
+print *, "--dfft"
+print *, "Error: ", err
+print *, "Init. time: ", time_i
+print *, "Forward time: ", time_f
+print *, "Backward time: ", time_b
+print *, ""
+
+print *, "Comparing to calls through rfft"
+call cpu_time(t1)
+y = rfft(x)
+call cpu_time(t2)
+time_f = t2-t1
+print *, "Init. & forward: done"
+
+call cpu_time(t1)
+y = irfft(y)
+call cpu_time(t2)
+time_b = t2-t1
+print *, "Backward: done"
+print *, ""
+
+err = maxval(abs(x-y/N))
+print *, "--rfft"
+print *, "Error: ", err
+print *, "Init. & forward time: ", time_f
+print *, "Backward time: ", time_b
+print *, ""
+end program