# CUDA backward real complex FFT

**URL:** <https://discourse.julialang.org/t/cuda-backward-real-complex-fft/137254>\
**Category:** GPU\
**Tags:** question, error\
**Created:** [May 23, 2026, 7:42am UTC](https://discourse.julialang.org/t/cuda-backward-real-complex-fft/137254 "2026-05-23T07:42:00Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![FerreolS](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ferreols/32/216854_2.png) [@FerreolS](https://discourse.julialang.org/u/FerreolS)\
**Post date:** [May 23, 2026, 7:42am UTC](https://discourse.julialang.org/t/cuda-backward-real-complex-fft/137254/1 "2026-05-23T07:42:00Z")

</div>

I’m trying to perform a convolution using CUDA fft and something puzzle me when comparing CPU and GPU result:

```julia
using Adapt, CUDA, FFTW, AbstractFFTs
T = Float32
sz = (2048,2048)
x = randn(T,sz);
x_gpu = adapt(CuArray,x);
d = randn(Complex{T},(1025,2048))
d_gpu = adapt(CuArray,d)

```

GPU and CPU output are not equal when the backward brfft gives a (2048,2048) output but they are equal for a (2049,2048) output:

```julia

julia> adapt(Array, brfft(d_gpu .* rfft(x_gpu), 2048)) ≈ brfft(d .* rfft(x),2048)
false

julia> adapt(Array, brfft(d_gpu .* rfft(x_gpu), 2049)) ≈ brfft(d .* rfft(x),2049)
true

julia> adapt(Array, brfft(rfft(x_gpu), 2048)) ≈ brfft(rfft(x),2048)
true

julia> adapt(Array, d_gpu) ≈ d
true

julia> adapt(Array, d_gpu .* rfft(x_gpu)) ≈ (d .* rfft(x))
true

```

and it is not just rounding error:

```julia
julia> adapt(Array, brfft(d_gpu .* rfft(x_gpu), 2048)) .- brfft(d .* rfft(x),2048)
2048×2048 Matrix{Float32}:
   -8465.38 13274.8 -119964.0 1.24876f5 7778.25 -1.24958f5 … 33341.4 18242.2 -75873.0 -30010.5 -1052.38 228162.0 -1.99176f5
      -1.00414f5 -51067.1 1.1154f5 -54599.9 25838.5 -96901.0 -18502.0 26938.0 -22623.0 -31004.0 -36856.9 -5554.5 48698.5
   -8464.75 13277.2 -1.19968f5 1.24872f5 7778.88 -124956.0 33340.8 18242.8 -75874.0 -30015.0 -1050.0 2.28162f5 -1.99173f5
      -1.00412f5 -51064.5 1.11541f5 -54601.5 25842.0 -96899.5 -18504.0 26935.0 -22622.5 -31004.0 -36852.8 -5557.5 48695.5
   -8466.75 13277.8 -1.19967f5 1.24874f5 7777.0 -1.24954f5 33341.5 18242.5 -75875.1 -30013.0 -1050.0 2.2816f5 -199175.0
      -1.00414f5 -51066.6 111541.0 -54601.5 25840.0 -96897.4 … -18502.2 26937.6 -22622.8 -31005.4 -36857.1 -5553.75 48695.0
       ⋮ ⋮ ⋱ ⋮                     
   -8465.0 13274.6 -1.19964f5 1.24875f5 7781.88 -1.24954f5 33339.8 18240.2 -75877.8 -30018.4 -1054.5 2.2816f5 -1.99176f5
 -100414.0 -51064.0 1.11544f5 -54599.1 25837.2 -96902.5 -18507.0 26936.2 -22623.2 -31008.0 -36857.5 -5552.5 48692.8
   -8463.5 13273.8 -1.19966f5 1.24875f5 7776.12 -1.24952f5 33341.0 18241.5 -75874.0 -30012.0 -1052.75 2.28162f5 -199176.0
      -1.00414f5 -51064.0 1.11542f5 -54601.0 25840.9 -96903.0 … -18503.0 26940.1 -22624.0 -31004.8 -36853.8 -5556.0 48693.8
   -8462.0 13276.5 -119968.0 1.24873f5 7778.75 -1.24956f5 33337.2 18241.0 -75875.8 -30011.4 -1052.75 2.28161f5 -1.99174f5
 -100413.0 -51062.0 1.1154f5 -54601.5 25843.5 -96899.5 -18498.5 26935.5 -22623.8 -31006.8 -36856.8 -5554.5 48695.5

```

I can’t see where is my issue. Any hint?

---

<div class="post-metadata">

**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [May 23, 2026, 1:54pm UTC](https://discourse.julialang.org/t/cuda-backward-real-complex-fft/137254/2 "2026-05-23T13:54:37Z")

</div>

> [@FerreolS](#):
>
> I can’t see where is my issue. Any hint?

The input to a complex-to-real FFT is not arbitrary — it has some conjugate-symmetries implied by the real-to-complex FFT (because the output has slightly more data than the input). When you give it an input that does not have these symmetries, the FFT algorithm performs some kind of projection, but exactly what projection it performs might differ between implementations.

Try setting `d = rfft(randn(T, sz))` to ensure that `d` has the correct symmetries.

(For example, if you have a 1d real array `x` of length 8 and perform an `rfft`, the output is a complex array `z` of length 5 in which the first and last elements are purely real. If you perform a `brfft(z, 8)`, it expects `z` to be a length-5 array with the first and last elements purely real. If you give it a `randn(ComplexF64, 5)` array, most likely it just ignores the imaginary parts of the first and last elements, but this may be implementation-dependent. For multi-dimensional transforms, the “edges” of the array have some more complicated conjugate symmetries.)

---

<div class="post-metadata">

**Author:** ![FerreolS](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ferreols/32/216854_2.png) [@FerreolS](https://discourse.julialang.org/u/FerreolS)\
**Post date:** [May 23, 2026, 9:12pm UTC](https://discourse.julialang.org/t/cuda-backward-real-complex-fft/137254/3 "2026-05-23T21:12:58Z")

</div>

Thank you, I was so focused on why my (slightly different) unit tests failed that didn’t think I was just testing implementation of FFT in that case. I guess that CUDA implementation differs from FFTW.